UNIT 3: ACIDS, BASES AND METALS
3(b) Salt preparation
Reactions of acids with bases
The reaction of acids with bases is called neutralisation.
 Neutralisation reactions always produce water.
Metal oxides, metal hydroxides and metal carbonates are examples of bases.
They react with acids according to the following equations:
METAL OXIDE + ACID → SALT + WATER
MEATL HYDROXIDE + ACID → SALT + WATER
METAL CARBONATE + ACID → SALT + WATER + CARBON DIOXIDE
In the reaction of an acid and metal hydroxide, the H+(aq) ions of the acid react
with the OH-(aq) ions of the metal hydroxide to produce water:
H+(aq) + OH-(aq) → H2O(l)
In the reaction of an acid and metal oxide, the H+(aq) ions of the acid react
with the O2-(s) ions of the metal oxide to produce water:
2H+(aq) + O2-(s) → H2O(l)
In the reaction of an acid and metal carbonate, the H+(aq) ions of the acid
react with the CO32-(aq) ions of the metal carbonate to produce water and
carbon dioxide:
H+(aq) + CO32-(aq) → H2O(l) + CO2(g)
Everyday examples of neutralisation include the treatment of acid indigestion
and using lime to reduce acidity in soil and lochs.
Reaction of acids with metals
Acids also react with metals. However, this is not an example of a neutralisation
reaction because it does not produce water.
MATAL + ACID → SALT + HYDROGEN
This type of reaction is called a redox reaction or displacement reaction.
Only metals above hydrogen in the Electrochemical Series react with acids.
Notes written by Mr B. Robertson

For example, iron does react with acids but copper does not.
In this reaction, H+(aq) ions form H2(g) molecules.
The test for hydrogen gas is that it burns with a “pop”.
Acid rain
Non-metal oxides which dissolve in water produce acids.
 Sulphur dioxide produced by burning fossil fuels and nitrogen dioxide
produced by the sparking of air in car engines dissolve in water in the
atmosphere to produce acid rain.
Acid rain has damaging effects on buildings made from carbonate rock,
structures made from iron and steel, soils and plant and animal life.
Salts
A salt is a compound in which the hydrogen ions of an acid have been replaced
by metal ions or ammonium ions.
Salts are formed in the reactions of acids with bases or metals.

Hydrochloric acid forms –chloride salts e.g. sodium chloride is formed in
the reaction of hydrochloric acid with sodium hydroxide.

Sulphuric acid forms –sulphate salts e.g. sodium sulphate is formed in the
reaction of sulphuric acid with sodium hydroxide.

Nitric acid forms –nitrate salts e.g. sodium nitrate is formed in the reaction
of nitric acid with sodium hydroxide.
Some salts which contain nitrogen e.g. ammonium nitrate, NH4NO3, ammonium
sulphate, (NH4)2SO4, and potassium nitrate, KNO3 are made by neutralisation
reactions for use as fertilisers.
 These salts are all soluble in water.
When preparing soluble salts, an insoluble metal carbonate or metal oxide
should be reacted with the acid.
 You can check the acid is completely neutralised using pH paper.
 The unreacted metal carbonate or metal oxide can be removed by
filtration.
 The salt can be obtained by evaporation.
Notes written by Mr B. Robertson
Precipitation reactions
Precipitation is the reaction of two solutions to form an insoluble product, called
a precipitate.
Insoluble salts are produced by precipitation reactions.
Ionic equations
Sometimes neutralisation reactions and precipitation reactions can be written
using ionic equations e.g.
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Cl-(aq) + Na+(aq) + H2O(l)
Spectator ions are ions which are unchanged in the reaction.
Te equation can be rewritten omitting spectator ions. For the above example,
this would be written as:
H+(aq) + OH-(aq) → H2O(l)
Titrations
Volumetric titrations can be used to calculate the concentration of an unknown
acid or alkali.
 The volume of the solution of known concentration is accurately measured
using a pipette and placed in a conical flask with a few drops of indicator.
 The solution of unknown concentration is added from a burette and the
volume required to neutralise the other solution is determined accurately
by experiment.
 The results of the first titration are ignored in calculating the average
volume added from the burette (the titre) because the first titration is a
rough titration.
 The other two results must be concordant i.e. within 0.2 cm3 of one another.
 The concentration of the unknown solution can be determined from the
results of the titration and the balanced equation using the n = c x v
relationship.
Notes written by Mr B. Robertson