3.31 AS Inorganic Chemistry - Inorganic analysis

Chemical analysis is used to determine either the identity or the quantity of a species in a sample. This section deals with the qualitative analysis, i.e. the identity of ions in aqueous solutions.

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Cations - positive ions

Most chemical tests for ions take place in aqueous solution, relying on the formation of insoluble salts from a combination of the ion being sought with a suitable counter-ion.

However, the ions formed by group 1 metals do not make precipitates with any common counter-ions, so another form of analysis must be carried out.

Flame tests

The group 1 metal ions (and some group 2 metal ions) give characteristic colours to a bunsen flame when a small sample of a crystalline salt is placed at the edge of the flame..

Group I
Lithium ions Li+ red
Sodium ions Na+ yellow/orange
Potassium ions K+ lilac
Rubidium ions Rb+ red/purple
Caesium ions Cs+ blue
Group II
Calcium ions Ca2+ brick red
Strontium ions Sr2+ crimson
Barium ions Ba2+ apple green

Analysis by precipitation

Most metal ions (apart from group I) give precipitates with hydroxide ions. The colour of the precipitate and its behavious towards excess base help to identify the metal ion.

There are three possibilities on addition of dilute sodium hydroxide solution to an unknown metal ion.

If 1 - no precipitate, then a flame test must be carried out.

If 2 - white precipitate, this could indicate either Mg2+, Zn2+, Al3+, Pb2+

On addition of excess NaOH solution the precipitate formed by Al3+ redissolves.

Al(OH)3(s) + OH-(aq) [Al(OH)4]-(aq)

Addition of concentrated ammonia solution dissolves the precipitate formed by Zn2+.

Zn(OH)2(s) + NH3(aq) [Zn(NH3)4]2+(aq) + 2OH-

Neither Mg(OH)2 nor Pb(OH)2 are affected by excess NaOH or NH3(aq). These two ions can be differentiated by the addition of potassium iodide solution. This gives a yellow precipitate with lead ions:

Pb2+(aq) + I-(aq) PbI2(s)

If 3 - coloured precipitate:

Blue precipitate - indicative of copper(II) ions:

Cu2+(aq) + 2OH-(aq) Cu(OH)2(s)

copper(II) hydroxide precipitate

Green gelatinous precipitate - indicative of iron(II) ions - precipitate turns brown at the surface due to aerial oxidation to iron(III):

iron(II) hydroxide precipitate

Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)

Brown gelatinous precipitate - indicative of iron(III) ions:

Fe3+(aq) + 3OH-(aq) Fe(OH)3(s)

iron(III) hydroxide precipitate

Blue precipitate from a red solution - indicative of cobalt(II) ions - turns grey at surface due to aerial oxidation to cobalt(III):

Co2+(aq) + 2OH-(aq) Co(OH)2(s)

cation analysis summary

Specific cation reactions

Copper(II) ions in aqueous solution react with ammonia solution forming a light blue precipitate of copper(II) hydroxide.

[Cu(H2O)6]2+(aq) + 4NH3(aq) [Cu(H2O)4(OH)2](s) + 2NH4+

copper(II) hydroxide precipitate

This precipitate redissolves forming a deep blue solution of the tetramminecopper(II) complex ion on addition of excess ammonia solution.

[Cu(H2O)4(OH)2](s) + 4NH3(aq) [Cu(NH3)4(H2O)2]2+(aq) + 2OH-

tetramminecopper(II) solution


Anions- negative ions

The negative ions that we are interested in are:

chloride, bromide , iodide, hydroxide, carbonate, sulfate, sulfite, nitrate.

A straightforward scheme allows all anions to be analysed:

Addition of barium chloride (or barium nitrate)

Barium chloride solution contains soluble barium ions, Ba2+(aq). These can interact with sulfate, sulfite and carbonate ions in solution to form precipitates:

Ba2+(aq) + SO42-(aq) BaSO4(s)

Initially, there are two possibilities

If 1 - white precipitate, add dilute hydrochloric acid. There are three possibilities:

If A - the anion is a carbonate

BaCO3(s) + 2HCl(aq) BaCl2(aq) + CO2(g) + H2O(l)

If B - the anion is a sulfite (although sulfur(IV) oxide is formed, it is too soluble for effervescence)

BaSO3(s) + 2HCl(aq) BaCl2(aq) + SO2(g) + H2O(l)

If C - the anion is a sulfate

BaSO4(s) + 2HCl(aq) no reaction

If 2 - take a fresh sample and add silver nitrate solution in dilute nitric acid.

Addition of silver nitrate/dilute nitric acid

Silver nitrate solution contains soluble silver ions, Ag+(aq). These can interact and form precipitates with sulfates, sulfites, carbonates and halide ions. For this reason the barium chloride test must be carried out first to eliminate the possibility of sulfate, sulfite and carbonate.

Initially, there are two possibilities:

If A - examine the colour of the precipitate.

Yellow precipitate = iodide

Ag+(aq) + I-(aq) AgI(s)

Cream coloured precipitate = bromide

Ag+(aq) + Br-(aq) AgBr(s)

White precipitate = chloride

Ag+(aq) + Cl-(aq) AgCl(s)

Note that all three precipitates darken slowly on exposure to light.

If B - no precipitate, take a fresh sample of the unknown and add sodium hydroxide and aluminium powder.

Addition of sodium hydroxide/aluminium powder

Warm the mixture and test for any gas given off using damp indicator paper. There are two possibilities:

analysis of anions


AS Inorganic Chemistry Experiments

Task Possible context
Make up a volumetric solution The preparation of a standard solution of sodium carbonate
Carry out a simple acid-base titration Determine the concentration of unknown hydrochloric acid by titration
Carry out some inorganic tests Tests for anions

AS Physical Chemistry Experiments

Task Possible context
Measure an enthalpy change Use Hess's law to find an unknown enthalpy change, such as the reaction of anhydrous copper(II) sulfate with water to produce hydrated crystals
Determine the Mr of a volatile liquid or the Mr of a gas Determine the Mr of hexane or the Mr of carbon dioxide
Investigate how the rate of a reaction changes with temperature. Investigate the rate of reaction of sodium thiosulfate with acid at different temperatures

AS Organic Chemistry

Task Possible context
Distil a product from a reaction The preparation of ethanal from the oxidation of ethanol or the preparation of cyclohexene from the dehydration of cyclohexanol
Carry out some organic tests Tests for alkene, alcohol, acid, aldehyde
Investigate the combustion of alcohols Use a calorimetric method to measure the enthalpies of combustion in an homologous series of alcohols