Chemguide: Support for CIE A level Chemistry

Learning outcome 14: Hydrocarbons

14.1: Alkanes

Statement 14.1.6

This statement deals with the environmental consequences of using hydrocarbon fuels.

We will look quickly at each of the major problems in turn, and then the common solution.

Carbon monoxide

Carbon monoxide is produced by the incomplete combustion of any hydrocarbon. This happens if there isn't enough air or oxygen present to give carbon dioxide and water.

Carbon monoxide is dangerous because it is poisonous. You will find this discussed towards the bottom of the page about the combustion of alkanes.

Oxides of nitrogen

Several different oxides of nitrogen are produced when petrol (gasoline) is burnt in car engines. These include nitrogen monoxide, NO, and nitrogen dioxide, NO2. They are often grouped together as NOx.

They are formed by direct combination of nitrogen and oxygen in the air in the presence of the electrical spark which ignites the petrol-air mixture.

The nitrogen-nitrogen triple bond in N2 is very strong, which is why nitrogen is generally quite unreactive. But the energy in the spark is enough to break the bond, and so provide the activation energy for the reaction between the nitrogen and oxygen.

If you just burned petrol in a dish in the open air, this doesn't happen. The temperature never gets high enough to provide the energy needed to break the nitrogen-nitrogen triple bond.

Oxides of nitrogen are poisonous, and also contribute to acid rain. If you have forgotten about acid rain, then you could explore the US Environmental Protection Agency site.

The role of nitrogen oxides (together with unburnt hydrocarbons) in the formation of photochemical smog is described in statement 12.1.4

Unburnt hydrocarbons

Small amounts of unreacted hydrocarbons from the petrol-air mixture are found in the exhaust fumes. Hydrocarbons in the atmosphere contribute to photochemical smog, as well as absorbing infra-red radiation and playing a part in the greenhouse effect.

Removing these pollutants - catalytic converters

You will find a short explanation of how catalytic converters work about half-way down the page that you will get to by following this link to a page about types of catalysis.

Catalytic converters aren't a perfect solution, because they only work effectively when they are really hot. That means that they aren't effective during short journeys before the exhaust system has warmed up properly.

Other possible pollutants

Sulphur dioxide and lead compounds

There is no indication in the new or previous syllabus that CIE want any more than I have described above. However, there was a question which went beyond this in June 2009.

The question asked for two toxic pollutants that can be produced during the complete combustion of a hydrocarbon in an internal combustion engine. The fact that it is complete combustion stops you giving carbon monoxide among your answers.

The answers given in the mark scheme gave 1 mark for an oxide of nitrogen - either in general terms as NOx or more specifically as NO or NO2. So what else could you give?

CIE suggested either SO2 or lead compounds. I can't see the justification of this.

Sulphur dioxide is produced when sulphur-containing fuels are burnt. This isn't mentioned in this specific bit of the syllabus, although it is in section 12.2. However, the main problem is that, at least in the UK, and probably elsewhere in the world, petrol is now "low-sulphur" - and so produces virtually no sulphur dioxide.

Similarly, the addition of lead compounds to petrol isn't mentioned anywhere in the present syllabus, and leaded petrol hasn't been sold in the UK for years.

If you lived somewhere that used only low-sulphur petrol and unleaded petrol, you couldn't answer this question.

Neither sulphur dioxide nor lead compounds are removed by catalytic converters. In fact, lead compounds actually stop the catalyst from working. You can't use leaded petrol in a car with a catalytic converter.


This was included as a possible answer to a question in June 2008. Again, there is no reference to this in the current syllabus. Carbon, like carbon monoxide, results from incomplete combustion of the hydrocarbon.

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© Jim Clark 2020