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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