Chemistry, Third edition

22 · ENVIRONMENTAL CHEMISTRY

Atmospheric pollution

Ozone layer depletion

Ozone (O 3 ) exists in the atmosphere, 20-40 km above sea level. Ozone absorbs UV

radiation which would otherwise harm living things. The effects of ozone loss could

include: increased human cataracts and skin cancer, reduction of plankton in ocean

waters and destruction of plants, including crops. Ozone layer destruction in the

Antarctic was reported in 1985; a major cause of this was believed to be the release of

chlorofluorocarbon compounds, such as CCl 2 F 2 (CFCs). These compounds are

chemically unreactive, non-toxic and odourless, properties which at one time caused

them to be used as solvents, aerosol propellants, refrigerant fluids and blowing

agents for expanded plastic foams. They are so stable, however, that they persist in

the atmosphere for years and eventually enter its upper layers, where they are broken

down by the powerful UV radiation emitted by the sun. Their decomposition prod-

ucts can then destroy ozone:

1.The decomposition of the chlorofluorocarbon products a highly reactive chlo-

rine atom (free radical). This reacts with ozone:

Cl•+ O 3  ClO•+ O 2

2.Oxygen molecules split up into atomic oxygen in the upper atmosphere:

O 2 + + h(240 nm)  2O•

3.Reaction with atomic oxygen produces another chlorine free radical:

ClO•+ O• Cl•+ O 2

Notice that Cl•radicals are regenerated in this reaction, and are free to react with

more ozone in step 1. It has been shown that over one thousand ozone molecules

can be destroyed by one Cl•.

CFCs have been phased out in recent years and replaced by hydrogen-containing

hydrochlorofluorocarbons (HCFCs), such as CHCl 2 CF 3 ; these compounds are more

likely to be destroyed by atmospheric reactions before they reach the ozone layer.

Also HFCs (hydrofluorocarbons) cannot produce chlorine free radicals – for this

reason CH 2 FCF 3 is now used as a domestic refrigerant.

22.2

412

Nitrate in drinking water

The maximum safe limit of nitrate in drinking water is a concentration of 45 ppm (mg dm^3 ).

A sample of water (volume 250 cm^3 ) was analysed and found to contain 0.012 g of NO 3 .

(i) What is the concentration of NO 3  in the sample in mol dm^3?

(ii)Would the water be considered safe to drink?

(iii)Ammonium nitrate (NH 4 NO 3 ) is a common, solid fertilizer. It supplies nitrogen to the soil

which is an essential plant nutrient. What percentage of nitrogen by mass does it contain?

Exercise 22A