Chemguide: Support for CIE A level Chemistry

These statements are about the environmental consequences of the use of nitrogen and sulphur compounds. I am including all of these on this one page, because they are often related to each other.

Before you go on, you should find and read the statements in your copy of the syllabus.

Statement 9.6(e)

This is about what happens if there is too great a use of nitrate fertilisers.

All nitrates are soluble in water, and so are all ammonium salts. That means that any fertiliser not used by the growing crop will dissolve in rainwater and eventually find its way into streams, rivers and lakes.

This will fertilise the growth of water plants and algae which can choke the surface of the water, cutting off light to other plants underneath. When this excess growth dies and decomposes, the process can use up all the oxygen in the water.

Loss of dissolved oxygen will cause fish to die.

This process of excess growth leading to the destruction of life in the water is known as eutrophication.

Because all nitrates are soluble, removing them from water intended for drinking can be expensive. HIgh levels of nitrates in drinking water can cause a disease in young babies called "blue baby syndrome" - so-called because the nitrates interfere with the way oxygen is carried in the blood. Un-oxygenated blood has a bluish colour.

There is also a theoretical (and, I believe, so far unproved) possibility that nitrates could cause stomach cancer.

Statement 9.6(f)

This is about the formation of oxides of nitrogen, and their removal using catalytic converters.

Nitrogen monoxide, NO, is formed when an electrical spark is passed through a mixture of nitrogen and oxygen. This happens in the atmosphere during lightning storms, but the syllabus is mainly concerned with its formation in car engines.

In a petrol (gasoline) engine, a spark is passed through a mixture of petrol vapour and air. The point of doing that, of course, is to ignite the petrol. But it also causes some of the oxygen and nitrogen in the air to combine:

If this escapes into the atmosphere, it causes problems (see below), but it can be removed using a catalytic converter.

A catalytic converter uses expensive metals like platinum coated on to a honeycomb structure to give a high surface area. The platinum catalyses various reactions which help to get rid of pollutants from the exhaust gases.

In this case, it converts harmful carbon monoxide and nitrogen monoxide into carbon dioxide and nitrogen.

Statement 9.6(g)

This statement is about why gases such as nitrogen monoxide or nitrogen dioxide are pollutants.

The most obvious problem is their role in the formation of acid rain.

Nitrogen monoxide reacts with oxygen in the atmosphere to form nitrogen dioxide.

The main cause of acid rain is sulphur dioxide. It is oxidised to give sulphur trioxide which reacts with rain water to give very dilute sulphuric acid.

Nitrogen dioxide acts as a catalyst in the conversion of sulphur dioxide into sulphur trioxide.

The nitrogen dioxide first oxidises sulphur dioxide to sulphur trioxide. In the process, the nitrogen dioxide is reduced to nitrogen monoxide.

Now the nitrogen monoxide is converted back to nitrogen dioxide again by reaction with oxygen.

So, the nitrogen dioxide is regenerated at the end of the reaction, and can go on to do the same thing again, and again . . .

Nitrogen dioxide is also mentioned in the teacher support material as contributing to the formation of photochemical smog. It has a role in the formation of ozone, and of chemicals called peroxyacetylnitrates (PAN).

Ozone in smog is dangerous to health and so are peroxyacetylnitrates. These last substances are formed by complex reactions involving nitrogen dioxide, unburnt hydrocarbons present in exhaust gases, and oxygen from the air.

There is no suggestion in the syllabus or the teacher support material that you should know the chemistry of either ozone or PAN formation.

Statement 9.6(h)

This statement is about the formation of atmospheric sulphur dioxide from burning fossil fuels.

Fossil fuels like coal and oil all contain sulphur compounds, and when the coal or the oil product (petrol or diesel or whatever) are burned, sulphur dioxide is produced.

This leads to acid rain unless some method is used to prevent it.

Burning coal in power stations was a major source of sulphur dioxide, but there are ways of removing this from the flue gases, known as "flue gas desulphurisation". The best of these methods allow the sulphur dioxide to be recovered and then converted into sulphuric acid.

Sulphur compounds in petrol (gasoline) or diesel oil were also an important source of sulphur dioxide pollution. Nowadays, these sulphur compounds can be removed at the refinery to produce low-sulphur (or even ultra-low sulphur) fuels.

Statement 9.6(i)

This statement deals with the role of sulphur dioxide in producing acid rain, and some consequences of acid rain.

Apart from the consequences of acid rain, most of this has been dealt with above.

The role of sulphur dioxide includes:

• its formation during the combustion of fossil fuels.

• the oxidation of sulphur dioxide to sulphur trioxide catalysed by nitrogen dioxide.

• the reaction of the sulphur trioxide with water to produce very dilute sulphuric acid.

The important equations for this have already been given.

The consequences of acid rain include:

• the corrosion of limestone buildings as the calcium carbonate reacts with the acid.

• the corrosion of ironwork as the iron reacts with the acid.

• the acidification of lakes and rivers leading to the death of aquatic life. This is complicated by the fact that a fall in pH dissolves aluminium ions from the soil. Aluminium ions are toxic to fish.

• damage to trees. This again is partly the result of aluminium ions being toxic to plants.

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