Elements, compounds and experimental techniques - Chemistry - page 14

Water molecules contain two atoms of hydrogen
and one atom of oxygen, and hence water has
the chemical formula H 2 O. Elements other than
hydrogen will also react with oxygen to form
compounds called oxides. For example, magnesium
reacts violently with oxygen gas to form the white
powder magnesium oxide (Figure 2.7). This reaction
is accompanied by a release of energy as new chemical
bonds are formed.

A redox reaction is one which involves the two
processes of reduction and oxidation. For example,
the oxygen has to be removed in the extraction
of iron from iron( iii ) oxide. This can be done in a
blast furnace with carbon monoxide. The iron( iii )
oxide loses oxygen to the carbon monoxide and is
reduced to iron. Carbon monoxide is the reducing
agent . A reducing agent is a substance that reduces
another substance during a redox reaction. Carbon
monoxide is oxidised to carbon dioxide by the
iron( iii ) oxide. The iron( iii ) oxide is the oxidising
agent . An oxidising agent is a substance which
oxidises another substance during a redox reaction.
iron( iii )
oxide

+ → +

iron carbon
dioxide
For a further discussion of oxidation and reduction
see Chapter 3 (p. 39) and Chapter 5 (p. 73).

carbon
monoxide

Both reduction and oxidation have taken place in
this chemical process, and so this is known as a redox
reaction.

More about formulae
The formula of a compound is made up from the
symbols of the elements present and numbers to show
the ratio in which the different atoms are present.
Carbon dioxide has the formula CO 2 . This tells you
that it contains one carbon atom for every two oxygen
atoms. The 2 in the formula tells you that there are
two oxygen atoms present in each molecule of carbon
dioxide. For further discussion see p. 43.
Table 2.5 shows the names and formulae of some
common compounds which you will meet in your
study of chemistry.

Table 2.5 Names and formulae of some common compounds.

Compound Formula

Figure 2.7 Magnesium burns brightly in oxygen to produce magnesium
oxide.

Ammonia NH 3

Calcium hydroxide Ca(OH) 2

Carbon dioxide CO 2

When a new substance is formed during a chemical
reaction, a chemical change has taken place.
magnesium + oxygen → magnesium oxide
When substances such as hydrogen and magnesium
combine with oxygen in this way they are said to have
been oxidised . The process is known as oxidation .
Reduction is the opposite of oxidation. In this
process oxygen is removed instead of being added.

Copper sulfate CuSO 4

Ethanol (alcohol) C 2 H 5 OH

Glucose C 6 H 12 O 6

Hydrochloric acid HCl

Nitric acid HNO 3

Sodium carbonate Na 2 CO 3

Sodium hydroxide NaOH

Sulfuric acid H 2 SO 4