Pblock class 12th notes for iitjee

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ObjectivesAfter studying this Unit, you will be able to

• appreciate general trends in the chemistry

of elements of groups

15,16,17 and 18;

• learn the preparation, properties and uses

of dinitrogen and phosphorus and some of

their important compounds;

• describe the preparation, properties and

uses of dioygen and o!one and chemistry

of some simple oides;

• "now allotropic forms of sulphur,

chemistry of its important compounds and

the structures of its ooacids;

• describe the preparation, properties and

uses of chlorine and hydrochloric acid;

• "now the chemistry of interhalogens and

structures of ooacids of halogens;

• enumerate the uses of noble gases;

• appreciate the importance of these

elements and their compounds in our day

to day life#

Unit7

The p-Block

Elements

Diversity in chemistry is the hallmark of p–block elements manifested in their

ability to react with the elements of s–, d– and f–blocks as well as with their

own.

$n %lass &$, you ha'e learnt that the p(bloc" elements are placed

in groups 1) to 18 of the periodic table# *heir 'alence shell

electronic configuration is ns+np16 -ecept .e which has 1 s+

configuration/# *he properties of p(bloc" elements li"e that of

others are greatly influenced by atomic si!es, ionisation enthalpy,

electron gain enthalpy and electronegati'ity# *he absence of d (

orbitals in second period and presence of d and0or f orbitals in

hea'ier elements -starting from third period onwards/ ha'e

significant effects on the properties of elements# $n addition, the

presence of all the three types of elements; metals, metalloids and

non(metals bring di'ersification in chemistry of these elements#

.a'ing learnt the chemistry of elements of roups 1) and 12of the p(bloc" of periodic table in %lass &$, you will learn the

chemistry of the elements of subse3uent groups in this Unit#

7.1 Group 15

Elements

7.1.1 Occurrence

roup 15 includes nitrogen, phosphorus, arsenic, antimony and bismuth# As we godown the group, there is a shift from non(metallic to metallic through metalloidic

character# 4itrogen and phosphorus are non(metals, arsenic and antimony metalloids

and bismuth is a typical metal#

olecular nitrogen comprises 78 by 'olume of the atmosphere# $n the earths crust,

it occurs as sodium nitrate, 4a4) -called %hile saltpetre/ and potassium nitrate

-$ndian saltpetre/# $t is found in the form of proteins in plants and animals#9hosphorus occurs in minerals



of the apatite family, %a:-92/6# %a&+ -& <, %l or ./ -e#g#, fluorapatite %a :

-92/6# %a<+/ which are the main components of phosphate roc"s# 9hosphorus is an

essential constituent of animal and plant matter# $t is present in bones as well as inli'ing cells# 9hosphoproteins are present in mil" and eggs# Arsenic, antimony and

bismuth are found mainly as sulphide minerals#

*he important atomic and physical properties of this group elements along with

their electronic configurations are gi'en in *able 7#1#

Table 7.1: Atomic and Physical Properties of Group 15 Elements

Atomic number

Atomic mass0g mol 1

=lectronic configuration$onisation

- i H 0-">

=lectronegati'ity

%o'alent radius0pma

$onic radius0pm

elting

?oiling

@ensity0g cm )-+:8

a E

III single bond (E = element!

b E

"– !

c E

"#!

d $hite phosphor%s!

e &rey α 'form at ".) atm!

f *%blimation temperat%re!

g +t )" !

h&rey α 'form! - olec%lar / 0.

*rends of some of the atomic, physical and chemical properties of the group are

discussed below#

7.1.2 Electronic *he 'alence shell electronic configuration of these elements is ns+np)# onfi!uration *he s

orbital in these elements is completely filled and p orbitals are

half(filled, ma"ing their electronic configuration etra stable#

7.1." Atomic and %o'alent and ionic -in a particular state/ radii increase in si!e #onic $adii down the group#

*here is a considerable increase in co'alent radius

from 4 to 9# .owe'er, from As to ?i only a small increase in co'alent radius is

obser'ed# *his is due to the presence of completely filled d and0or f orbitals in

hea'ier members#

7.1.% #onisation $onisation enthalpy decreases down the group due to gradual increase Enthalpy in atomic

si!e# ?ecause of the etra stable half(filled p orbitals electronic

configuration and smaller si!e, the ionisation enthalpy of the group 15 elements is

much greater than that of group 12 elements in the corresponding periods# *he order

of successi'e ionisation enthalpies, as epected is i.1 B i.+ B i.) -*able 7#1/#

%hemistry 166



7.1.5 *he electronegati'ity 'alue, in general, decreases down the group with

Electrone!ati&ity increasing atomic si!e# .owe'er, amongst the hea'ier elements, the

difference is not that much pronounced#

7.1.' Physical All the elements of this group are polyatomic# @initrogen is a diatomic gas Properties while allothers are solids# etallic character increases down the group#

7.1.7 hemical

Properties

4itrogen and phosphorus are non(metals, arsenic and antimony metalloids and

bismuth is a metal# *his is due to decrease in ionisation enthalpy and increase in

atomic si!e# *he boiling points, in general, increase from top to bottom in the group

but the melting point increases upto arsenic and then decreases upto bismuth# =cept

nitrogen, all the elements show allotropy#

Oxidation states and trends in chemical reactivity

*he common oidation states of these elements are ), C) and C5# *he tendency to

ehibit ) oidation state decreases down the group due to increase in si!e and

metallic character# $n fact last member of the group, bismuth hardly forms any

compound in ) oidation state# *he stability of C5 oidation state decreases downthe group# *he only well characterised ?i -D/ compound is ?i<5# *he stability of C5

oidation state decreases and that of C) state increases -due to inert pair effect/ down

the group# 4itrogen ehibits C 1, C +, C 2 oidation states also when it reacts with

oygen# 9hosphorus also shows C1 and C2 oidation states in some ooacids#

$n the case of nitrogen, all oidation states from C1 to C2 tend to disproportionate

in acid solution# <or eample,

).4+ → .4) C .+ C +4

Eimilarly, in case of phosphorus nearly all intermediate oidation states

disproportionate into C5 and ) both in al"ali and acid# .owe'er C) oidation state

in case of arsenic, antimony and bismuth become increasingly stable with respect todisproportionation#

4itrogen is restricted to a maimum co'alency of 2 since only four -one s and

three p/ orbitals are a'ailable for bonding# *he hea'ier elements ha'e 'acant d orbitals in the outermost shell which can be used for bonding -co'alency/ and hence,

epand their co'alence as in 9<6 #

Anomalous properties of nitrogen

4itrogen differs from the rest of the members of this group due to its smaller si!e,

high electronegati'ity, high ionisation enthalpy and non(a'ailability of d orbitals#

4itrogen has uni3ue ability to form p ( p multiple bonds with itself and with other

elements ha'ing small si!e and high electronegati'ity -e#g#, %, /# .ea'ier elements

of this group do not form pπ ( pπ bonds as their atomic orbitals are so large and

diffuse that they cannot ha'e effecti'e o'erlapping# *hus, nitrogen eists as adiatomic molecule with a triple bond -one s and two p/ between the two atoms#

%onse3uently, its bond enthalpy -:21#2 "> mol 1/ is 'ery high# n the contrary,

phosphorus, arsenic and antimony form single bonds as 99, AsAs and EbEb while bismuth forms metallic bonds in elemental state# .owe'er, the single 44 bond is

wea"er than the single 99 bond because of high interelectronic repulsion of the non(

bonding electrons, owing to the small bond length# As a result the catenation

tendency is wea"er in

167 *he p(?loc" =lements



nitrogen# Another factor which affects the chemistry of nitrogen is the absence of d

orbitals in its 'alence shell# ?esides restricting its co'alency to four, nitrogen cannot

form d ) p bond as the hea'ier elements can e#g#, F )9 or F )9 %.+ -F al"yl

group/# 9hosphorus and arsenic can form d ) d bond also with transition metals

when their compounds li"e 9-%+.5/) and As-%6.5/) act as ligands#

-i/ 1eactivity towards hydrogenG All the elements of roup 15 form hydrides of the

type =.) where = 4, 9, As, Eb or ?i# Eome of the properties of these hydrides

are shown in *able 7#+# *he hydrides show regular gradation in their properties#

*he stability of hydrides decreases from 4.) to ?i.) which can be obser'ed

from their bond dissociation enthalpy# %onse3uently, the reducing character of

the hydrides increases# Ammonia is only a mild reducing agent while ?i. ) is

the strongest reducing agent amongst all the hydrides# ?asicity also decreases

in the order 4.) H 9.) H As.) H Eb.) H ?i.)#

Table 7.2: Properties of *ydrides of Group 15 Elements

Property

elting point0I

?oiling point0I

-=./ @istance0pm

.=. angle -J/

f H

V

0"> mol

1

diss H V-=./0"> mol

1

-ii/ 1eactivity towards o2ygenG All these elements form two types of oidesG =+)

and =+5# *he oide in the higher oidation state of the element is more acidic

than that of lower oidation state# *heir acidic character decreases down the

group# *he oides of the type =+) of nitrogen and phosphorus are purely

acidic, that of arsenic and antimony amphoteric and those of bismuth is

predominantly basic#

-iii/ 1eactivity towards halogensG *hese elements react to form two series of halidesG =&) and =&5# 4itrogen does not form pentahalide due to non(

a'ailability of the d orbitals in its 'alence shell# 9entahalides are more co'alent

than trihalides# All the trihalides of these elements ecept those of nitrogen are

stable# $n case of nitrogen, only 4<) is "nown to be stable# *rihalides ecept

?i<) are predominantly co'alent in nature#

-i'/ 1eactivity towards metalsG All these elements react with metals to form their

binary compounds ehibiting ) oidation state, such as, %a) 4+ -calcium

nitride/ %a)9+ -calcium phosphide/, 4a)As+ -sodium arsenide/, Kn)Eb+ -!inc

antimonide/ and g)?i+ -magnesium bismuthide/#

%hemistry 168



*hough nitrogen ehibits C5 oidation state, it does not form

pentahalide# i'e reason#

4itrogen with n +, has s and p orbitals only# $t does not ha'e d

orbitals to epand its co'alence beyond four# *hat is why it does not

form pentahalide#

9.) has lower boiling point than 4.)# LhyM

Unli"e 4.), 9.) molecules are not associated through hydrogen

bonding in li3uid state# *hat is why the boiling point of 9.) is lower

than 4.)#

Example7.1

Solution

Example7.2

Solution

Intext

Questions

7.1 Lhy are pentahalides more co'alent than trihalides M

7.2 Lhy is ?i.) the strongest reducing agent amongst all the hydrides of roup 15elements M

7.2 Dinitrogen Preparation

@initrogen is produced commercially by the li3uefaction and fractional distillation of

air# Ni3uid dinitrogen -b#p# 77#+ I/ distils out first lea'ing behind li3uid oygen -b#p#

:O I/#$n the laboratory, dinitrogen is prepared by treating an a3ueous solution of

ammonium chloride with sodium nitrite#

4.2%$-a3/ C 4a4+-a3/ → 4+-g/ C +.+-l/ C 4a%l -a3/

Emall amounts of 4 and .4) are also formed in this reaction; these

impurities can be remo'ed by passing the gas through a3ueous sulphuric acid

containing potassium dichromate# $t can also be obtained by the thermal

decomposition of ammonium dichromate#

-4.2/+%r +7 ???.eat→ 4+ C 2.+ C %r +)

Dery pure nitrogen can be obtained by the thermal decomposition of sodium or

barium a!ide#

?a-4)/+ →?a C )4+

Properties

@initrogen is a colourless, odourless, tasteless and non(toic gas# $t has two stable

isotopesG 12 4 and 15 4# $t has a 'ery low solubility in water -+)#+ cm) per litre of

water at +7) I and 1 bar pressure/ and low free!ing and boiling points -*able 7#1/#

@initrogen is rather inert at room temperature because of the high bond enthalpy

of 4≡ 4 bond# Feacti'ity, howe'er, increases rapidly with rise in temperature# At

higher temperatures, it directly combines with some metals to form predominantly

ionic nitrides and with non(metals, co'alent nitrides# A few typical reactions areG



6Ni C 4+ ⎯⎯⎯ .eat→ +Ni) 4

)g C 4+ ⎯⎯⎯ .eat→ g) 4+

16: *he p(?loc" =lements



$t combines with hydrogen at about 77) I in the presence of a catalyst -.abers

9rocess/ to form ammoniaG 4+-g/ C ).+-g/ ZZZZZX +4.)-g/;

77) "

YZZZZZ

@initrogen combines with dioygen only at 'ery high temperature -at about +OOOI/ to form nitric oide, 4#

4 C -g/ ZZZZZ.eatX +4-g/

+ + Y

Uses: *he main use of dinitrogen is in the manufacture of ammonia and other industrial

chemicals containing nitrogen, -e#g#, calcium cyanamide/# $t also finds use where an inert

atmosphere is re3uired -e#g#, in iron and steel industry, inert diluent for reacti'e chemicals/#

Ni3uid dinitrogen is used as a refrigerant to preser'e biological materials, food items and in

cryosurgery#

Example 7.3 Lrite the reaction of thermal decomposition of sodium a!ide#

Solution *hermal decomposition of sodium a!ide gi'es dinitrogen gas# +4a4 )

→ +4a + )4+

Intext Question

7.3Ammonia

7." Lhy is 4+ less reacti'e at room temperatureM

Preparation

Ammonia is present in small 3uantities in air and soil where it is formed

by the decay of nitrogenous organic matter e#g#, urea#

4.+ %4.+ + +.+ →( 4.2 )+%) U +4.) + .+ + %+

n a small scale ammonia is obtained from ammonium salts which decompose when treated

with caustic soda or lime#

+4.2%l C %a-./+ →+4.) C +.+ C %a%l+

-4.2/+ E2 C +4a. →+4.) C +.+ C 4a+E2

n a large scale, ammonia is manufactured by .abers process# 4+-g/ C ).+-g/ Ö +4.)-g/; f

H 0 26#1 "> mol 1

$n accordance with Ne %hateliers principle, high pressure would fa'our the formation of

ammonia# *he optimum conditions for the production of ammonia are a pressure of +OO P 1O 5 9a

-about +OO atm/, a temperature of Q 7OO I and the use of a catalyst such as iron oide with small

amounts of I + and Al+) to increase the rate of attainment of e3uilibrium# *he flow chart for the

production of ammonia is shown in <ig# 7#1#

%hemistry 17O

.+



4+

Fig. 7.1

3low chart for

the

man%fact%re of

ammonia

Propert

ies

-4+ C ).+ C +4.)/ li3uid

4.)

4

. .

.is trigonal pyramidal with the nitrogen atom at the ape# $t has three bond pairs andone lone pair of electrons as shown in the structure#

Ammonia gas is highly soluble in water# $ts a3ueous solution is wea"ly

basic due to the formation of . ions#

4.)-g/ C .+-l/ l 4.2C -a3/ C . -a3/

$t forms ammonium salts with acids, e#g#, 4.2%l, -4.2/+ E2, etc# As a

wea" base, it precipitates the hydroides of many metals from their salt

solutions# <or eample,

+<e%l ) (a3 ) + )4. 2 . (a3 ) →<e + ) # 2. + ( s) + )4.

2%l (a3) ( brown ppt)

KnE2 (a3) + +4.2 . (a3) →Kn ( .) + ( s) + ( 4.2 )+ E2

(a3) ( white ppt )

*he presence of a lone pair of electrons on the nitrogen atom of the

ammonia molecule ma"es it a Newis base# $t donates the electron pair and

forms lin"age with metal ions and the formation of such

comple compounds finds applications in detection of metal ions such as

%u+C, AgCG

%u+C -a3/ C 2 4.)-a3/ Ö %u-4.)/2R+C-a3/

-blue/

Ag+ (a3) + %l

− (a3) → Ag%l ( s)

-colourless/

Ag%l ( s) + +4.)



-white ppt/




Uses: Ammonia is used to produce 'arious nitrogenous fertilisers -ammonium nitrate,

urea, ammonium phosphate and ammonium sulphate/ and in the manufacture of some

inorganic nitrogen compounds, the most important one being nitric acid# Ni3uid ammonia

is also used as a refrigerant#

Example 7.4 Lhy does 4.) act as a Newis base M

Solution 4itrogen atom in 4.) has one lone pair of electrons which is a'ailablefor donation# *herefore, it acts as a Newis base#

Intext Questions

7 . 4 Oxieso!

"itrogen

7.% ention the conditions re3uired to maimise the yield of ammonia#

7.5 .ow does ammonia react with a solution of %u+CM

4itrogen forms a number of oides in different oidation states# *he

names, formulas, preparation and physical appearance of these

oides are gi'en in *able 7#)#

Table 7.": O+ides of ,itro!en

,ame

@initrogen oide

4itrogen-$/ oideR

4itrogen monoide

4itrogen-$$/ oideR

@initrogen trioide

4itrogen-$$$/ oideR

4itrogen dioide

4itrogen-$D/ oideR

@initrogen tetroide

4itrogen-$D/ oideR

@initrogen pentoide



4itrogen-D/ oideR

%hemistry 17+



Newis dot main resonance structures and bond parameters of oides are gi'en in

*able 7#2#

Table 7.%: -tructures of O+ides of ,itro!en

Lhy does 4+ dimerise M Example 7.5

4+ contains odd number of 'alence electrons# $t beha'es as a typical Solution odd molecule#

n dimerisation, it is con'erted to stable 4+2 moleculewith e'en number of electrons#

Intext Question

7.' Lhat is the co'alence of nitrogen in 4+5 M

7 . 5 "itri# A#i 4itrogen forms ooacids such as .+ 4++ -hyponitrous acid/, .4+ -nitrous acid/ and

.4) -nitric acid/# Amongst them .4) is the most important#

17) *he p(?loc" =lements



Preparation

$n the laboratory, nitric acid is prepared by heating I4) or 4a4) and concentrated

.+E2 in a glass retort#

4a4 ) + . + E 2 → 4a.E 2 + .4)

n a large scale it is prepared mainly by stwalds process#

*his method is based upon catalytic oidation of 4.) by atmospheric oygen#

24.) (g) + 5+ (g) ⎯⎯⎯⎯⎯⎯⎯⎯→9t0Fhgaugecatalyst 24 (g) + 6.+ (g) 5OO I , : bar

-from air/

4itric oide thus formed combines with oygen gi'ing 4+#

+4 (g ) + + (g ) U +4 + (g)

4itrogen dioide so formed, dissol'es in water to gi'e .4)# )4 + (g )

+ .+ (l ) →+.4) (a3 ) + 4 (g) 4 thus formed is recycled and the a3ueous .4) can be concentrated by

distillation upto Q 68 by mass# <urther concentration to :8 can be achie'ed by

dehydration with concentrated .+E2#

Properties

$t is a colourless li3uid -f#p# +)1#2 I and b#p# )55#6 I/# Naboratory grade nitric

acid contains Q 68 of the .4) by mass and has a specific gra'ity of 1#5O2#

$n the gaseous state, .4) eists as a planar molecule with the structure asshown#

$n a3ueous solution, nitric acid beha'es as a strong acid gi'ing hydronium and

nitrate ions#

.4)-a3/ C .+-l/ → .)C-a3/ C 4)

-a3/

%oncentrated nitric acid is a strong oidising agent and attac"s most metals

ecept noble metals such as gold and platinum# *he products of oidation depend

upon the concentration of the acid, temperature and the nature of the material

undergoing oidation#

)%u C 8 .4)-dilute/ →)%u-4)/+ C +4 C 2.+ %u C

2.4)-conc#/ →%u-4)/+ C +4+ C +.+

Kinc reacts with dilute nitric acid to gi'e 4 + and with concentrated acid to gi'e

4+#

2Kn C 1O.4)-dilute/ →2 Kn -4)/+ C 5.+ C 4+ Kn C

2.4)-conc#/ →Kn -4)/+ C +.+ C +4+

Eome metals -e#g#, %r, Al/ do not dissol'e in concentrated nitric acid because of

the formation of a passi'e film of oide on the surface#

%oncentrated nitric acid also oidises nonmetals and their compounds# $odine is

oidised to iodic acid, carbon to carbon dioide, sulphur to .+E2, and phosphorus to

phosphoric acid#

%hemistry 172



$+ C 1O.4) → +.$) C 1O 4+ C 2.+

% C 2.4) → %+ C +.+ C 24+

E8 C 28.4)-conc#/ →8.+E2 C 284+ C 16.+

92 C +O.4)-conc#/ → 2.)92 C +O 4+ C 2.+ 4rown 1ing 5est G *he familiar brown ring test for nitrates depends on the ability

of <e+C to reduce nitrates to nitric oide, which reacts with <e +C to form a brown

coloured comple# *he test is usually carried out by adding dilute ferrous sulphate

solution to an a3ueous solution containing nitrate ion, and then carefully adding

concentrated sulphuric acid along the sides of the test tube# A brown ring at the

interface between the solution and sulphuric acid layers indicate the presence of

nitrate ion in solution#

4)( C )<e+C C 2.C → 4 C )<e)C C +.+

<e -.+/6 R++

C 4 → <e -.+/5 -4/R+C C .+

-brown/

Uses: *he maSor use of nitric acid is in the manufacture of ammonium nitrate for fertilisers

and other nitrates for use in eplosi'es and pyrotechnics# $t is also used for the preparation of

nitroglycerin, trinitrotoluene and other organic nitro compounds# ther maSor uses are in the

pickling of stainless steel, etching of metals and as an oidiser in roc"et fuels#

7.$ %&osp&or

us '

Allotropi#

(orms

P

6OJ

P P

P

Fig. 7.2

$hite phosphor%s

9hosphorus is found in many allotropic forms, the important ones being

white, red and blac"#

hite phosphorus is a translucent white way solid# $t is poisonous,

insoluble in water but soluble in carbon disulphide and glows in dar" -chemiluminescence/# $t dissol'es in boiling 4a. solution in an inert

atmosphere gi'ing 9.)#

92 + )4a. + ). + →9.) + )4a. + 9+

( sodium hypophosphite)

Lhite phosphorus is less stable and therefore, more reacti'e than theother solid phases under normal conditions because of angular strain in the

92 molecule where the angles are only 6OJ# $t readily catches fire in air to

gi'e dense white fumes of 921O#

92 + 5 + →92 1O

$t consists of discrete tetrahedral 92 molecule as shown in <ig# 7#+#

$ed phosphorus is obtained by heating white phosphorus at 57)I in

an inert atmosphere for se'eral days# Lhen red phosphorus is heated under

high pressure, a series of phases of blac" phosphorus are formed# Fed

phosphorus possesses iron grey lustre# $t is odourless, non(poisonous and

insoluble in water as well as in carbon disulphide# %hemically, red

phosphorus is much less reacti'e than white phosphorus# $t does not glow

in the dar"#



P P

P P P

P P

Fig.7.3:

crystals# $t does not oidise in air# β(?lac" phosphorus is prepared by heating white

phosphorus at 27) I under high pressure# $t does not burn in air upto 67) I#

7.7%&osp&ine

Preparation

9hosphine is prepared by the reaction of calcium phosphide with water or dilute

.%l#

%a)9+ C 6.+ →)%a-./+ C +9.)

%a)9+ C 6.%l →)%a%l+ C +9.)

$n the laboratory, it is prepared by heating white phosphorus with concentrated 4a.

solution in an inert atmosphere of %+#

92 + )4a. + ). + →9. ) + )4a. + 9+

( sodium hypophosphite)

Lhen pure, it is non inflammable but becomes inflammable owing to the presence of

9+.2 or 92 'apours# *o purify it from the impurities, it is absorbed in .$ to form

phosphonium iodide -9.2$/ which on treating with I. gi'es off phosphine#9. 2 $ + I. →I$ + . + + 9.)

Properties

$t is a colourless gas with rotten fish smell and is highly poisonous# $t eplodes in contact

with traces of oidising agents li"e .4), %l+ and ?r + 'apours#

$t is slightly soluble in water# *he solution of 9. ) in water decomposes in presence of

light gi'ing red phosphorus and .+# Lhen absorbed in copper sulphate or mercuric chloride

solution, the corresponding phosphides are obtained#

)%uE2 + +9.) →%u) 9+ + ).+ E2

).g%l+ + +9.) →.g) 9+ + 6.%l

9hosphine is wea"ly basic and li"e ammonia, gi'es phosphonium compounds with acids

e#g#,

9.) + .?r →9.2 ?r

Uses: *he spontaneous combustion of phosphine is technically used in Holme6s signals# %ontainers

containing calcium carbide and calcium phosphide are pierced and thrown in the sea when the gases

e'ol'ed burn and ser'e as a signal# $t is also used in smoke screens#

%hemistry 176



$n what way can it be pro'ed that 9.) is basic in natureM Example 7.$

9.) reacts with acids li"e .$ to form 9.2$ which shows that Solution it is basic

in nature#

9.) + .$ →9.2$

@ue to lone pair on phosphorus atom, 9.) is acting as a Newis base inthe abo'e reaction#

Intext Questions

7.7 ?ond angle in 9.2C is higher than that in 9.)# LhyM

7./ Lhat happens when white phosphorus is heated with concentrated 4a. solution in an

inert atmosphere of %+ M

7 . ) %&osp&orus

9

%l

7./.2 Phosphorus Preparation

Pentachloride 9hosphorus pentachloride is prepared by the reaction of white

phosphorus with ecess of dry chlorine#

92 + 1O%l+ →29%l5

$t can also be prepared by the action of E+%l+ on phosphorus# 92 + 1OE+ %l+

→29%l5 + 1OE+

Properties

9%l5 is a yellowish white powder and in moist air, it hydrolyses to

9%l) and finally gets con'erted to phosphoric acid#

9%l5 + .+ →9%l) + +.%l9%l) + ).+ →.) 92 + ).%l



Lhen heated, it sublimes but decomposes on stronger heating# 9%l 5 ??.eat?→

9%l ) + %l+

%l

9

%l

%l

$t reacts with organic compounds containing .

group con'erting them to chloro deri'ati'es#%+ .5 . + 9%l5 →%+

.5 %l + 9%l) + .%l

%.) %. + 9%l5 →

%.) %%l + 9%l)

C.%l

<inely di'ided metals on heating with 9%l5 gi'e

corresponding chlorides#

+Ag + 9%l 5 →+Ag%l + 9%l)

En + +9%l 5 →En%l 2 + +9%l)

$t is used in the synthesis ofsome organic



%l

$n g

bip

e3u

+O+

pm

bon

the

%l

$n t

9%



Table 7.5: O+oacids of Phosphorus

,ame

.ypophosphorous

-9hosphinic/

rthophosphorous

-9hosphonic/

9yrophosphorous

.ypophosphoric

rthophosphoric

9yrophosphoric

etaphosphoric0

E2ists in polymeric forms only. 7haracteristic bonds of (H89" " have been given in the 5able.

*he compositions of the ooacids are interrelated in terms of loss or gain of .+molecule or (atom#

*he structures of some important ooacids are gi'en belowG

9

. .

.

.)92

rthophosphoric acid



Fig. 7.!

*tr%ct%res of some

important o2oacids of

phosphor%s



$n ooacids phosphorus is tetrahedrally surrounded by other atoms# All these

acids contain one 9 and at least one 9. bond# *he ooacids in which

phosphorus has lower oidation state -less than C5/ contain, in addition to 9 and

9. bonds, either 99 -e#g#, in .29+6/ or 9. -e#g#, in .)9+/ bonds but not both#

*hese acids in C) oidation state of phosphorus tend to disproportionate to higher and lower oidation states# <or eample, orthophophorous acid -or phosphorous acid/

on heating disproportionates to gi'e orthophosphoric acid -or phosphoric acid/ and

phosphine#

2.) 9) →).) 92 + 9.)

*he acids which contain 9. bond ha'e strong reducing properties# *hus,

hypophosphorous acid is a good reducing agent as it contains two 9. bonds and

reduces, for eample, Ag4) to metallic sil'er#

2 Ag4) C +.+ C .)9+ → 2Ag C 2.4) C .)92

*hese 9. bonds are not ionisable to gi'e .

C

and do not play any role in basicity# nly those . atoms which are attached with oygen in 9. form are

ionisable and cause the basicity# *hus, .)9) and .)92 are dibasic and tribasic,

respecti'ely as the structure of .)9) has two 9. bonds and .)92 three#

Example 7.*

Solution

.ow do you account for the reducing beha'iour of .)9+ on the

basis of its structure M

$n .)9+, two . atoms are bonded directly to 9 atom whichimparts reducing character to the acid#

Intext Questions7.11 Lhat is the basicity of

.)92M

7.12 Lhat happens when

.)9) is heatedM

7.1+ Group 1$ygen, sulphur, selenium, tellurium and polonium constitute

roup

Elements

16 of the periodic table# *his is sometimes "nown as group of

chalcogens# *he name is deri'ed from the ree" word for brass and

points to the association of sulphur and its congeners with copper#

ost copper minerals contain either oygen or sulphur andfre3uently the other members of the group#

7.1.1 Occurrence ygen is the most abundant of all the elements on earth# ygen forms about 26#6 by

mass of earths crust# @ry air contains +O#:26 oygen by 'olume#

.owe'er, the abundance of sulphur in the earths crust is only O#O)(O#1#

%ombined sulphur eists primarily as sulphates such as gyps%m %aE2#+.+, epsom

salt gE2#7.+, baryte ?aE2 and sulphides such as galena 9bE, :inc blende KnE,

copper pyrites %u<eE+# *races of sulphur occur as hydrogen sulphide in 'olcanoes#

rganic materials such as eggs, proteins, garlic, onion, mustard, hair and wool

contain sulphur#



%hemistry 18O



Eelenium and tellurium are also found as metal selenides and tellurides in

sulphide ores# 9olonium occurs in nature as a decay product of thorium and uranium

minerals#

*he important atomic and physical properties of roup16 along with electronic

configuration are gi'en in *able 7#6# Eome of the atomic, physical and chemical properties and their trends are discussed below#

7.1.2 Electronic *he elements of roup16 ha'e si electrons in the outermost shell and onfi!uration ha'e

ns+np2 general electronic configuration#

7.1." Atomic @ue to increase in the number of shells, atomic and ionic radii increase and #onic from top to

bottom in the group# *he si!e of oygen atom is, howe'er,

$adii eceptionally small#

7.1.% #onisation $onisation enthalpy decreases down the group# $t is due to increase in Enthalpy si!e#

.owe'er, the elements of this group ha'e lower ionisation enthalpy

'alues compared to those of roup15 in the corresponding periods# *his is due to the

fact that roup 15 elements ha'e etra stable half(filled p orbitals electronic

configurations#

7.1.5 Electron ?ecause of the compact nature of oygen atom, it has less negati'e

Gain electron gain enthalpy than sulphur# .owe'er, from sulphur onwards

Enthalpy the 'alue again becomes less negati'e upto polonium#

7.1.' 4et to fluorine, oygen has the highest electronegati'ity 'alue amongst

Electrone!ati&ity the elements# Lithin the group, electronegati'ity decreases with an

increase in atomic number# *his implies that the metallic character

increases from oygen to polonium#

=lements of roup 16 generally show lower 'alue of first ionisation enthalpy compared to

the corresponding periods of group 15# LhyM

@ue to etra stable half(filled p orbitals electronic configurations of roup 15 elements,

larger amount of energy is re3uired to remo'e electrons compared to roup 16 elements#

Example 7.1+

Solution

7.1.7 Physical Eome of the physical properties of roup 16 elements are gi'en in Properties *able 7#6#

ygen and sulphur are non(metals, selenium and tellurium metalloids, whereas polonium is a metal#

9olonium is radioacti'e and is short li'ed -.alf(life 1)#8 days/# All these elements ehibit allotropy#

*he melting and boiling points increase with an increase in atomic number down thegroup# *he large difference between the melting and boiling points of oygen and

sulphur may be eplained on the basis of their atomicity; oygen eists as diatomic

molecule -+/ whereas sulphur eists as polyatomic molecule -E8/#

7.1./ hemicalOxidation states and trends in chemical reactivityProperties

*he elements of roup 16 ehibit a number of oidation states -*able

7#6/# *he stability of (+ oidation state decreases down the group#

9olonium hardly shows + oidation state# Eince electronegati'ity of

oygen is 'ery high, it shows only negati'e oidation state as + ecept




Table 7.': -ome Physical Properties of Group 1' Elements

Property

Atomic number

Atomic mass0g mol 1

=lectronic configuration

%o'alent radius0-pm/a

$onic radius, =+ 0pm

=lectron gain enthalpy,

0 eg H "> mol 1

$onisation enthalpy - i H 1/

0"> mol 1

=lectronegati'ity

@ensity 0g cm ) -+:8 I/

elting point0I

?oiling point0I

idation states0

a*ingle bond!

b +ppro2imate val%e!

c +t the melting point!

d 1hombic s%lph%r!

e He2agonal grey!

f onoclinic form, );" .

92ygen shows o2idation states of #0 and #< in o2ygen fl%orides 93 0 and 90 3 0 respectively.

in the case of <+ where its oidation state is C +# ther elements of the group

ehibit C +, C 2, C 6 oidation states but C 2 and C 6 are more common# Eulphur,

selenium and tellurium usually show C 2 oidation state in their compounds with

oygen and C 6 with fluorine# *he stability of C 6 oidation state decreases down thegroup and stability of C 2 oidation state increase -inert pair effect/# ?onding in C2

and C6 oidation states are primarily co'alent#

Anomalous "ehaviour of oxygen

*he anomalous beha'iour of oygen, li"e other members of p(bloc" present in

second period is due to its small si!e and high electronegati'ity# ne typical eample

of effects of small si!e and high electronegati'ity is the presence of strong hydrogen

bonding in .+ which is not found in .+E#

*he absence of d orbitals in oygen limits its co'alency to four and in practice,

rarely eceeds two# n the other hand, in case of other elements of the group, the'alence shells can be epanded and co'alence eceeds four#

-i/ 1eactivity with hydrogenG All the elements of roup 16 form hydrides of the

type .+= -= E, Ee, *e, 9o/# Eome properties of hydrides are gi'en in *able

7#7# *heir acidic character increases from . + to .+*e# *he increase in acidic

character can be eplained in terms of decrease in bond -.=/ dissociationenthalpy down the group# wing to the decrease in bond -.=/ dissociation

enthalpy down the group, the thermal stability of hydrides also decreases from

.+ to .+9o# All the hydrides ecept water possess reducing property and this

character increases from .+E to .+*e#

%hemistry 18+



Table 7.7: Properties of *ydrides of Group 1' Elements

Property

m#p0I

b#p0I

.= distance0pm

.=. angle -J/

f H 0"> mol 1

diss H (H–E0"> mol 1

@issociation constanta

a +%eo%s sol%tion, 0>

-ii/ 1eactivity with o2ygenG All these elements form oides of the = + and =)

types where = E, Ee, *e or 9o# !one - )/ and sulphur dioide -E+/ are

gases while selenium dioide -Ee+/ is solid# Feducing property of dioidedecreases from E+ to *e+; E+ is reducing while *e+ is an oidising agent#

?esides =+ type, sulphur, selenium and tellurium also form = ) type oides

-E), Ee), *e)/# ?oth types of oides are acidic in nature#

-iii/ 1eactivity towards the halogensG =lements of roup 16 form a large number of

halides of the type, =&6, =&2 and =&+ where = is an element of the group and

& is a halogen# *he stability of the halides decreases in the order < H %l H ?r

H $ # Amongst heahalides, heafluorides are the only stable halides# All

heafluorides are gaseous in nature# *hey ha'e octahedral structure# Eulphur

heafluoride, E<6 is eceptionally stable for steric reasons#

Amongst tetrafluorides, E<2 is a gas, Ee<2 a li3uid and *e<2 a solid# *hesefluorides ha'e sp)d hybridisation and thus, ha'e trigonal bipyramidal structures in

which one of the e3uatorial positions is occupied by a lone pair of electrons# *hisgeometry is also regarded as see'saw geometry#

All elements ecept selenium form dichlorides and dibromides# *hese dihalides

are formed by sp) hybridisation and thus, ha'e tetrahedral structure# *he well "nown

monohalides are dimeric in nature# =amples are E+<+, E+%l+, E+?r +, Ee+%l+ and

Ee+?r +# *hese dimeric halides undergo disproportionation as gi'en belowG

+Ee+%l+ → Ee%l2 C )Ee

.+E is less acidic than .+*e# LhyM

@ue to the decrease in bond -=./ dissociation

enthalpy down the group, acidic character increases#

Example 7.11Solution

Intext Questions7.1" Nist the important sources of sulphur#

7.1% Lrite the order of thermal stability of the hydrides of roup 16 elements#

7.15 Lhy is .+ a li3uid and .+E a gas M

18) *he p(?loc" =lements

7.11 Diox,gen



Preparation

@ioygencan be

obtained in

the

laboratory bythe following

waysG

-i/ ?y

heating

oygen

containi

ng salts

such as

chlorate

s,

nitrates

and

perman

ganates#

+I %l) ???

?→.e

at

+I %l+ )+

n+

-ii/ ?y thethermal

decomp

osition

of the

oides

of

metals

low in

the electrochemical series and higher oides of some

metals#

+Ag+-s/ → 2Ag-s/ C +-g/; +9b)2-s/ →

69b-s/ C +-g/

+.g-s/ → +.g-l/ C +-g/ ; +9b+-s/ →

+9b-s/ C +-g/

-iii/ .ydrogen peroide is readily decomposed into water and

dioygen by catalysts such as finely di'ided metals and

manganese dioide#

+.++-a3/ →+.+-1/ C +-g/

-i'/ n large scale it can be prepared from water or air#

=lectrolysis of water leads to the release of hydrogen at

the cathode and oygen at the anode#

$ndustrially, dioygen is obtained from air by first

remo'ing carbon dioide and water 'apour and then, the

remaining gases are li3uefied and fractionally distilled to gi'edinitrogen and dioygen#

Properties

@ioygen is a colourless and odourless gas# $ts solubility in

water is to the etent of )#O8 cm) in 1OO cm) water at +:) I

which is Sust sufficient for the 'ital support of marine and

a3uatic life# $t li3uefies at :O I and free!es at 55 I# $t has three

stable isotopesG 16, 17 and 18# olecular oygen, + is

uni3ue in being paramagnetic inspite of ha'ing e'en number of

electrons -see %lass &$ %hemistry ?oo", Unit 2/#

@ioygen directly reacts with nearly all metals and non(

metals ecept some metals - e#g#, Au, 9t/ and some noble gases#

$ts combination with other elements is often strongly

eothermic which helps in sustaining the reaction# .owe'er, to

initiate the reaction, some eternal heating is re3uired as bond

dissociation enthalpy of ogyen(oygen double bond is high

-2:)#2 "> mol 1/#

Eome of the reactions of dioygen with metals, non(metals

and other compounds are gi'en belowG

+

%a

+

+

→

+

%

a

2

Al



+

)

+

→

+

A

l

+

)

92

+

5

+ →

92

1O

% + + →%+

+KnE C )+ →

+Kn C +E+

%.2 + ++ →

%+ + +.+

Eome compounds are catalytically oidised# <or e#g#,

+E + + + ???D+5?→+E)

2.%l + + ????→%u%l+ +%l + + +. +

%hemistry 182



Uses: $n addition to its importance in normal respiration and combustion processes, oygen

is used in oyacetylene welding, in the manufacture of many metals, particularly steel#

ygen cylinders are widely used in hospitals, high altitude flying and in mountaineering#

*he combustion of fuels, e#g#, hydra!ines in li3uid oygen, pro'ides tremendous thrust in

roc"ets#

Intext Questions

7.1' Lhich of the following does not react with oygen directlyM Kn, *i, 9t,

<e

7.17 %omplete the following reactionsG

-i/ %+.2 C + →

-ii/ 2Al C ) + →

7.12 Sim

ple

Oxi

es

A binary compound of oygen with another element is called oide# Asalready stated, oygen reacts with most of the elements of the periodic

table to form oides# $n many cases one element forms two or more

oides# *he oides 'ary widely in their nature and properties#

ides can be simple -e#g#, g, Al+) / or mied -9b)2, <e)2/#

Eimple oides can be classified on the basis of their acidic, basic or

amphoteric character# An oide that combines with water to gi'e an

acid is termed acidic oide -e#g#, E+, %l+7, %+, 4+5 /# <or

eample, E+ combines with water to gi'e .+E), an acid#

E + + . + →. + E)

As a general rule, only non(metal oides are acidic but oides of some metals in high oidation state also

ha'e acidic character -e#g#, n+7, %r), D+5/# *he

oides which gi'e a base with water are "nown as basic

oides -e#g#, 4a+, %a, ?a/# <or eample, %a

combines with water to gi'e %a-./+, a base#

%a + . + →%a ( .)+

$n general, metallic oides are basic#

Eome metallic oides ehibit a dual beha'iour# *hey

show

7 . 13 O-one



Preparation

Lhen a slow dry stream of oygen is passed through a silent electrical discharge,

con'ersion of oygen to o!one -1O/ occurs# *he product is "nown as o!onised

oygen#

)+ → +) .V -+:8 I/ C12+ "> mol 1

Eince the formation of o!one from oygen is an endothermic process, it is

necessary to use a silent electrical discharge in its preparation to pre'ent its

decomposition#

$f concentrations of o!one greater than 1O per cent are re3uired, a battery of

o!onisers can be used, and pure o!one -b#p# )85 I/ can be condensed in a 'essel

surrounded by li3uid oygen#

Properties

9ure o!one is a pale blue gas, dar" blue li3uid and 'iolet(blac" solid# !one has a

characteristic smell and in small concentrations it is harmless# .owe'er, if the

concentration rises abo'e about 1OO parts per million, breathing becomesuncomfortable resulting in headache and nausea#

!one is thermodynamically unstable with respect to oygen since its

decomposition into oygen results in the liberation of heat - . is negati'e/ and an

increase in entropy - E is positi'e/# *hese two effects reinforce each other, resulting

in large negati'e ibbs energy change - / for its con'ersion into oygen# $t is not

really surprising, therefore, high concentrations of o!one can be dangerously

eplosi'e#

@ue to the ease with which it liberates atoms of nascent oygen -) →+ C /, itacts as a powerful oidising agent# <or e#g#, it oidises lead sulphide to lead sulphateand iodide ions to iodine#

9bE-s/ C 2)-g/ → 9bE2-s/ C 2+-g/

+$ -a3/ C .+-l/ C )-g/ →+. -a3/ C $+-s/ C +-g/

Lhen o!one reacts with an ecess of potassium iodide solution buffered with a

borate buffer -p. :#+/, iodine is liberated which can be titrated against a standard

solution of sodium thiosulphate# *his is a 3uantitati'e method for estimating ) gas#

=periments ha'e shown that nitrogen oides -particularly nitric oide/ combine

'ery rapidly with o!one and there is, thus, the possibility that nitrogen oides emitted

from the ehaust systems of supersonic Set aeroplanes might be slowly depleting the

concentration of the o!one layer in the upper atmosphere#

4 (g ) + ) (g ) → 4+ (g ) + + (g)

Another threat to this o!one layer is probably posed by the use of freons which

are used in aerosol sprays and as refrigerants#

*he two oygen(oygen bond lengths in the o!one molecule are identical-1+8 pm/ and the molecule is angular as epected with a bond angle of

about 117o# $t is a resonance

hybrid of two main formsG

Uses: $t is used as a germicide, disinfectant and for sterilising water# $t is also used for

bleaching oils, i'ory, flour, starch, etc# $t acts as an oidising agent in the manufacture of

potassium permanganate#

%hemistry 186



Intext Questions

7.1/ Lhy does ) act as a powerful oidising agentM

7.1 .ow is ) estimated 3uantitati'elyM

7.14Sulp&ur ' Eulphur forms numerous allotropes of which the yello3 rhombic

Allotropi#

(orms

-α(sulphur/ and monoclinic -β (sulphur/ forms are the most important# *he stable form at

room temperature is rhombic sulphur, which transforms to monoclinic sulphur when heated

abo'e )6: I#

#hom"ic sulphur $ %sulphur&

*his allotrope is yellow in colour, m#p# )85#8 I and specific gra'ity +#O6# Fhombic sulphur crystals are formed

on e'aporating the solution of roll sulphur in %E +# $t is insoluble in water but dissol'es to some etent in

ben!ene, alcohol and ether# $t is readily soluble in %E+#

'onoclinic sulphur $

%sulphur&

$ts m#p# is ):) I and specific gra'ity 1#:8# $t is soluble in %E +# *his form of sulphur is prepared by melting

rhombic sulphur in a dish and cooling, till crust is formed# *wo holes are made in the crust and the remaining

li3uid poured out# n remo'ing the crust, colourless needle shaped crystals of β(sulphur are formed# $t is stable

abo'e )6: I and transforms into α(sulphur below it# %on'ersely, α(sulphur is stable below )6: I and transforms

into β(sulphur abo'e this# At )6: I both the forms are stable# *his temperature is called transition temperature#

?oth rhombic and monoclinic sulphur ha'e E8 molecules# *hese E8 molecules are pac"ed to gi'e different

crystal structures# *he E8 ring in both the forms is puc"ered and has a crown shape# *he molecular dimensions

are gi'en in <ig# 7#5-a/#

-a/ -b/

Fig. 7.(: 5he str%ct%res of (a * ring in rhombic s%lph%r

and (b * ) form

Ee'eral other modifications of sulphur containing 6(+O sulphur atoms per ring ha'e

been synthesised in the last two decades# $n

cyclo(E6, the ring adopts the chair form and

the molecular dimensions are as shown in

<ig# 7#5 -b/# At ele'ated temperatures

-Q1OOO I/, E+ is the dominant species and

is paramagnetic li"e +#

Lhich form of sulphur shows paramagnetic beha'iour M

$n 'apour state sulphur partly eists as E+ molecule which has two unpaired electrons in the

antibonding π T orbitals li"e + and, hence, ehibits paramagnetism#

Example 7.12

Solution


7.15 Sulp&ur Dioxie



Preparation

Eulphur dioide is formed together with a little -6(8/ sulphur trioide when sulphur

is burnt in air or oygenG

E- s/ C +- g / → E+ - g /

$n the laboratory it is readily generated by treating a sulphite with dilutesulphuric acid#

E)+(-a/ C +.C -a/ → .+-l/ C E+ - g /

$ndustrially, it is produced as a by(product of the roasting of sulphide ores#

2<eE+ ( s ) + 11+ ( g ) →+<e+ ) ( s) + 8E+ ( g )

*he gas after drying is li3uefied under pressure and stored in steel cylinders#

Properties

Eulphur dioide is a colourless gas with pungent smell and is highly soluble in water#

$t li3uefies at room temperature under a pressure of two atmospheres and boils at +6)

I#

Eulphur dioide, when passed through water, forms a solution of sulphurousacid#

E+ ( g) + .+ ( l) U .+ E) ( a )

$t reacts readily with sodium hydroide solution, forming sodium sulphite, which

then reacts with more sulphur dioide to form sodium hydrogen sulphite#

+4a. C E+ → 4a+E) C .+

4a+E) C .+ C E+ →+4a.E)

$n its reaction with water and al"alies, the beha'iour of sulphur dioide is 'ery similar to that

of carbon dioide#

Eulphur dioide reacts with chlorine in the presence of charcoal -which acts as a catalyst/ to

gi'e sulphuryl chloride, E+%l+# $t is oidised to sulphur trioide by oygen in the presence of 'anadium-D/ oide catalyst#

E+-g/ C %l+ -g/ → E+%l+-l /

+E+ (g) + + (g) ???D+5?→+E) (g)

Lhen moist, sulphur dioide beha'es as a reducing agent# <or eample, it con'erts iron-$$$/

ions to iron-$$/ ions and decolourises acidified potassium permanganate-D$$/ solution; the latter

reaction is a con'enient test for the gas#

+<e) + + E+ + +.+ →+<e

+ + + E

+2

− + 2.

+

5E+ + +n−2 + +.+ →5E

+2

− + 2.

+ + +n

++

*he molecule of E+ is angular# $t is a resonance hybrid of the two canonicalformsG

Uses: Eulphur dioide is used -i/ in refining petroleum and sugar -ii/ in bleaching wool and sil" and -iii/

as an anti(chlor, disinfectant and preser'ati'e# Eulphuric acid, sodium hydrogen sulphite and calcium

hydrogen sulphite -industrial chemicals/ are manufactured from sulphur dioide# Ni3uid E+ is used as a

sol'ent to dissol'e a number of organic and inorganic chemicals#

%hemistry 188



Intext Questions

7.2 Lhat happens when sulphur dioide is passed through an a3ueous solution of <e-$$$/

saltM

7.21 %omment on the nature of two E bonds formed in E+ molecule# Are the two E bonds in this molecule e3ual M

7.22 .ow is the presence of E+ detected M

7.1$ Oxoa#is o! Eulphur forms a number of ooacids such as .+E), .+E+), .+E+2,

Sulp&ur .+E+5, .+E6 - + to 5/, .+E2, .+E+7, .+E5, .+E+8 # Eome of these acids areunstable and cannot be isolated# *hey are "nown in

a3ueous solution or in the form of their salts# Etructures of some important ooacids are

shown in <ig# 7#6#

7.17 Sulp&uri#

A#i

Fig. 7.): *tr%ct%res of some important o2oacids of s%lph%r

'anufacture

Eulphuric acid is one of the most important industrial chemicals worldwide#

Eulphuric acid is manufactured by the ontact Process which in'ol'es three stepsG

-i/ burning of sulphur or sulphide ores in air to generate E+#

-ii/ con'ersion of E+ to E) by the reaction with oygen in the presence of a

catalyst -D+5/, and

-iii/ absorption of E) in .+E2 to gi'e 9le%m -.+E+7/#

A flow diagram for the manufacture of sulphuric acid is shown in -<ig# 7#7/# *he

E+ produced is purified by remo'ing dust and other impurities such as arseniccompounds#

*he "ey step in the manufacture of .+E2 is the catalytic oidation of E+ with+ to gi'e E) in the presence of D+5 -catalyst/#

+E+ (g) + +

*he reaction is eothermic, re'ersible and the forward reaction leads to a

decrease in 'olume# *herefore, low temperature and high pressure are the fa'ourable

conditions for maimum yield# ?ut the temperature should not be 'ery low otherwise

rate of reaction will become slow#

$n practice, the plant is operated at a pressure of + bar and a temperature of 7+O

I# *he E) gas from the catalytic con'erter is

18: *he p(?loc" =lements



Later

$mpurespray

E+C+

Eulphur

Air

Eulphur

burner

@ust

precipitator

Fig. 7.7: 3low diagram for the man%fact%re of s%lph%ric acid

absorbed in concentrated .+E2 to produce ole%m# @ilution of oleum with water

gi'es .+E2 of the desired concentration# $n the industry two steps are carried out

simultaneously to ma"e the process a continuous one and also to reduce the cost#

E) C .+E2 →.+E+7

-leum/

*he sulphuric acid obtained by %ontact process is :6(:8 pure#

Properties

Eulphuric acid is a colourless, dense, oily li3uid with a specific gra'ity of 1#82 at +:8

I# *he acid free!es at +8) I and boils at 611 I# $t dissol'es in water with the

e'olution of a large 3uantity of heat# .ence, care must be ta"en while preparing

sulphuric acid solution from concentrated sulphuric acid# *he concentrated acid must

be added slowly into water with constant stirring#

*he chemical reactions of sulphuric acid are as a result of the followingcharacteristicsG -a/ low 'olatility -b/ strong acidic character

-c/ strong affinity for water and -d/ ability to act as an oidising agent# $n a3ueous

solution, sulphuric acid ionises in two steps#

.+E2-a3/ C .+-l/ →.)C-a3/ C .E2

-a3/; a1 'ery large - a1 H1O/ .E2

-a3/ C .+-l/ →.)C-a3/ C E2

+(-a3/ ; a+ 1#+ P 1O +

*he larger 'alue of a - a H1O/ means that .+E2 is largely1 1

dissociated into .C and .E2 # reater the 'alue of dissociation constant - a/, the

stronger is the acid#

*he acid forms two series of saltsG normal sulphates -such as sodium sulphate

and copper sulphate/ and acid sulphates -e#g#, sodium hydrogen sulphate/#



Eulphuric acid, because of its low 'olatility can be used to manufacture more

'olatile acids from their corresponding salts#

%hemistry 1:O



+ & C .+E2 → + .& C +E2 -& <, %l, 4)/

- etal/

%oncentrated sulphuric acid is a strong dehydrating agent# any wet gases can

be dried by passing them through sulphuric acid, pro'ided the gases do not react with

the acid# Eulphuric acid remo'es water from organic compounds; it is e'ident by its

charring action on carbohydrates#

%1+.++11 ????→.+E2 1+% C 11.+

.ot concentrated sulphuric acid is a moderately strong oidising agent# $n thisrespect, it is intermediate between phosphoric and nitric acids# ?oth metals and non(

metals are oidised by concentrated sulphuric acid, which is reduced to E+#

%u C + .+E2-conc#/ →%uE2 C E+ C +.+ )E C

+.+E

2-conc#/ →)E

+ C +.

+

% C +.+E2-conc#/ →%+ C + E+ C + .+

Uses: Eulphuric acid is a 'ery important industrial chemical# A nations industrial

strength can be Sudged by the 3uantity of sulphuric acid it produces and consumes# $t is

needed for the manufacture of hundreds of other compounds and also in many industrial

processes# *he bul" of sulphuric acid produced is used in the manufacture of fertilisers

-e#g#, ammonium sulphate, superphosphate/# ther uses are inG

-a/ petroleum refining -b/ manufacture of pigments, paints and dyestuff intermediates -c/

detergent industry -d/ metallurgical applications -e#g#, cleansing metals before enameling,

electroplating and gal'anising

-e/ storage batteries -f/ in the manufacture of nitrocellulose products and -g/ as a

laboratory reagent#

Lhat happens when

-i/ %oncentrated .+E2 is added to calcium fluoride

-ii/ E) is passed through waterM

-i/ $t forms hydrogen fluoride %a<+ + . + E 2 →%aE 2 +

+.<

-ii/ $t dissol'es E) to gi'e .+E2 # E) + .+ →.+ E2

Example 7.13

Solution

Intext Questions

7.2" ention three areas in which .+E2 plays an important role#

7.2% Lrite the conditions to maimise the yield of .+E2 by %ontact process#

7.25 Lhy is a + a1 for .+E2 in water M

1:1 *he p(?loc" =lements

7.1) Group 17 Elements



<luorine,

chlorine,

bromine, iodine

and astatine are

members of

roup 17# *hese are collecti'ely "nown as the halo!ens -ree" halo means salt

and genes means born i#e#, salt producers/# *he halogens are highly reacti'e

non(metallic elements# Ni"e roups 1 and +, the elements of roup 17 show

great similarity amongst themsel'es# *hat much similarity is not found in the

elements of other groups of the periodic table# Also, there is a regular gradation

in their physical and chemical properties# Astatine is a radioacti'e element#

7.1/.1 Occurrence <luorine

andchlo

rine

are

fairly

abun

dant

while

bro

mine

and

iodine

less

so#

<luorine

is present

mai

nly

asinsol

uble

fluor ides

-fluo

rspa

r %a<

+,

cryolite

4a)

Al<6

and

fluor

oapatite

)%a)

-92

/+#%

a<+/ and small 3uantities are present in soil,

ri'er water plants and bones and teeth of animals# Eea water contains chlorides, bromides

and iodides of sodium, potassium, magnesium

and calcium, but is mainly sodium chloridesolution -+#5 by mass/# *he deposits of dried

up seas contain these compounds, e#g#, sodium

chloride and carnallite, I%l#g%l+#6.+#

%ertain forms of marine life contain iodine in

their systems; 'arious seaweeds, for eample,

contain upto O#5 of iodine and %hile saltpetre

contains upto O#+ of sodium iodate#

*he important atomic and physical

properties of roup 17 elements along with

their electronic configurations are gi'en in

*able 7#8#

Table 7./: Atomic and Physical Properties of *alo!ens

Property

Atomic number

Atomic mass0g mol 1


%o'alent radius0pm

$onic radius & 0pm

$onisation enthalpy0"> mol 1

=lectron gain enthalpy0"> mol 1

=lectronegati'ity b

.yd H -& /0"> mol

1

elting point0I

?oiling point0I

@ensity0g cm )

@istance & &0pm

?ond dissociation enthalpy

0-"> mol 1

/

E V ?@ e

a 1adioactive!

b 8a%ling scale! half'cell reaction is A 0(g # 0e

– →



c 3or the li%id at temperat%res (

given in the parentheses!d solid!

e 5he

0A – (a.

%hemistry 1:+



*he trends of some of the atomic, physical and chemical properties are discussed

below#

7.1/.2 Electronic All these elements ha'e se'en electrons in their outermost shell onfi!uration -ns+np5/

which is one electron short of the net noble gas#

7.1/." Atomic*he halogens ha'e the smallest atomic radii in their respecti'e periods

and #onic due to maimum effecti'e nuclear charge# *he atomic radius of fluorine $adii li"e the

other elements of second period is etremely small# Atomic and ionic radii increase from fluorine to

iodine due to increasing number

of 3uantum shells#

7.1/.% #onisation *hey ha'e little tendency to lose electron# *hus they ha'e 'ery high Enthalpy ionisation

enthalpy# @ue to increase in atomic si!e, ionisation enthalpy

decreases down the group#

7.1/.5 Electron.alogens ha'e maimum negati'e electron gain enthalpy in the

Gain corresponding periods# *his is due to the fact that the atoms of theseEnthalpy elements ha'e only one electron less than stable noble gas configurations#

=lectron gain enthalpy of the elements of the group becomes less negati'e

down the group# .owe'er, the negati'e electron gain enthalpy of fluorine

is less than that of chlorine# $t is due to small si!e of fluorine atom# As

a result, there are strong interelectronic repulsions in the relati'ely

small + p orbitals of fluorine and thus, the incoming electron does not

eperience much attraction#

7.1/.' *hey ha'e 'ery high electronegati'ity# *he electronegati'ity decreases

Electrone!ati&ity down the group# <luorine is the most electronegati'e element in the

periodic table#

.alogens ha'e maimum negati'e electron gain enthalpy in the Example 7.14 respecti'e

periods of the periodic table# LhyM

.alogens ha'e the smallest si!e in their respecti'e periods and therefore Solution high

effecti'e nuclear charge# As a conse3uence, they readily accept

one electron to ac3uire noble gas electronic configuration#

7.1/.7 Physical .alogens display smooth 'ariations in their physical properties# <luorine Properties and

chlorine are gases, bromine is a li3uid and iodine is a solid# *heir melting and boiling points steadily

increase with atomic number# All

halogens are coloured# *his is due to absorption of radiations in 'isible region whichresults in the ecitation of outer electrons to higher energy le'el# ?y absorbing

different 3uanta of radiation, they display different colours# <or eample, < +, has

yellow, %l+ , greenish yellow, ?r +, red and $+, 'iolet colour# <luorine and chlorine

react with water# ?romine and iodine are only sparingly soluble in water but are

soluble in 'arious organic sol'ents such as chloroform, carbon tetrachloride, carbon

disulphide and hydrocarbons to gi'e coloured solutions#

ne curious anomaly we notice from *able 7#8 is the smaller enthalpy of

dissociation of <+ compared to that of %l+ whereas &(& bond dissociation enthalpiesfrom chlorine onwards show the epected

1:) *he p(?loc" =lements



Example 7.15

Solution

trendG %l %l H ?r ?r H $ $# A reason for this anomaly is the relati'ely large electron(

electron repulsion among the lone pairs in <+ molecule where they are much closer to each

other than in case of %l+#

Although electron gain enthalpy of fluorine is less negati'e as compared to chlorine,

fluorine is a stronger oidising agent than chlorine# LhyM

$t is due to

-i/ low enthalpy of dissociation of <(< bond -*able 7#8/#

-ii/ high hydration enthalpy of < -*able 7#8/#

7.1/./ hemicalOxidation states and trends in chemical reactivity

Properties

.alogen atom

in ground state

-other than fluorine/

<irst ecited state

5 unpaired electrons accountEecond ecited state

for C5 oidation state

7 unpaired electrons*hird ecited state

account for C7 oidation state

*he higher oidation states of chlorine, bromine and iodine are realised mainly

when the halogens are in combination with the small and highly electronegati'e

fluorine and oygen atoms# e#g#, in interhalogens, oides and ooacids# *he oidation

states of C2 and C6 occur in the oides and ooacids of chlorine and bromine# *he

fluorine atom has no d orbitals in its 'alence shell and therefore cannot epand its

octet# ?eing the most electronegati'e, it ehibits only 1 oidation state#

All the halogens are highly reacti'e# *hey react with metals and non(metals toform halides# *he reacti'ity of the halogens decreases down the group#

*he ready acceptance of an electron is the reason for the strong oidising nature

of halogens# <+ is the strongest oidising halogen and it oidises other halide ions in

solution or e'en in the solid phase# $n general, a halogen oidises halide ions of higher atomic number#

<+ C +& → +< C &+ -& %l, ?r or $/

%l+ C +& →+%l C &+ -& ?r or $/ ?r + C

+$ →+?r C $+

*he decreasing oidising ability of the halogens in a3ueous solution down the

group is e'ident from their standard electrode potentials -*able 7#8/ which are

dependent on the parameters indicated belowG



%hemistry 1:2



1

& +( g) ⎯⎯⎯⎯⎯⎯→ & ( g) ⎯⎯ 10 + diss H

V

+

*he relati'e oidising power of halogens can further be illustrated by their

reactions with water# <luorine oidises water to oygen whereas chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids#

*he reaction of iodine with water is non(spontaneous# $n fact, $ can be oidised by

oygen in acidic medium; Sust the re'erse of the reaction obser'ed with fluorine#

+<+ (g) + +.+ ( l ) →2.+ (a3) + 2<

− (a3) + + (g)

&+ ( g) + .+ ( l ) →.& ( a3) + .& ( a3)

( where & %l or ?r )

2$− ( a3) + 2.

+ ( a3) + + ( g) →+$+ ( s) + +.+ ( l )

Anomalous "ehaviour of fluorine

Ni"e other elements of p(bloc" present in second period of the periodic table, fluorine

is anomalous in many properties# <or eample, ionisation enthalpy, electronegati'ity,

enthalpy of bond dissociation and electrode potentials are all higher for fluorine than

epected from the trends set by other halogens# Also, ionic and co'alent radii, m#p#

and b#p# and electron gain enthalpy are 3uite lower than epected# *he anomalous

beha'iour of fluorine is due to its small si!e, highest electronegati'ity, low <(< bond

dissociation enthalpy, and non a'ailability of d orbitals in 'alence shell#

ost of the reactions of fluorine are eothermic -due to the small and strong

bond formed by it with other elements/# $t forms only one ooacid while other

halogens form a number of ooacids# .ydrogen fluoride is a li3uid -b#p# +:) I/ due

to strong hydrogen bonding# ther hydrogen halides are gases#

-i/ 1eactivity towards hydrogenG *hey all react with hydrogen to gi'e hydrogen

halides but affinity for hydrogen decreases from fluorine to iodine# *hey

dissol'e in water to form hydrohalic acids# Eome of the properties of hydrogen

halides are gi'en in *able 7#:# *he acidic strength of these acids 'aries in the

orderG .< B .%l B .?r B .$# *he stability of these halides decreases down the

group due to decrease in bond -.&/ dissociation enthalpy in the orderG .< H

.%l H .?r H .$#

Table 7.: Properties of *ydro!en *alides

Property

elting point0I

?oiling point0I

?ond length -. &/0pm

diss H V0"> mol

1

p a

-ii/ 1eactivity towards o2ygenG .alogens form many oides with oygen but mostof them are unstable# <luorine forms two oides <+ and +<+# .owe'er, only

<+ is thermally stable at +:8 I# *hese oides




are essentially oygen fluorides because of the higher electronegati'ity of

fluorine than oygen# ?oth are strong fluorinating agents# +<+ oidises

plutonium to 9u<6 and the reaction is used in remo'ing plutonium as 9u<6 fromspent nuclear fuel#

%hlorine, bromine and iodine form oides in which the oidation states of these halogens range from C1 to C7# A combination of "inetic and

thermodynamic factors lead to the generally decreasing order of stability of

oides formed by halogens, $ H %l H ?r# *he higher oides of halogens tend to

be more stable than the lower ones#

%hlorine oides, %l+, %l+, %l+6 and %l+7 are highly reacti'e oidising

agents and tend to eplode# %l+ is used as a bleaching agent for paper pulpand tetiles and in water treatment#

*he bromine oides, ?r +, ?r+ , ?r) are the least stable halogen oides

-middle row anomally/ and eist only at low temperatures# *hey are 'ery

powerful oidising agents#

*he iodine oides, $+2 , $+5, $+7 are insoluble solids and decompose onheating# $+5 is a 'ery good oidising agent and is used in the estimation of carbon monoide#

-iii/ 1eactivity towards metalsG .alogens react with metals to form metal halides#

<or e#g#, bromine reacts with magnesium to gi'e magnesium bromide#

g (s) + ?r + (l) →g?r + (s)

Example7.1$

Solution

*he ionic character of the halides decreases in the order <H %l H ?r H $ where is a mono'alent metal# $f a metal

ehibits more than one oidation state, the halides in higher

oidation state will be more co'alent than the one in lower

oidation state# <or e#g#, En%l2, 9b%l2, Eb%l5 and U<6 are more

co'alent than En%l+, 9b%l+, Eb%l) and U<2 respecti'ely#

-i'/ 1eactivity of halogens towards other halogensG .alogens

combine amongst themsel'es to form a number of compounds

"nown as interhalogens of the types &&′ , &&)

′, &&5

′ and

&&7′ where & is a larger si!e halogen and &

′ is smaller si!e

halogen#

<luorine ehibits only 1 oidation state whereas other halogens

ehibit C 1, C ), C 5 and C 7 oidation states also# =plain#

<luorine is the most electronegati'e element and cannot ehibit any

positi'e oidation state# ther halogens ha'e d orbitals and

therefore, can epand their octets and show C 1, C ), C 5 and C 7

oidation states also#

Intext Questions

7.2' %onsidering the parameters such as bond dissociation enthalpy, electron gain

enthalpy and hydration enthalpy, compare the oidising power of <+ and %l+#

7.27 i'e two eamples to show the anomalous beha'iour of fluorine#

7.2/ Eea is the greatest source of some halogens# %omment#



%hemistry 1:6

7.1*&lorine

%hlorine was disco'ered in 1772 by Echeele by the action of

.%l on n+# $n 181O @a'y established its elementary natureand suggested the name chlorine on account of its colour -ree", chloros greenish yellow/#

Preparation

$t can be prepared by any one of the following methodsG

-i/ ?y heating manganese dioide with concentratedhydrochloric acid# n+ C 2.%l →n%l+ C %l+ C

+.+

.owe'er, a miture of common salt and concentrated

.+E2 is used in place of .%l#

24a%l C n+ C 2.+E2 →n%l+ C 24a.E2 C

+.+ C %l+ -ii/ ?y the action of .%l on potassium

permanganate#

+In2 C 16.%l → +I%l C +n%l+ C 8.+ C 5%l+

'anufacture of chlorine

-i/ Deacon6s processG ?y oidation of hydrogen chloride gas

by atmospheric oygen in the presence of %u%l+

-catalyst/ at 7+) I#

2.%l + + ????→%u%l+ +%l + + +.+

-ii/ Electrolytic processG %hlorine is obtained by the

electrolysis of brine -concentrated 4a%l solution/#

%hlorine is liberated at anode# $t is also obtained as a by

product in many chemical industries#

Properties

$t is a greenish yellow gas with pungent and suffocating odour#$t is about +(5 times hea'ier than air# $t can be li3uefied easily

into greenish yellow li3uid which boils at +): I# $t is soluble in

water#

%hlorine reacts with a number of metals and non(metals to

form chlorides#

+Al C )%l+ → +Al%l) ;

+4a C %l+ →+4a%l;

+<e C )%l+ → +<e%l) ;

$t has great affinity for hydrogen# $t reacts with compounds

containing hydrogen to form .%l#

.+ + %l+ →+.%l

.+ E + %l+ →+.%l + E

%1O .16 + 8%l+ →16.%l + 1O%

Lith ecess ammonia, chlorine gi'es nitrogen and

ammonium chloride whereas with ecess chlorine, nitrogen

trichloride -eplosi'e/ is formed#

84.) C )%l+ → 64.2%l C 4+ ; 4.) C )%l+

→ 4%l) C ).%l-ecess/

-ecess/

Lith cold and dilute al"alies chlorine produces a mitureof chloride and hypochlorite but with hot and concentrated



al"alies it

gi'es

chloride and

chlorate#

+4a. C %l+ → 4a%l C 4a%l C .+

-cold and dilute/

6 4a. C )%l+ → 54a%l C 4a%l) C ).+

-hot and conc#/

Lith dry sla"ed lime it gi'es

bleaching powder# +%a-./+ C+%l+ →%a-%l/+ C %a%l+ C

+.+




*he composition of bleaching powder is %a-%l/+#%a%l+#%a-./+#+.+# %hlorine

reacts with hydrocarbons and gi'es substitution products with saturated hydrocarbons

and addition products with unsaturated

hydrocarbons# <or eample,%.2 C %l+

ethane%+.2 C %l+

=thene

%hlorine water on standing loses its yellow colour due to the formation of .%l

and .%l# .ypochlorous acid -.%l/ so formed, gi'es nascent oygen which is

responsible for oidising and bleaching properties of chlorine#

-i/ $t oidises ferrous to ferric, sulphite to sulphate, sulphur dioide to sulphuric

acid and iodine to iodic acid#

+<eE2 C .+E2 C %l+ →<e+-E2/) C +.%l 4a+E) C %l+ C .+ → 4a+E2 C +.%l E+ C

+.+ C %l+ →.+E2 C +.%l

$+ C 6.+ C 5%l+ → +.$) C 1O.%l

-ii/ $t is a powerful bleaching agent; bleaching action is due to oidation# %l+ C .+

→+.%l C

%oloured substance C →%olourless substance

$t bleaches 'egetable or organic matter in the presence of moisture# ?leaching

effect of chlorine is permanent#

Uses: $t is used -i/ for bleaching woodpulp -re3uired for the manufacture of paper and

rayon/, bleaching cotton and tetiles, -ii/ in the etraction of gold and platinum -iii/ in themanufacture of dyes, drugs and organic compounds such as %%l2, %.%l), @@*, refrigerants,

etc# -i'/ in sterilising drin"ing water and -'/ preparation of poisonous gases such as phosgene

-%%l+/, tear gas -%%l) 4+/, mustard gas -%l%.+%.+E%.+%.+%l/#

Example7.17

Solution

Lrite the balanced chemical e3uation for the reaction of %l+ with hot and

concentrated 4a.# $s this reaction a disproportionation reactionM >ustify#

)%l+ C 64a. → 54a%l C 4a%l) C ).+

es, chlorine from !ero oidation state is changed to 1 and C5 oidation states#

Intext Questions

7.2 i'e the reason for bleaching action of %l+#

7." 4ame two poisonous gases which can be prepared from chlorine gas#7.2+ /,rogen &lorie



lauber

prepared this

acid in 1628 by heating common salt with concentrated sulphuric acid#

@a'y in 181O showed that it is a compound of hydrogen and chlorine#

%hemistry 1:8



Preparation

$n laboratory, it is prepared by heating sodium chloride with concentrated sulphuric

acid#

4a%l C .+E2 ⎯⎯⎯⎯→

2+OI

4a.E2 C .%l 4a.E2 C 4a%l ⎯⎯⎯⎯→

8+)I 4a+E2 C .%l

.%l gas can be dried by passing through concentrated sulphuric acid#

Properties

$t is a colourless and pungent smelling gas# $t is easily li3uefied to a colourless li3uid

-b#p#18: I/ and free!es to a white crystalline solid -f#p# 15: I/# $t is etremely

soluble in water and ionises as belowG

.%l (g) + .+ ( l ) →.) + (a3) + %l

− (a3) a = 1O

7

$ts a3ueous solution is called hydrochloric acid# .igh 'alue of dissociation

constant - a/ indicates that it is a strong acid in water# $t reacts with 4.) and gi'es

white fumes of 4.2%l#

4.) C .%l → 4.2%l

Lhen three parts of concentrated .%l and one part of concentrated .4 ) aremied, a3ua regia is formed which is used for dissol'ing noble metals, e#g#, gold, platinum#

Au + 2.+ + 4)

− + 2%l

− →Au%l

−2 + 4 + +.+

)9t + 16.+ + 24)

− + 18%l

− →)9t%l

+6

− + 24 + 8.+

.ydrochloric acid decomposes salts of wea"er acids, e#g#, carbonates,

hydrogencarbonates, sulphites, etc#

4a+%) C +.%l →+4a%l C .+ C %+

4a.%) C .%l → 4a%l C .+ C %+

4a+E) C +.%l → +4a%l C .+ C E+

Uses: $t is used -i/ in the manufacture of chlorine, 4. 2%l and glucose -from corn starch/,

-ii/ for etracting glue from bones and purifying bone blac", -iii/ in medicine and as a

laboratory reagent#

Lhen .%l reacts with finely powdered iron, it forms ferrous chloride and not

ferric chloride# LhyM

$ts reaction with iron produces .+# <e + +.%l→ <e%l + + .+

Niberation of hydrogen pre'ents the formation of ferric chloride#

Example 7.1)

Solution

7.21 Oxoa#is

o!

/alogens

@ue to high electronegati'ity and small si!e, fluorine forms only one

ooacid, .< "nown as fluoric -$/ acid or hypofluorous acid# *he other

halogens form se'eral ooacids# ost of them cannot be isolated in pure

state# *hey are stable only in a3ueous solutions or in the form of their salts#

*he ooacids of halogens are gi'en in *able 7#1O and their structures are

gi'en in <ig# 7#8#

1:: *he p(?loc" =lements



Table 7.1: O+oacids of *alo!ens

*alic 4# acid

-.ypohalous acid/

*alic 4### acid

-.alous acid/

*alic 46 acid

-.alic acid/

*alic 46## acid

-9erhalic acid/

Fig. 7.*

5he str%ct%res of

o2oacids of chlorine

7.22 Inter&alogen ompouns

Lhen two different halogens react with each other, interhalogen compounds are formed# *hey can be assigned

general compositions as &&′ , &&)

′, &&5

′ and &&7

′ where & is halogen of larger si!e and &

′ of smaller si!e

and & is more electropositi'e than &′# As the ratio between radii of & and &

′ increases, the number of atoms

per molecule also increases# *hus, iodine -D$$/ fluoride should ha'e maimum number of atoms as the ratio of

radii between $ and < should be maimum# *hat is why its formula is $<7 -ha'ing maimum number of atoms/#

Preparation*he interhalogen compounds can be prepared by the direct combination or by the action of halogen on lower

interhalogen compounds# *he product formed depends upon some specific conditions, <or e#g#,

%l+ + <+ ????→+%l<;

2)7 I

-e3ual 'olume/

%l+

-ecess/

$+ + %l+ →+$%l;

-e3uimolar /

%hemistry +OO



Properties

Eome properties of interhalogen compounds are gi'en in *able 7#11#

Table 7.11: -ome Properties of #nterhalo!en ompounds

Type ormula

&&′ 1 %l<

?r<

$<a

?r%l b

$%l

$?r

&&′ ) %l<)

?r<)

$<)

$%l)c

&&′ 5 $<5

?r<5

%l<5

&&′ 7 $<7

a@ery %nstable!

b5he p%re solid is known at room temperat%re!

c Dimerises as 7l–bridged dimer (I 07l )

*hese are all co'alent molecules and are diamagnetic in nature# *hey are 'olatile

solids or li3uids ecept %$< which is a gas at +:8 I# *heir physical properties are

intermediate between those of constituent halogens ecept that their m#p# and b#p# are

a little higher than epected#

*heir chemical reactions can be compared with the indi'idual halogens# $n

general, interhalogen compounds are more reacti'e than halogens -ecept fluorine/#

*his is because &&′ bond in interhalogens is wea"er than && bond in halogensecept << bond# All these undergo hydrolysis gi'ing halide ion deri'ed from the

smaller halogen and a hypohalite - when &&′ /, halite - when &&′ )/, halate -when

&&′ 5/ and perhalate -when &&′ 7/ anion deri'ed from the larger halogen#

&&B + . + →.&

B + .&

*heir molecular structures are 'ery interesting which can be eplained on the basis of DE=9F theory -=ample 7#1:/# *he &&) compounds ha'e the bent V*

shape, &&5 compounds s3uare pyramidal and $<7 has pentagonal bipyramidalstructures -*able 7#11/#

+O1 *he p(?loc" =lements



Example 7.1* @educe the molecular shape of ?r<) on the basis of DE=9F theory#

Solution *he central atom ?r has se'en electrons in the

'alence shell# *hree of these will form electron(pair bonds withthree fluorine atoms lea'ing behind four electrons# *hus, there

are three bond pairs and two lone pairs# According to DE=9F

theory, these will occupy the corners of a trigonal bipyramid#

*he two lone pairs will occupy the e3uatorial positions to

minimise lone pair(lone pair and the bond pair(lone pair

repulsions which are greater than the bond pair(bond pair

repulsions# $n addition, the aial fluorine atoms will be bent

towards the e3uatorial fluorine in order to minimise the lone(

pair(lone pair repulsions# *he shape would be that of a slightly

bent V*#

Uses: *hese compounds can be used as non a3ueous sol'ents# $nterhalogen

compounds are 'ery useful fluorinating agents# %l<) and ?r<) are used for the production ofU<6 in the enrichment of +)5U#

U-s/ C )%l<)-l/ → U<6-g/ C )%l<-g/

7.23 Group1)

Elements

7.2".1 Occurrence

Intext Question

7."1 Lhy is $%l more reacti'e than $+M

roup 18 consists of si elementsG helium, neon, argon, "rypton, enon

and radon# All these are gases and chemically unreacti'e# *hey form 'ery

few compounds# ?ecause of this they are termed noble gases#

All the noble gases ecept radon occur in the atmosphere# *heir

atmospheric abundance in dry air is Q 1 by 'olume of which argon is the

maSor constituent# .elium and sometimes neon are found in minerals of

radioacti'e origin e#g#, pitchblende, mona!ite, cle'eite# *he main

commercial source of helium is natural gas# &enon and radon

are the rarest elements of the group# Fadon is obtained as a decay product

of ++6Fa#

++688 Fa →

+++86 Fn +2

+ .e

Example7.2+

Solution

Lhy are the elements of roup 18 "nown as noble gases M

*he elements present in roup 18 ha'e their 'alence shell orbitals completely

filled and, therefore, react with a few elements only under certain conditions#

*herefore, they are now "nown as noble gases#

%hemistry +O+



*he important atomic and physical properties of the roup 18 elements along

with their electronic configurations are gi'en in *able 7#1+# *he trends in some of the

atomic, physical and chemical properties of the group are discussed here#

Table 7.12: Atomic and Physical Properties of Group 1/ Elements

Propery

Atomic number

Atomic mass0 g mol 1


Atomic radius0pm

$onisation enthalpy

0">mol(1

=lectron gain enthalpy

0">mol(1

@ensity -at E*9/0gcm )

elting point0I

?oiling point0I

Atmospheric content

- by 'olume/

radioactive

7.2".2 Electronic All noble gases ha'e general electronic configuration ns+np6 ecept onfi!uration

helium which has 1s+ -*able 7#1+/# any of the properties of noble gases including their inacti'e nature

are ascribed to their closed

shell structures#

7.2"." #onisation @ue to stable electronic configuration these gases ehibit 'ery high Enthalpy ionisationenthalpy# .owe'er, it decreases down the group with increase

in atomic si!e#

7.2".% AtomicAtomic radii increase down the group with increase in atomic

$adii number#

7.2".5 Electron Eince noble gases ha'e stable electronic configurations, they ha'e no Gain tendency to accept

the electron and therefore, ha'e large positi'e 'alues Enthalpy of electron gain enthalpy#

Physical Properties

All the noble gases are monoatomic# *hey are colourless, odourless and tasteless#

*hey are sparingly soluble in water# *hey ha'e 'ery low melting and boiling points because the only type of interatomic interaction in these elements is wea" dispersion

forces# .elium has the lowest boiling point -2#+ I/ of any "nown substance# $t has an

unusual property of diffusing through most commonly used laboratory materials such

as rubber, glass or plastics#

4oble gases ha'e 'ery low boiling points# LhyM

4oble gases being monoatomic ha'e no interatomic forces ecept wea" dispersion forces

and therefore, they are li3uefied at 'ery low temperatures# .ence, they ha'e low boiling

points#

Example 7.21

Solution



+O) *he p(?loc" =lements



+hemical Properties

$n general, noble gases are least reacti'e# *heir inertness to chemical reacti'ity is

attributed to the following reasonsG

-i/ *he noble gases ecept helium -1s+/ ha'e completely filled ns+np6 electronic

configuration in their 'alence shell#

-ii/ *hey ha'e high ionisation enthalpy and more positi'e electron gain enthalpy#

*he reacti'ity of noble gases has been in'estigated occasionally, e'er since their

disco'ery, but all attempts to force them to react to form the compounds, were

unsuccessful for 3uite a few years# $n arch 1:6+, 4eil ?artlett, then at theUni'ersity of ?ritish %olumbia, obser'ed the reaction of a noble gas# <irst, he

prepared a red compound which is formulated as +C9t<6

# .e, then realised that the

first ionisation enthalpy of molecular oygen -1175 ">mol 1/ was almost identical

with that of enon -117O "> mol 1/# .e made efforts to prepare same type of

compound with &e and was successful in preparing another red colour compound

&eC

9t<6

by miing 9t<6 and enon# After this disco'ery, a number of enoncompounds mainly with most electronegati'e elements li"e fluorine and oygen,

ha'e been synthesised#

*he compounds of "rypton are fewer# nly the difluoride -Ir<+/ has beenstudied in detail# %ompounds of radon ha'e not been isolated but only identified

-e#g#, Fn<+/ by radiotracer techni3ue# 4o true compounds of Ar, 4e or .e are yet"nown#

$a& ,enon%fluorine compounds

&enon forms three binary fluorides, &e<+, &e<2 and &e<6 by the direct reactionof elements under appropriate eperimental conditions#

&e -g/ C <+ -g/

-enon in ecess/

&e -g/ C +<+ -g/

-1G5 ratio/

&e -g/ C )<+ -g/

-1G+O ratio/

&e<6 can also be prepared by the interaction of &e<2 and +<+ at 12)I# &e<2 +

+<+ →&e<6 + +

&e<+, &e<2 and &e<6 are colourless crystalline solids and sublime readily at +:8

I# *hey are powerful fluorinating agents# *hey are readily hydrolysed e'en by tracesof water# <or eample, &e<+ is hydrolysed to gi'e &e, .< and +#

+&e<+ -s/ C +.+-l/ → +&e -g/ C 2 .<-a3/ C +-g/

*he structures of the three enon fluorides can be deduced from DE=9F and

these are shown in <ig# 7#:# &e<+ and &e< 2 ha'e linear and s3uare planar structures

respecti'ely# &e<6 has se'en electron pairs -6 bonding pairs and one lone pair/ andwould, thus, ha'e a distorted octahedral structure as found eperimentally in the gas phase#

&enon fluorides react with fluoride ion acceptors to form cationic species and

fluoride ion donors to form fluoroanions#

&e<+ C 9<5 →&e<RC 9<6R ; &e<2 C Eb<5 →&e<)R

C Eb<6R &e<6 C <

→C &e<7R - 4a, I, Fb or %s/

%hemistry +O2



$"& ,enon%oxygen compounds

.ydrolysis of &e<2 and &e<6 with water gi'es &eO)#

8e

-a/ Ninear

Fig. 7.-

5he str%ct%res of

(a Ae3 0 (b Ae3 C

(c Ae3 ) (d Ae93 C

and (e Ae9"

@oes the hydrolysis of &e<6 lead to a redo reactionM

4o, the products of hydrolysis are &e<2 and &e+<+ where the oidation states of all the elements remainthe same as it was in the reacting state#O O

O

-e/ 9yramidal

Example 7.22

Solution

Uses: .elium is a non(inflammable and light gas# .ence, it is used in filling balloons for

meteorological obser'ations# $t is also used in gas(cooled nuclear reactors# Ni3uid helium -b#p# 2#+ I/

finds use as cryogenic agent for carrying out 'arious eperiments at low temperatures# $t is used to

produce and sustain powerful superconducting magnets which form an essential part of modern 4F

spectrometers and agnetic Fesonance $maging -F$/ systems for clinical diagnosis# $t is used as a

diluent for oygen in modern di'ing apparatus because of its 'ery low solubility in blood#

4eon is used in discharge tubes and fluorescent bulbs for ad'ertisement display purposes# 4eon

bulbs are used in botanical gardens and in green houses#



Argon is used mainly to pro'ide an inert atmosphere in high temperature metallurgical processes

-arc welding of metals or alloys/ and for filling electric bulbs# $t is also used in the laboratory for

handling substances that are air(sensiti'e#

*here are no significant uses of &enon and Irypton# *hey are used in light bulbs designed for special

purposes#

Intext Questions7."2 Lhy is helium used in di'ing apparatusM

7."" ?alance the following e3uationG &e<6 C .+ →&e+<+ C .<

7."% Lhy has it been difficult to study the chemistry of radonM




Summar,roups 1) to 18 of the periodic table consist of p(bloc9 elements with their 'alence shell electronic

configuration ns+np16# roups 1) and 12 were dealt with in %lass &$# $n this Unit remaining groups of the p(bloc" ha'e been discussed#

Group 15 consists of fi'e elements namely, 4, 9, As, Eb and ?i which ha'e general electronic

configuration ns+np)# 4itrogen differs from other elements of this group due to small si!e, formation of

p ) p multiple bonds with itself and with highly electronegati'e atom li"e or % and non(

a&ailability of d orbitals to epand its 'alence shell# =lements of group 15 show gradation in

properties# *hey react with oygen, hydrogen and halogens# *hey ehibit two important oidationstates, C ) and C 5 but C) oidation is fa'oured by hea'ier elements due to Vinert pair effect#

@initrogen can be prepared in laboratory as well as on industrial scale# $t forms oides in 'arious

oidation states as 4+, 4, 4+), 4+, 4+2 and 4+5# *hese oides ha'e resonatin! structures

and ha'e multiple bonds# Ammonia can be prepared on large scale by *abers process# .4) is an

important industrial chemical# $t is a strong monobasic acid and is a powerful oidising agent# etals

and non(metals react with .4) under different conditions to gi'e 4 or 4+#

9hosphorus eists as 92 in elemental form# $t eists in se'eral allotropic forms# $t forms hydride,

9.) which is a highly poisonous gas# $t forms two types of halides as 9& ) and 9&5# 9%l) is prepared by

the reaction of white phosphorus with dry chlorine while 9%l5 is prepared by the reaction of phosphorus

with E+%l+# 9hosphorus forms a number of ooacids# @epending upon the number of 9. groups,

their basicity 'aries# *he ooacids which ha'e 9. bonds are good reducing agents#

The Group 1' elements ha'e general electronic configuration ns+np2# *hey show maimum

oidation state, C6# radation in physical and chemical properties is obser'ed in the group 16 elements#

$n laboratory, dioygen is prepared by heating I%l) in presence of n+# $t forms a number of oides

with metals# Allotropic form of oygen is ) which is a highly oidising agent# Eulphur forms a number

of allotropes# f these, α and β forms of sulphur are the most important# Eulphur combines with

oygen to gi'e oides such as E+ and E)# E+ is prepared by the direct union of sulphur withoygen# E+ is used in the manufacture of .+E2# Eulphur forms a number of ooacids# Amongst them,

most important is .+E2# $t is prepared by contact process# $t is a dehydrating and oidising agent# $t is

used in the manufacture of se'eral compounds#

Group 17 of the periodic table consists of the following elements <, %l, ?r, $ and At#*heseelements are etremely reacti'e and as such they are found in the combined state only# *he common

oidation state of these elements is 1# .owe'er, highest oidation state can be C7# *hey show regular gradation in physical and chemical properties# *hey form oides, hydrogen halides, interhalogen

compounds and ooacids# %hlorine is con'eniently obtained by the reaction of .%l with In 2# .%l

is prepared by heating 4a%l with concentrated .+E2# .alogens combine with one another to form

interhalo!en compounds of the type & &1n -n 1, ), 5, 7/ where & 1 is lighter than &# A number of

ooacids of halogens are "nown# $n the structures of these ooacids, halogen is the central atom whichis bonded in each case with one . bond as &.# $n some cases & O bonds are also found#

Group 1/ of the periodic table consists of noble !ases# *hey ha'e ns+ np

6 'alence shell electronic

configuration ecept .e which has 1 s+# All the gases ecept Fn occur in atmosphere# Fn is obtained as

the decay product of ++6Fa#@ue to complete octet of outermost shell, they ha'e less tendency to form compounds# *he best

characterised compounds are those of enon with fluorine and oygen only under certain conditions#

*hese gases ha'e se'eral uses# Argon is used to pro'ide inert atmosphere, helium is used in filling

balloons for meteorological obser'ations, neon is used in discharge tubes and fluorescent bulbs#

%hemistry +O6



Exercises7.1 @iscuss the general characteristics of roup 15 elements with reference to their electronic

configuration, oidation state, atomic si!e, ionisation enthalpy and electronegati'ity#

7.2 Lhy does the reacti'ity of nitrogen differ from phosphorusM

7." @iscuss the trends in chemical reacti'ity of group 15 elements#

7.% Lhy does 4.) form hydrogen bond but 9.) does notM

7.5 .ow is nitrogen prepared in the laboratoryM Lrite the chemical e3uations of the reactions

in'ol'ed#

7.' .ow is ammonia manufactured industriallyM

7.7 $llustrate how copper metal can gi'e different products on reaction with .4)#

7./ i'e the resonating structures of 4+ and 4+5#

7. *he .4. angle 'alue is higher than .9., .As. and .Eb. angles# LhyM

*int: %an be eplained on the basis of sp)

hybridisation in 4.) and only s–p bonding betweenhydrogen and other elements of the groupR#

7.1 Lhy does F )9 eist but F ) 4 does not -F al"yl group/M

7.11 =plain why 4.) is basic while ?i.) is only feebly basic#

7.12 4itrogen eists as diatomic molecule and phosphorus as 92# LhyM

7.1" Lrite main differences between the properties of white phosphorus and red phosphorus#

7.1% Lhy does nitrogen show catenation properties less than phosphorusM

7.15 i'e the disproportionation reaction of .)9)#

7.1' %an 9%l5 act as an oidising as well as a reducing agentM >ustify#

7.17 >ustify the placement of , E, Ee, *e and 9o in the same group of the periodic table in terms of

electronic configuration, oidation state and hydride formation#

7.1/ Lhy is dioygen a gas but sulphur a solidM

7.1 Inowing the electron gain enthalpy 'alues for → and →

+ as 121 and 7O+ "> mol

1

respecti'ely, how can you account for the formation of a large number of oides ha'ing +

species and not M-*int: %onsider lattice energy factor in the formation of compounds/#

7.2 Lhich aerosols deplete o!oneM

7.21 @escribe the manufacture of .+E2 by contact processM

7.22 .ow is E+ an air pollutantM

7.2" Lhy are halogens strong oidising agentsM

7.2% =plain why fluorine forms only one ooacid, .<#

7.25 =plain why inspite of nearly the same electronegati'ity, oygen forms hydrogen bonding while

chlorine does not#

7.2' Lrite two uses of %l+#

7.27 Lhy are halogens colouredM

7.2/ Lrite the reactions of <+ and %l+ with water#

7.2 .ow can you prepare %l+ from .%l and .%l from %l+M Lrite reactions only#

7." Lhat inspired 4# ?artlett for carrying out reaction between &e and 9t<6M

7."1 Lhat are the oidation states of phosphorus in the followingG

-i/ .)9) -ii/ 9%l) -iii/ %a)9+ -i'/ 4a)92 -'/ 9<)M




7."2 Lrite balanced e3uations for the followingG

-i/ 4a%l is heated with sulphuric acid in the presence of n+#

-ii/ %hlorine gas is passed into a solution of 4a$ in water#

7."" .ow are enon fluorides &e<+, &e<2 and &e<6 obtainedM

7."% Lith what neutral molecule is %l isoelectronicM $s that molecule a Newis baseM

7."5 .ow are &e) and &e<2 preparedM

7."' Arrange the following in the order of property indicated for each setG

-i/ <+, %l+, ?r +, $+ ( increasing bond dissociation enthalpy#

-ii/ .<, .%l, .?r, .$ ( increasing acid strength#

-iii/ 4.), 9.), As.), Eb.), ?i.) increasing base strength#

7."7 Lhich one of the following does not eistM

-i/ &e<2 -ii/ 4e<+ -iii/ &e<+ -i'/ &e<6

7."/ i'e the formula and describe the structure of a noble gas species which is isostructural withG

-i/ $%l2

-ii/ $?r +

-iii/ ?r)

7." Lhy do noble gases ha'e comparati'ely large atomic si!esM

7.% Nist the uses of neon and argon gases#

Ans3ers to -ome #nte+t ;uestions

7.1 .igher the positi'e oidation state of central atom, more will be its polarising power which, in

turn, increases the co'alent character of bond formed between the central atom and the other

atom#

7.2 ?ecause ?i.) is the least stable among the hydrides of roup 15#

7." ?ecause of strong pπ pπ o'erlap resulting into the triple bond, 4≡ 4#

7.' <rom the structure of 4+5 it is e'ident that co'alence of nitrogen is four#

7.7 ?oth are sp) hybridised# $n 9.2C all the four orbitals are bonded whereas in 9.) there is a lone

pair of electrons on 9, which is responsible for lone pair(bond pair repulsion in 9. ) reducing the

bond angle to less than 1O:J +8′ #

7.1 9%l5 C @+ → 9%l) C +@%l

7.11 *hree 9. groups are present in the molecule of .)92# *herefore, its basicity is three#

7.15 ?ecause of small si!e and high electronegati'ity of oygen, molecules of water are highly

associated through hydrogen bonding resulting in its li3uid state#

7.21 ?oth the E bonds are co'alent and ha'e e3ual strength due to resonating structures#

7.25 .+E2 is a 'ery strong acid in water largely because of its first ionisation to .)C and .E2

# *he

ionisation of .E2 to .)

C and E2+ is 'ery 'ery

small# *hat is why I a + << I a1 #

7."1 $n general, interhalogen compounds are more reacti'e than halogens due to wea"er && 1

bonding than && bond# *hus, $%l is more reacti'e than $+#

7."% Fadon is radioacti'e with 'ery short half(life which ma"es the study of chemistry of radon

Pblock class 12th notes for iitjee

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