Chemistry AQA GCSE Revision Notes - Analysis and quantitative chemistry

Further analysis and quantitative chemistry

A range of chemical tests can be used for the detection and identification of elements and compounds. Titrations can be used to find the amounts of acid or alkali in a solution.

Analysing substances

a) Flame tests can be used to identify metal ions. Lithium, sodium, potassium, calcium and barium compounds produce distinctive colours in flame tests:

■ lithium compounds result in a crimson flame

■ sodium compounds result in a yellow flame

■ potassium compounds result in a lilac flame

■ calcium compounds result in a red flame

■ barium compounds result in a green flame.

b) Aluminium, calcium and magnesium ions form white precipitates with sodium hydroxide solution but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.

c) Copper(II), iron(II) and iron(III) ions form coloured precipitates with sodium hydroxide solution.

Copper forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.

d) Carbonates react with dilute acids to form carbon dioxide. Carbon dioxide produces a white precipitate with limewater. This turns limewater cloudy.

e) Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid.

Silver chloride is white, silver bromide is cream and silver iodide is yellow.

The production of ammonia

In industrial processes, energy requirements and emissions need to be considered both for economic reasons and for sustainable development.

Learn to:

■ interpret results of the chemical tests in this specification

■ interpret and evaluate the results of analyses carried out to identify elements and compounds for forensic, health or environmental purposes.

·       carry out titrations using strong acids and strong alkalis only (sulfuric, hydrochloric and nitric acids only).

·       (HT only) calculate the chemical quantities in titrations involving concentrations (in moles per dm3) and masses (in grams per dm3).

Making ammonia

a) The raw materials for the Haber process are nitrogen and hydrogen. Nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources.

b) The purified gases are passed over a catalyst of iron at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres). Some of the hydrogen and nitrogen reacts to form ammonia.

The reaction is reversible so ammonia breaks down again into nitrogen and hydrogen:

nitrogen + hydrogen ammonia

On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen are recycled.

c) When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.

d) The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.

e) If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases.

f) If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases.

g) In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.

h) These factors, together with reaction rates, are important when determining the optimum conditions in industrial processes, including the Haber process. HT only

f) Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.

g) The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator.

h) If the concentration of one of the reactants is known, the results of a titration can be used to find the concentration of the other reactant.