Chem istry Modu le Three Handout

Qualitativ e Inorganic A naly sis

Identification of A nions (Specific Objectives 6.5–6.6)

Carbonates (CO32-)

Two tests are available

· Add dilute HCl or concentrated H2SO4: CO2 (g) is liberated with effervescence. Carbonates of

the cations Pb2+, Ca2+ and Ba2+ do not react with H2SO4 as these cations form insoluble

sulfates.

CO32-(s) + 2H+(aq)àH2O(l)+ CO2(g)

· Add Ba2+(aq) or Ca2+(aq) followed by dilute acid: a white precipitate of BaCO3(s) or CaCO3(s) is

formed. The precipitate is soluble in dilute acid with the liberation of CO2 (g).

M2+(aq) + CO32-(aq)àMCO3(s)

Ba2+ ions can be obtained from BaCl2 (aq) and Ba(NO3)2 (aq); Ca2+ ions can be obtained from

CaCl2 (aq).

Nitrate (NO3-)

Four tests are available

· Add concentrated H2SO4 (aq): on warming, HNO3 (g) and red brown nitrogen dioxide gas are

given off. The HNO3 (g) formed undergoes thermal decomposition to produce nitrogen

dioxide and oxygen gas.

NO3-(s) + H2SO4 (aq)àHNO3 g) + HSO4-(aq)

4HNO3 (g)à2H2O (l) + 4NO2 (g)+ O2 (g)

· Add concentrated H2SO4 (aq) in the presence of Cu: NO2 (g) and HNO3 (g) are liberated. The

HNO3 (g) reacts with the Cu to produce NO2 (g) and the resulting solution is green-blue in

colour due to the presence of Cu2+ (aq) ions.

4HNO3 (g) + Cu (s)àCu (NO3)2 (aq) + 2NO2 (g)+ 2H2O (l)

· Add powdered Al or Zn (reducing agents) and Sodium Hydroxide: On heating, NH3 (g) is

given off. The NO3- (aq) ions are reduced by Al/Zn.

NO3- (s) + 3Al (s) + 3OH- (aq) + 6H2O (l)à3 [Al (OH)4]- (aq) + NH3 (g)

NO3- (s)+ 4Zn (s) + 7OH- (aq) + 6H2O (l)à4 [Zn (OH) 4]2- (aq) + NH3 (g)

· Add iron (II) sulfate and concentrated sulphuric acid: crystals of FeSO4 are mixed with the

nitrate solution and then concentrated sulphuric acid is gently added to the mixture such tha

the sulphuric acid forms a layer above the aqueous solution. A brown ring forms at the

junction of the two liquids owing to the presence of Fe(NO)SO4; this test is known as the

brown ring test. The NO3- ion is reduced by iron (II) which is oxidized to iron (III) and forms

a nitrosyl complex.

NO3- (aq) + 3Fe2+ (s) + 4H+ (aq)àNO(g) + 3Fe3+ (aq) + 2H2O (l)

NO (g) + FeSO4 (s)àFe(NO)SO4 (s)

Sulfate (SO42-)

Two tests are available:

· Add Ba2+ (aq)/H+ (aq): a white precipitate of BaSO4 (s) is formed. It is soluble in warm

concentrated HCl.

Ba2+ (aq) + SO42- (aq)àBaSO4 (s)

This test is carried out in an acidified medium to inhibit the precipitation of carbonate and

sulfite.

· Add Pb2+ (aq): a white precipitate of PbSO4 (s) is formed which is soluble in hot concentrated

sulphuric acid.

Pb2+ (aq) + SO42- (aq)à PbSO4 (s)

Pb2+ ions can be obtained from lead (II) ethanoate.

Sulfite (SO32-)

Three tests are available:

· Add dilute hydrochloric acid or concentrated sulphuric acid: SO2 (g) is evolved on warming.

SO32- (aq)+ 2H+ (aq)à SO2 (g) + H2O (l)

· Add Ba2+ (aq): a white precipitate of Barium Sulfite is formed which is readily soluble in dilute

hydrochloric acid with the liberation of Sulphur Dioxide gas.

Ba2+ (aq) + SO32- (aq)àBaSO3 (s)

· Add Silver Nitrate solution: a white precipitate of Silver Sulfite is formed which turns from

grey to black on warming as a result of the decomposition to silver.

2Ag+ (aq) + SO32- (aq)àAg2SO3 (s)

Chlorides (Cl-)

Three tests are available:

· Add concentrated sulphuric acid: the pungent, colourless hydrogen chloride gas is evolved.

H2SO4 (aq) + Cl- (s)àHCl (g) + HSO4- (aq)

· Add Silver Nitrate solution followed by aqueous ammonia: a white precipitate of Silver

Chloride is formed which is soluble in aqueous ammonia to form Ag(NH3)2+ (aq).

Ag+ (aq) + Cl- (aq)àAgCl (s)

· Add Pb2+ (aq): a white precipitate of lead chloride is formed which dissolves on heating and re-

precipitates on cooling.

Pb2+ (aq) + 2Cl- (aq)àPbCl2 (s)

Bromides (Br -)

Three tests are available

· Add Silver nitrate solution followed by aqueous ammonia: a white or cream precipitate of

Silver Bromide is formed which is partially soluble in aqueous ammonia.

Ag+ (aq) + Br-(aq)àAgBr (s)

· Add concentrated sulphuric acid: red brown vapours of Bromine gas and Hydrogen Bromide

gas are seen. The Hydrogen Bromide gas that is formed is oxidized to Bromine gas.

H2SO4 (aq) + Br- (aq)àHBr (g) + HSO4- (aq)

· Add Pb2+(aq): a white precipitate of Lead Bromide is formed. This precipitate dissolves on

heating and re-precipitates on cooling.

Pb2+ (aq) + 2Br- (aq)àPbBr2 (s)

Iodides (I-)

Three tests are available:

· Add Silver Nitrate solution followed by aqueous ammonia: a yellow or cream precipitate of

Silver Iodide is formed which is insoluble in aqueous ammonia.

Ag+ (aq) + I- (aq)àAgI (s)

· Add concentrated sulphuric acid: Iodine is formed as a black or violet precipitate. On

warming, violet vapours of iodine are evolved. Hydrogen Iodide gas is initially formed but is

oxidized to Iodine.

H2SO4 (aq) + I- (s)àHI (g) + HSO4- (aq)

2HI (g) + [o]à I2 (s) + H2O (l)

· Add Pb2+ ions: a yellow precipitate of Lead Iodide is formed which is soluble in excess of the

iodine solution.

Pb2+ (aq) + 2I- (aq)àPbI2 (s)

PbI2 (s) + 2I- (aq)à [PbI4]2- (aq)

Chromate (CrO42-)

Three tests are available:

· Add Silver Nitrate solution: a red-brown precipitate of Silver Chromate is formed which is

soluble in aqueous ammonia.

2Ag+ (aq) + CrO42- (aq)àAg2CrO4 (s)

· Add Ba2+ ions: a pale yellow precipitate of Barium Chromate is formed which is soluble in

strong acids.

Ba2+ (aq) + CrO42- (aq)àBaCrO4 (s)

· Add Pb2+ ions: a yellow precipitate of Lead Chromate is formed.

Pb2+ (aq) + CrO42- (aq)à PbCrO4 (s)

Testing for Gases

ü Carbon Dioxide

i) Colourless, odourless

ii) When bubbles through lime water (Calcium Hydroxide Solution), the mixture turns

milky. The milky appearance is due to the formation of a solid precipitate of Calcium

Carbonate.

Ca(OH)2 (aq) + CO2 (g)àCaCO3 (s) + H2O (l)

ü Hydrogen

i) Colourless, odourless

ii) Pops with lighted splint or may burn with a blue flame. Hydrogen gas is recognized

by the ‘pop’when it burns. This ‘pop’is the sound of a small explosion since H2 is

extremely flammable.

ü Hydrogen Chloride

i) Colourless, pungent

ii) Hydrogen Chloride gas is an acidic gas; it dissolves in water to form hydrochloric

acid, HCl (aq)

iii) Fumes in moist air; turns moist blue litmus red; fumes with ammonia gas. The fumes

are due to the formation of Ammonium Chloride.

HCl (g) + NH3 (g)àNH4Cl (g)

ü Hydrogen Sulfide

i) Colourless, odour of rotten eggs

ii) Turns Pb2+ ions black due to the formation of Lead Sulfide.

ü Ammonia

i) Colourless, pungent

ii) Turns moist red litmus blue; fumes with Hydrogen Chloride gas.

iii) Ammonia is an alkaline gas

ü Nitrogen dioxide

i) Red-brown, pungent

ii) Turns moist blue litmus red

ü Oxygen

i) Colourless, odourless

ii) Relights a glowing splint

iii) Oxygen is the only gas which supports burning and hence will relight a glowing splint

ü Sulfur dioxide

i) Colourless, choking odour

ii) Turns moist blue litmus red

iii) Turn KMnO4/H+ colourless

iv) Turns K2Cr2O7/H+ green

v) Sulfur dioxide is an acidic, reducing gas.

ü Chlorine

i) Pale yellow-green, pungent, choking odour

ii) Bleaches moist blue litmus

iii) Chlorine gas has a bleaching effect

ü Bromine

i) Red-brown, pungent

ii) Moist blue litmus turns red then bleached; fumes in moist air

iii) Bromine gas has a bleaching effect

ü Iodine

i) Violet-black solid and violet vapours

ii) Bleaches moist litmus

iii) Turns starch/iodide paper blue-black