Chemistry, Third edition

REDOX REACTIONS IN NATURE

zinc oxide, which protects it from further corrosion – the zinc becomes passive.

Covering a metal with a zinc layer is known as galvanizing.

3.It is not practical to galvanize large objects, such as a ships or pipes. Instead a block

of a reactive metal, such as magnesium (or zinc), is attached to the large object

and, again, preferentially loses electrons to oxygen. This method of protecting the

metal is known as sacrificial protection.

Redox reactions in nature

Nitrogen fixation

Living organisms need nitrogen to make proteins, the ‘building bricks’ of plants and

animals. Although nitrogen gas is abundant, making up approximately four-fifths of

the atmosphere, most living organisms cannot obtain it directly from the air because

it is an unreactive gas. Nitrogen becomes available to plants and animals via the

nitrogen cycle(Fig. 7.7).

Atmospheric nitrogen is particularly unreactive; however, it is oxidized to NO

when lightning flashes. Oxygen in the atmosphere oxidizes NO to NO 2 , which reacts

with rain water to form the acids HNO 2 and HNO 3. These acids react with metal

oxides and carbonates in the soil, to form nitrate and nitrite salts.

Plants obtain nitrogen in the form of nitrate and ammonium ions. Nitrates are

very soluble in water and reach the roots of plants easily; the nitrate ion is then taken

up and reduced to ammonia by the plant. Nitrogen-fixing bacteria that live in the

soil, or in noduleson the roots of plants, convert (i.e. biologically fix) atmospheric

nitrogen into ammonium salts. Animals eat plants and, when both of these organ-

isms die, their organic matter eventually decays into ammonium compounds. Nitri-

fying and denitrifying bacteria convert ammonium compounds into NO 3 and

NO 2 , then N 2 O and N 2. In this way nitrogen is returned to the atmosphere and the

cycle is complete.

7.8

115

Tinning

Steel food cans coated

with tin used to be

common. Unfortunately,

when damaged they

corrode more rapidly than if

the coating was not

present – explain this

phenomenon.

Exercise 7L

Fig. 7.7The nitrogen cycle.