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OCR AS Chemistry A H032 for �rst assessment in 2016
Complete Tutor NotesSections:3.1.2 Group 2 page 77
3.1.3 The halogens page 81
3.1.4 Qualitative analysis page 88
5Group 2 Elements Page 77
18 Group 2 Elements
Be4
beryllium9.0
Mg12
magnesium24.3
Ca20
calcium40.1
Sr38
strontium87.6
Ra88
radium(226)
Ba56
barium137.3
The group 2 elements are also known as the alkaline earthmetals.
Mg Mg
They all have 2 electrons in their outer shell which are lostwhen they react to form 2+ ions.
2+
Since electrons are lost, group 2 metals undergo oxidationwhen they react.
-2e-
Oxidation is the loss of electrons.
Reaction with oxygen
Group 2 metals will react with oxygen to produce metal oxides. Theredox nature of this reaction can be shown by investigating oxidationnumbers.
2Mg(s) + O2(g) 2MgO(s)0 0
0+2
REDUCTION
Oxidation number of magnesium has increasedit has therefore been oxidised
REDUCTION
Oxidation number of oxygenhas been reduced therefore ithas been reduced
Mg, being an element,has an oxidation No. of 0 initially
O2, being an element has an oxidation No.of 0 initially
Combined oxygen always has an oxidation No. of -2. Therefore theoxidation number of magnesium must be +2 to make the compound neutral overall
-2
OXIDATION
5
Page 78 Group 2 Elements
18
OXIDATION
REDUCTION
Group 2 elements react with water to produce hydroxides which are alkaline.
For example, strontium will react with water to produce strontiumhydroxide and hydrogen gas:
Reaction with water
Sr(s) + 2H2O(l) Sr(OH)2(s) + H2(g)0 -2 -2+1
-2+1+1+1
Oxidation number of strontium has increasedit has therefore been oxidised
Oxidation number of hydrogenhas been reduced therefore ithas been reduced
Sr, being an element,has an oxidation No. of 0 initially
Combined O alwayshas an oxidation No.of -2 and combined
hydrogen an oxidationNo. of +1
Combined oxygen always has an oxidation No. of -2 and hydrogen +1. Therefore the oxidation number of strontium must be +2 to make the compound neutral overall
+2 00
Hydrogen being anelement has anoxidation No. of 0
ALK
ALIN
ITY O
F HYDROXID
ES INCR
EASES
SOLU
BILITY O
F HYDROXID
ES INCR
EASES
REA
CTIVITY O
F ELEMEN
TS INCR
EASES
+
+
+ Be
Mg
Ca
Reactivity Reactivity increasesas you go down thegroup as the outer2 electrons are moreeasily lost due tothe fact there is lessattraction from thenucleus due toincreased electronshielding and becausethe electrons arefurther away fromthe nucleus.
Solubility and alkalinity of hydroxides
Be4
beryllium9.0
Mg12
magnesium24.3
Ca20
calcium40.1
Sr38
strontium87.6
Ra88
radium(226)
Ba56
barium137.3
5
Group 2 Elements Page 79
18You need to learn the following reactions for group 2 metals and theircompounds and be able to write balanced equations for them:
Sr Sr(OH)2
SrO SrCl2
+HCl
+HCl
SrCO3
SrCO3(s) SrO(s) + CO2(g)
+H2O
+H2O
-CO2
+O2
heat+HCl-CO2
+HCl
Thermal decomposition of group 2 carbonates
EASE O
F THEM
AL D
ECOMPO
SITION O
FCA
RBO
NATES D
ECREA
SES
BeCO3 is unstableat room temperature
BaCO3 requires a huge amount of heat to decompose it.
Example of equation for thermaldecomposition:
heat
Thermal decomposition is the breaking up of a chemical with heat into at least two other chemical substances.
Be4
beryllium9.0
Mg12
magnesium24.3
Ca20
calcium40.1
Sr38
strontium87.6
Ra88
radium(226)
Ba56
barium137.3
5
Page 80 Group 2 Elements
18 Uses of groups 2 compounds
Calcium hydroxide, Ca(OH)2, is used in agriculture to neutralise acid soil:
Ca(OH)2
Ionic equation for this:
H+(aq) + OH-(aq) H20(l)
hydrogen ionsresponsible forthe acidity insoil
Similarly, magnesium hydroxide, Mg(OH)2, is used in Milk of Magnesia medicine to neutralise excess stomach acid.
Phillips’MILK OF
MAGNESIALIQUID
GENTLE SOOTHING RELIEFfor upset stomach and indigestion
Magnesium Hydroxide
Group 7 Elements Page 81
19&20Group 7 (17)* Elements
F9
Fluorine19.0
Cl17
Chlorine35.5
Br35
Bromine79.9
I53
Iodine126.9
At85
Astatine(210)
The group 7 (17) elements are known as the halogens.
They all have 7 electrons in their outermost shell andwhen they react, they acquire an electron to give 8electrons in their outer shells.
Because they gain an electron, they are reduced when they react since reduction is the gaining of electrons.
The reactivity of halogens decreases going down the groupsince there is less attraction to electrons in the outer shellsfrom the nucleus as you movedown the group due to increasedatomic radius and increasedelectron shielding (number ofelectrons in inner shells).
* According to the IUPAC system for numbering groups in the periodic table, the halogens are referred to as group 17 as with this system they number the groups of the transition elements in the middle of the periodic table. Older versions refer to the halogens as groups 7 elements. We will adopt the older version of the table when referring to groups since then the group number will correspond to the number of outer electrons (except for helium).
+
+
+ F
Cl
Br
REA
CTIVITY D
ECREA
SES
Cl
-
Chloride ion
Page 82 The halogens
19&20
++
Bromide ionChlorine atom
-
This electron will be more stronglyattracted to the chlorine than thebromine
Chloride ion
-
Bromineatom
A more reactive halogen can displacea less reactive one.
+ +
+
Chlorinatedwater
Potassiumbromide solution
Bromine + Potassium chloride
Cl2(aq) + 2KBr(aq) Br2(aq) + 2KCl(aq)In this reaction you would see the solution turn orange asbromine is liberated.
For example,
The halogens Page 83
BB
Cl2(aq)+2K+Br-(aq) Br2(aq) + 2K+Cl-(aq)
This reaction can be written as an ionic equation. Let’s consider the ions present:
With an ionic equation you only include the species that have changed.If we look at the above equation, we see the potassium ion has notchanged and so this can be left out of the ionic equation: (Thepotassium ion in this equation is a spectator ion as it does not take part in the reaction.)
Cl2(aq) + 2Br-(aq) Br2(aq) + 2Cl-(aq)
If we assign oxidation numbers we can work out what has beenoxidised and what has been reduced:
Cl2(aq)+2Br-(aq) Br2(aq) + 2Cl-(aq)0 -1 0 -1
19&20
OXIDATIONOxidation number ofbromine has increasedit has therefore beenoxidised
We say that chlorine is a stronger oxidising agent than bromine since chlorine can oxidise bromine.
REDUCTION
Oxidation number of chlorinehas been reduced therefore ithas been reduced
Cl2, being an element,has an oxidation No. of 0 initially
The oxidation number of the bromide ion will be the same as its charge, that is -1
Br2, an element,has an oxidationnumber of 0
Chloride ion hasan oxidation numberof -1 (same as itscharge)
Page 84 The halogens
19&20 Order of Reactivity Halogens -By displacement reactions
Add a halogen, in water, to a solution of halide ions - a more reactivehalogen will displace the less reactive one.
+
Chlorine water Potassiumbromide(colourless)
Bromine + potassium chloride
Cl2(aq) 2KBr(aq) Br2(aq) + 2KCl(aq)
+
Chlorine water Potassiumiodide(colourless)
Iodine + potassium chloride
Cl2(aq) 2KI(aq) I2(aq) + 2KCl(aq)
+
Bromine water Potassiumiodide(colourless)
Iodine + potassium bromide
Br2(aq) 2KI(aq) I2(aq) + 2KBr(aq)
+
Iodine water Potassiumchloride(colourless)
Iodine + potassium chloride
I2(aq) KCl(aq) I2(aq) + KCl(aq)
When iodine is added to both chloride and bromide ions there is no reactionas both chlorine and bromine are more reactive than iodine and the colour ofthe resulting solution in both cases will be brown due to unreacted iodine.
NO REACTION
The halogens Page 85
19&20
+
Iodine water Potassiumbromide(colourless)
Iodine + potassium bromide
I2(aq) KBr(aq) I2(aq) + KBr(aq)
NO REACTION
+
Bromine water Potassiumchloride(colourless)
Bromine + potassium chloride
Br2(aq) KCl(aq) Br2(aq) + KCl(aq)
NO REACTION
To distinguish between the different halogens in solution, an organicsolvent like cyclohexane can be added. The halogen will go into theupper organic layer and will give a distinct colour.
Chlorine waterCl2(aq)
Pale green
Chlorine water +cyclohexane
Cl2(aq)
Pale green
Bromine waterBr2(aq)
Orange
Bromine water +cyclohexane
Br2(aq)
OrangeIodine water
I2(aq)Brown
Iodine water +cyclohexane
I2(aq)
Violet
Page 86 The halogens
19&20
+
+
+ F
Cl
Br
Boiling Points
BOILIN
G P
OIN
T IN
CREA
SES
The boiling points increase as yougo down the group due to strongerinduced dipole-dipole interactions (alsoknown as London forces) that occurbecause the number of electronsincreases as you move down the group.
Chlorine in drinking water
Chlorine is added to drinking water in small amounts to kill waterbourne bacteria.
Drawbacks
Chlorine is toxic
Also, chlorine may react with organicmaterial to produce chlorinated hydrocarbonswhich cause cancer. (Note: chlorine itselfdoes not cause cancer.)
Cl2 + organicmatter
chlorinatedhydrocarbonscancer causing
Overall, the risk from water bourne bacteria is greater and soadding chlorine to water does make it safer to drink.
The halogens Page 87
19&20 Disproportionation reactions of chlorine
When chlorine is added to water, chloric (I) acid is produced alongwith hydrochloric acid.
Disproportionation is the simultaneous oxidation and reductionof the same element in a redox reaction.
Cl2(aq) + H2O HClO(aq) + HCl(aq)(l)0 +1+1 -2 +1-1+1 -2
+10
OXIDATION
REDUCTION
Similarly, chlorine disproportionates when it reacts with sodium hydroxide to form bleach
Cl2(aq) +2NaOH(aq) NaClO(aq) +NaCl(aq) + H2O(l)0 +1+1 -2 +1-1+1 -2+10
OXIDATION
REDUCTION
Page 88 Qualitative Analysis
21 Qualitative AnalysisTesting for the presence of halide ionsAdding silver nitrate (AgNO3(aq)) solution to a solution of halide ionswill produce a precipitate.
Cl-(aq)+AgNO3(aq)
white precipitate
+ dil. NH3(aq)
precipitatedissolves
Br-(aq)+AgNO3(aq)
cream precipitate
+ conc. NH3(aq)
precipitatedissolves
Cl-(aq) + Ag+(aq) AgCl(s)
Br-(aq)+ Ag+(aq) AgBr(s)
I-(aq)+AgNO3(aq)
yellow precipitate
+ conc. NH3(aq)
precipitatedoes not dissolveI-(aq) + Ag+(aq) AgI(s)
We know from our work with acids and group 2 metals, that whenyou add a dilute acid to a carbonate, carbon dioxide is released:
2H+(aq) + CO32-(aq) CO2(g) + H2O(l)
Testing for the presence of Carbonate CO32- ions
The presence of carbon dioxide gas can be confirmed by bubbling thegas through lime water (calcium hydroxide solution) where it will gocloudy.
Qualitative Analysis Page 89
21Testing for the presence of Sulfate, SO4
2-(aq), ions1. Add HCl 2. Add barium chloride
solution BaCl2(aq)Precipitate appears ifsulfate, SO4
2-(aq) present
SO42-(aq) + Ba2+(aq) BaSO4(s)
Testing for the presence of Ammonium, NH4+(aq), ions
1. Add sodium hydroxidesolution, NaOH(aq)
2. Heat and test anygas evolved with dampred litmus paper Litmus paper will
turn blue if NH4+(aq)
present
Heat
NH4+(aq) + OH-(aq) NH3(g) + H2O(l)
causes precipitate
Page 90 Qualitative Analysis
21 Order of testing and false positivesWhen testing for anions (negatively charged ions), it is important youtest in the correct order to avoid false positive results.
It is important to rule out the carbonate ion, CO32-(aq), before testing
for the sulfate ion, SO42-(aq) since the barium ion used in the
sulfate test will react with the carbonate ion and produce barium carbonate, BaCO3(s), which will form a precipitate and a give falsepositive result for sulfate.
Similarly, it is important to rule out the sulfate ion, SO42-(aq),
before proceeding to test for halides as the silver ion from the silvernitrate used in the halide test will form a precipitate of silver sulfatewith the sulfate ion.
A sample should therefore be tested in this order:
1. Test for carbonates
2. Test for sulfates
3. Test for halides
negative
negative
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