8.7–8.8, 8.11, 8.13Atmospheric pollutants from fuels
The combustion of fuels is a major source of atmospheric pollutants.
Most fuels, including coal, contain carbon and/or hydrogen, and may also contain some sulfur.
The gases released into the atmosphere when a fuel is burned may include carbon dioxide, water vapour, carbon monoxide, sulfur dioxide and oxides of nitrogen. Solid particles and unburned hydrocarbons may also be released, that form particulates in the atmosphere.
You should be able to describe how carbon monoxide, soot (carbon particles), sulfur dioxide and oxides of nitrogen are produced by burning fuels. Each has a different cause, and the mark is for the cause, not the name:
The oxides of nitrogen are the odd ones out, and worth pausing on. The nitrogen does not come from the fuel at all — it is the nitrogen in the air drawn into the engine, which is normally unreactive but reacts with oxygen at the very high temperatures inside a running engine.
You should be able to predict the products of combustion of a fuel given appropriate information about the composition of the fuel and the conditions in which it is used.
Worked example
A fuel contains carbon, hydrogen and a small amount of sulfur. It is burned in an engine with a limited supply of air. Predict the products.
- Every element in the fuel will be oxidised as far as the oxygen allows.
- Hydrogen gives water vapour.
- Carbon with a limited supply of oxygen gives carbon dioxide, but also carbon monoxide and soot, because there is not enough oxygen for complete combustion.
- The sulfur in the fuel is oxidised to sulfur dioxide.
- The engine is hot, so nitrogen and oxygen from the air react to form oxides of nitrogen.
Answer: Water vapour, carbon dioxide, carbon monoxide, soot, sulfur dioxide and oxides of nitrogen — plus some unburned hydrocarbons.
Common mistake
Read the conditions before answering. With plenty of air the products are only carbon dioxide and water, plus sulfur dioxide if the fuel contains sulfur. Carbon monoxide and soot appear only when the oxygen supply is limited.
8.9–8.10, 8.12Properties and effects of atmospheric pollutants
Carbon monoxide is a toxic gas. It is colourless and odourless, and so is not easily detected.
Sulfur dioxide and oxides of nitrogen cause respiratory problems in humans and cause acid rain.
Particulates cause global dimming and health problems for humans.
You should be able to describe and explain the problems caused by increased amounts of these pollutants in the air.
Common mistake
Acid rain and global dimming are separate problems with separate causes: acid rain comes from sulfur dioxide and oxides of nitrogen dissolving in water, while global dimming comes from solid particulates reflecting sunlight. Neither is the greenhouse effect.
- Carbon monoxide
- a toxic gas produced by incomplete combustion, which is colourless and odourless and so not easily detected
- Acid rain
- rain made acidic by sulfur dioxide and oxides of nitrogen released when fuels are burned
- Global dimming
- the reduction in sunlight reaching the Earth's surface caused by particulates in the atmosphere
8.14Hydrogen as a fuel for cars
Burning hydrogen produces water and nothing else:
hydrogen + oxygen → water
2H₂ + O₂ → 2H₂O
No carbon dioxide, no carbon monoxide, no soot, no sulfur dioxide. Set against petrol, which produces all four, that is the whole case for hydrogen — and an evaluation question wants the case against it as well.
That last row is the one that separates a good answer from an average one. Hydrogen is clean at the point of use; whether it is clean overall depends entirely on how the hydrogen was made. Made by electrolysis using electricity from a coal-fired power station, it moves the carbon dioxide rather than removing it.
Common mistake
"Hydrogen produces no pollution" is too strong. It produces no pollution when it burns — say where the emissions have gone instead, because that is what an evaluate question is testing.
Before you practise
- Complete combustion gives carbon dioxide and water; incomplete combustion, in a limited supply of oxygen, gives carbon monoxide and soot as well.
- Carbon monoxide is toxic because it binds to haemoglobin in place of oxygen — and it is colourless and odourless, so it is not noticed.
- Soot causes breathing problems and blackens buildings, and it blocks appliances so they burn less completely still.
- Sulfur dioxide comes from sulfur impurities in the fuel and dissolves in rain water to make acid rain, which damages trees, lakes and limestone buildings.
- Oxides of nitrogen form when the nitrogen and oxygen in the air react in the heat of an engine.
- Burning hydrogen gives water only — but the storage, distribution and the energy used to make it are the arguments against.