Chemistry, Third edition

414 22 · ENVIRONMENTAL CHEMISTRY

Global warming and the greenhouse effect

The glass of a greenhouse ‘traps’ infrared radiation. Similarly, greenhouse gases in

the atmosphere are so called because they can absorb infrared radiation and reduce

heat loss from the Earth, as shown in Figure 22.3. Such gases include: water vapour,

CO 2 , CH 4 and CFCs and HFCs. Whilst the concentration of water vapour in the

atmosphere has not changed appreciably in recent years, there has been a marked

increase in the levels of the other three. Deforestation and the large-scale burning of

fossil fuels, for example, have contributed to the steady rise in the atmospheric con-

centration of CO 2 since the last century. If these trends continue, in the future there

could be a marked increase in the temperature of the atmosphere resulting in cli-

matic change: for example, less rainfall in temperate zones and more rainfall in the

drier areas of the world. However, increased CO 2 levels in the atmosphere should

cause plants undergoing photosynthesis to take up the gas at a greater rate; plants

living in a warmer climate with adequate rainfall should grow faster.

Predictions of climatic change have been made using complex computer models,

which incorporate many assumptions. The future situation is still by no means clear.

Radon

Pollution caused by radionuclides can be divided into natural pollution (caused

chiefly by radon gas) and man-made pollution (mainly caused by the nuclear power

industry).

Radon gas (mainly^22286 Rn) is the biggest single source of radiation for most of the

population. Its half-life is 3.82 days. Radon is continually being produced as a daughter

The greenhouse

effect

Suggest some alternatives

to fossil fuels that would

notincrease the

greenhouse effect.

Exercise 22C

Removing polluting gases from car exhausts

Catalystic converters in car exhaust systems, which contain mixtures of transition metals and

their oxides, convert toxic CO (from incomplete combustion of hydrocarbons) and NO, present

in the exhaust gases, into CO 2 and N 2 respectively.

Write an equation for these two reactions and classify each reaction as catalytic oxidation,

or catalytic reduction.

Exercise 22B

BOX 22.2

Heavy metals and aluminium concentrations in river water

The term ‘heavy metal’, to a chemist, means

one of a group of the metals in the lower

right hand corner of the Periodic Table

(metals in Groups 13–16 that are in periods

4 and greater). Examples include Fe, Cu, Pb,

Cd and Hg. One reason that these metals

are considered toxic is that they have a great

affinity for sulfur and attack sulfur bonds in

enzymes, making them unable to function

properly.

Aluminium is considered to be a light metal,

but it can still be toxic at high

concentrations. Concentrations of the

aqueous aluminium ion have been found to

be higher in ‘acid’ (pH 4–5) river and lake

waters and this has been found to adversely

affect the trout population in those waters,

particularly if the calcium concentration is

low.

Fig. 22.3The greenhouse
effect.