A Level Notes on Gp6

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Group 6/16 Introduction

down group 6/16 ===>property\Zsymbol, name 8O Oxygen 16S Sulphur 34Se Selenium 52Te Tellurium 84Po Polonium (radioactive)

Period 2 3 4 5 6Appearance (RTP) colourless

gasyellow

solid(monoclinic/rhombic

allotropes)

silver metalloid or red

powder allotropessilver white

metalloid radioactive silvery solidmelting pt./

oC -218 117 217 450 254

boiling pt./oC -183 445 685 990 962

density/gcm-3


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principal oxidation states -2, +1, (+2) -2, +2, +4, +6 -2, +2, +4, +6 -2, +2, +4, +6 -2, +2, +4, +6property\Zsymbol, name 8O Oxygen 16S Sulphur 34Se Selenium 52Te Tellurium 84Po Polonium Generally speaking down a p block group the element becomes more metallic in chemical character. Oxygen and sulphur are non-metals, selenium and tellurium are semi-metals, polonium is essentially a metal

OXYGEN - brief summary of a few points The structure of the element:

o Non-metal existing as diatomic molecule, O2, with a double covalent bond.o It has two allotropes: 'normal oxygen' O2 (dioxygen above) and the highly unstable and reactive gas ozone, O3 (trioxygen).

Physical properties of the element:

o O2 is a colourless gas; mpt -218oC; bpt -183

oC; poor conductor of heat/electricity.

O3 is a pale blue gas. Group, electron configuration (and oxidation states):

o Gp6; e.c. 2,6 or1s22s

22p

4; Normally (-2) e.g. H2O, CO2 etc. but can have other oxidation states ...

o e.g. H2O2 (-1), F2O (+2). Reaction of element with oxygen:
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o O2 molecules won't react with themselves BUT in the upper atmosphere oxygen atoms are formed by high energy

radiation/particle collision with oxygen molecules causing homolytic bond fission to produce free oxygen atom (free radicals).

These combine with oxygen molecules to form ozone. Ozone can be synthesised by an electric discharge through oxygen. (i) O2 = hv => 2O

.(ii) O

.+ O2 ==> O3

Reaction of oxide with water, acids or bases/alkalis: Not applicable. Reaction of element with chlorine:

o None, but unstable chlorine(I) oxide (chlorine monoxide) can be made indirectly and there are other chlorine oxides.

(seechlorine) Reaction of chloride with water:

o Slowly hydrolyses to form weak chloric(I) acid. Cl2O(g) + H2O(l) ==> 2HClO(aq)

Reaction of element with water:o Slightly soluble but no reaction.

Other comments:o Formed in plant photosynthesis. Consumed in respiration.

Links to other pages on site:

o Preparation of oxygen gas:Test for gas:

SULFUR/SULPHUR - brief summary of a few points
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The structure of the element:o Three solid allotropes. Two are crystalline lattices based on S8 molecules (rhombic and monoclinic sulfur). A 3rd form is an

unstable dark brown-black polymeric form called plastic sulphur, formed when boiling sulphur is poured onto cold water, great fun,

but of little use! Physical properties:

o Colourless gas; mpt 117oC; bpt 445

oC; poor conductor of heat/electricity.

Group, electron configuration (and oxidation states):

o Gp6; e.c. 2,6 or1s22s

22p

63s

23p

4; ranges from (-2 to +6) e.g.

o Na2S (-2), S2Cl2 (+1), SO2 (+4) and H2SO4, SF6, SO3 (all +6). Reaction of element with oxygen:

o Burns in air with a pale blue flame to form sulphur dioxide (sulphur(IV) oxide), with a little sulphur trioxide. S(s) + O2(g) ==> SO2(g)

o Sulphur trioxide(sulphur(VI) oxide) has to be made by the industrial Contact Process. 2SO2(g) + O2(g) == V2O5 catalyst, 450

oC ==> 2SO3(g)

Reaction of the oxides with water: Both dissolve to form acid solutions.o Sulphur dioxide forms the weak 'fictitious' sulphurous acid.

SO2(g) + H2O(l) ==> H2SO3(aq)

the reaction is better represented ionically as .. SO2(aq) + H2O(l) H

+(aq) + HSO3

-(aq)

o Sulphur trioxide reacts very violently and exothermically to form the oily liquid, strong sulphuric acid. SO3(g) + H2O(l) ==> H2SO4(l)

In water, the sulphuric acid is almost fully ionised. H2SO4(aq) + 2H2O(l) ==> 2H3O

+(aq) + SO4

2-(aq)

Reaction of oxide with acids:


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o None, only acidic in nature. Reaction of oxide with bases/alkalis:

o Sulphur dioxide dissolves in strong bases to form sulphites/sulphate(IV)so 2NaOH(aq) + SO2(g) ==> Na2SO3(aq) + H2O(l) formation of sodium sulphite/sulphate(IV)o ionic equation: 2OH

-(aq) + SO2(g) ==> SO3

2-(aq) + H2O(l)

o You would NOT attempt to react sulphur trioxide with water, the reaction is very violent and exothermic.o but theoretically: 2NaOH(aq) + SO3(g) ==> Na2SO4(aq) + H2O(l)

Reaction of element with chlorine:

o When chlorine is passed over molten sulphur a variety of chlorides are formed.o The main product is disulphur dichlorideo 2S(s) + Cl2(g) ==> S2Cl2(l) (also SiCl2, SiCl4)

Reaction of chloride with water:o Slowly hydrolyses in water, via a complex reaction, to form an acid solution of several products (not meant to be a balanced

equation). S2Cl2(g) + H2O(l) ==> HCl(aq), S(s), SO2(aq), H2SO3(aq), H2SO4(aq),H2S(aq) - complex redox/hydrolysis reaction!

Reaction of element with water:o None.


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Shapes and bond angles of molecules and ions of oxygen and sulfur

oxygen molecule O2

electrons: two bond pairs and two lone pairs, ANGULAR or BENT

shape: e.g. hydrogen sulphide, H2S, or water H2O, i.e. H2X with H-X-H bond angle of approximately 109oand similarly ions like NH2

-.

Note: the exact H-O-H angle is 104.5o

due to the extra repulsion of two lone pairs. (Q = H, X = O, S etc. in group 6)


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electrons: 6 bond pairs, OCTAHEDRAL SHAPE: e.g. sulfur(VI) fluoride (sulphur hexafluoride molecule) SF6 with Q-X-Q bond angles of

90o

and 180o

(Q = F, X = S)


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Sulfur(IV) oxide/sulphur(VI) oxide, SO2 (sulphur dioxide/sulfur

dioxide) molecule is a bent shape (angular), O-S-O bond angle ~120o

due

to two groups of bonding electrons and one non-bonding lone pair of

electrons.The sulfate(IV) ion/sulphate(IV) ion, SO3

2-(sulfite ion/sulphite ion) is a

trigonal pyramid shape, O-S-O bond angle ~109o

due to three groups of

bonding electrons and one lone pair of electrons.Sulphur(VI) oxide/sulfur(VI) oxide, SO3 (sulfur trioxide/sulphur trioxide), is a

trigonal planar shape, O-S-O bond angle of 120o

due to three bonding

groups of electrons and no lone pairs of electrons.Sulfate(VI) ion/sulfate(VI) ion, SO4

2-(sulphate ion/sulfate ion) is tetrahedral

in shape, O-S-O angle of 109.5o. due to four groups of bonding electrons

and no lone pairs of electrons.The shapes are deduced below using dot and cross diagrams and VSEPR

theory and illustratedvalence bond dot and cross diagrams below.

-


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Why is sulphuric acid a useful material? How is it made? Contact ProcessBecause sulfuric acid has so many uses the industrial development of a country is sometimes measured by the amount of sulphuric acid that is

used each year. Sulphuric acid is made starting from the element sulphur which is found in the Earth's crust. Sulphuric acid is used as car battery acid and is used to make fertilisers, dyes and detergents.

o e.g. ammonia + sulphuric acid ==> ammonium sulphate (a fertiliser salt)o 2NH3(aq) + H2SO4(aq)==> (NH4)2SO4(aq) => evaporation to get crystalso Its acid action make it good forcleaning metal surfaces in industry.

Sulphuric acid is manufactured from the raw materials sulphur, air and waterand involves the production of sulphur trioxide in

the Contact Process. (1) Sulfur is burned in air to form sulphur dioxide (exothermic).

o In the reaction the sulphur is oxidised (O gain) (1a) S(s) + O2(g)==> SO2(g)o Sulfur dioxide can also be indirectly obtained from the process of extracting copper from copper sulphide ores e.g. in a copper

smelter: (1b) Cu2S(s) + O2(g)==> 2Cu(l) + SO2(g) Note: Sulphur dioxide itself is a useful chemical in i ts own right:

o It is used as a bleach in the manufacture of wood pulp for paper manufactureo and its toxic nature makes it useful as a food preservative by killing bacteria.


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(2) The Contact Process of sulphur trioxide production must be economically efficient for the manufacture of the important

industrial chemical sulphuric acid.o In the Contact Process reactor the sulphur dioxide is mixed with air (the required stoichiometric volume/mole SO2:O2 ratio is 2:1, in

practice 1-2:1 is used) and the mixture passed over a catalyst of vanadium(V) oxide V205 at a relatively high temperature of about

450C and at a pressure of between 1-2 atm. In the reactor the sulphur dioxide is oxidised in the reversible exothermic reaction ...

o (2) 2SO2(g) + O2(g) 2SO3(g)o

Kp =pSO3

-------------------pSO2

2pO2

The reaction forms sulphur trioxide and the equilibrium is very much to the right hand side ...o The reaction forms sulphur trioxide and the equilibrium is very much to the right hand side because despite the reaction being

exothermic a relatively high temperature is used which favours the reverse reaction R to L, from the energy change

equilibrium rule, i.e. increasing temperature shifts the equilibrium in the endothermic direction. However the value of Kp is high

enough to give a 99% yield.o The reaction is favoured by high pressure (pressure equilibrium rule, 3 => 2 gas molecules, LHS ==> RHS), but only a small

increase in pressure is used to give high yields of sulphur trioxide, because the formation of SO3 on the right hand side is so

energetically favourable (approx. 99% yield, i.e. only about 1% SO2 unreacted).o The use of the V2O5 catalyst ensures a fast reaction without having to use too a higher temperature which would begin to

favour the left hand side too much (energy change equilibrium rule), but remember a catalyst does not affect the % yield or

equilibrium concentration of SO3, you just get there more economically faster.
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o Multiple reactor beds are used to ensure the maximum % conversion and heat exchange systems are used to control the

temperature, and pre-heat incoming reactant gases. (3) The sulphur trioxide is dissolved in concentrated sulphuric acid to form fuming sulphuric acid (oleum).

o SO3(g) + H2SO4(l)==> H2S2O7(l) (4) Water is then carefully added to the oleum to produce concentrated sulphuric acid (98%).

o H2S2O7(l) + H2O(l)==> 2H2SO4(l)o If the sulphur trioxide is added directly to water an acid mist forms which is difficult to contain because the reaction to form

sulphuric acid solution is very exothermic!o If you 'add' equations (3) + (4) you get

(5)SO3(g) + H2O(l)==> H2SO4(l) Good anti-pollution measures need to be in place since the sulphur oxides are harmful and would cause local acid rain! To help this

situation AND help the economics of the process, any unreacted sulphur dioxide is recycled through the reactor. Concentrated sulphuric acid can be used in the laboratory as a dehydrating agent.

o Dehydration is the removal of water or the elements of water from a compound and can be described as an elimination

reaction. Usually and adjacent H and OH in a molecule are removed to form the water.o When added to some organic compounds containing hydrogen and oxygen, e.g. sugar, concentrated sulphuric acid removes the

elements of water from the compound leaving a 'spongy' black carbon residue.o Ifalcohols are heated with conc. sulphuric acid, they are dehydrated to alkenes.

e.g. ethanol ==> ethene + water CH3CH2OH ==> CH2=CH2 + H2O

o When added to blue copper sulphate crystals concentrated sulphuric acid removes the water of crystallisation

leaving white anhydrous copper sulphate. In this case the water already exists BUT not in a mixture and so the following reaction

is classified as a chemical change. CuSO4.5H2O(s) CuSO4(s) + 5H2O(absorbed into the H2SO4 which it reacts with)


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o (ii) the reduction half-reaction is: Br2(aq) + 2e-==> 2Br

-(aq)

Two bromine atoms (as molecule) change from ox. state 0 to -1.o The hydrogen (+1) and oxygen (-2) do not change oxidation state.

(i) + (ii) equals the balanced equation, 2 electrons gained and lost or an ox. state rise and fall of 2 units. Bromine is the oxidising agent (gain/accept e

-s, lowered ox. state),

and sulphur dioxide is the reducing agent (loses e-s, inc. ox. state of S).

o Sulphur dioxide does ionise to a small extent in water to give the sulphite ion, and adding a strong non-oxidising acid like dilute

hydrochloric acid to sodium metabisulphite produces the ion, which means another equation can also adequately describe the

redox change in terms of sulphur and bromine. e.g. if the sulphite ion acts as the reducing agent the reaction with chlorine would be written as: SO3

2-(aq) + Cl2(aq) + H2O(l) ==> SO4

2-(aq) + 2Cl

-(aq) + 2H

+(aq)

The oxidation of hydrogen sulfide by iron(III) ionso If an iron(III) salt (old name, ferricsalt) is added to hydrogen sulphide solution a precipitate of sulphur forms and the orange-brown

solution turns pale green.o H2S(aq) + 2Fe

3+(aq) ==> 2Fe

2+(aq) + 2H

+(aq) + S(s)

o Oxidation: 1 S at (-2) change to 1 S at (0), H2S ==> S, a loss of 2 electrons, inc. 2 ox. state units.o Reduction: 2 Fe at (+3) change to 2 Fe at (+2), gain in total of 2 electrons, decrease in 2 ox. state units.o No change in the oxidation state of the 2H's (+1) involved.o The iron(III) ion acts as the oxidising agent (gains/accepts e

-s, lowered ox. state of Fe) and the hydrogen sulphide is the

reducing agent(loses/donates e-s, inc. ox. state of S). The decomposition of hydrogen peroxide

o Hydrogen peroxide decomposition, catalysed by the black solid manganese(IV) oxide, MnO2. 2H2O2(aq) ==> O2(g) + 2H2O(l) Ox. state changes: 4O at (-1) change to 2O at (0) in O2 and 2O at (-2) in H2O and H is unchanged at (+1).


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A Level Notes on Gp6

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