Chemguide: Support for CIE A level Chemistry


Learning outcomes 15.3(a), 15.3(b) and 15.3(c)

These statements are about the use of alkanes as fuels, and some of the environmental problems associated with them.

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


Statement 15.3(a)

This statement deals with the use of alkanes as fuels. If you haven't already read it for statement 15.1(d) about fractional distillation, look at this page from the BBC which relates the boiling points of the various fractions to their uses.

You will need to be able to write an equation for the combustion of any alkane whose formula you are given (or can work out). You will already have read this page, but look again at the complete combustion part of the page about the combustion of alkanes.

Alkanes are useful as fuels because they are readily available, convenient to transport and use, burn cleanly (as long as there is an excess of air or oxygen), and have high enthalpy changes of combustion.


Statement 15.3(b)(i)

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.


Unburnt hydrocarbons

Small amounts of unreacted hydrocarbons from the petrol-air mixture are found in the exhaust fumes. Hydrocarbons in the atmosphere can absorb infra-red radiation and play a part in the greenhouse effect (see below).


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 13.2. 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.

Carbon

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.


Statement 15.3(b)(ii)

This is about gases contributing to the enhanced greenhouse effect. I will explain the word "enhanced" in a minute.


The greenhouse effect

When sunlight hits the surface of the earth, some of it is re-emitted as infra-red (IR) radiation. Some molecules in the atmosphere are particularly good at absorbing the energy in this infra-red radiation. These molecules include water vapour, carbon dioxide and hydrocarbons such as methane.

The extra energy absorbed by these molecules is passed on to other molecules in the atmosphere during collisions, and so the whole atmosphere warms up.

This process has been going on throughout the history of the Earth, and is the reason why the Earth's temperature is suitable for life. The greenhouse effect on its own is a good thing!


The enhanced greenhouse effect

The amount of carbon dioxide in the atmosphere has been increasing quite rapidly for various reasons, including the burning of fossil fuels like petrol.

The extra carbon dioxide will increase ("enhance") the greenhouse effect and cause the Earth to warm.


Note:  Everything I have said about the relationship between carbon dioxide levels and the greenhouse effect is factually accurate. The questions arise about the possible extent of the future warming, and how much of the warming that took place during the last 30 years of the twentieth century was due to carbon dioxide or to natural cycles involving the sun and ocean currents, for example.

At the time of writing (June 2014), that question is still open, and there has been no significant increase in world temperatures in the last 16 or so years despite increases in carbon dioxide concentration. Predictions are based on computer models of a fantastically complex system which is incompletely understood.

You don't need to worry about this for CIE A level chemistry purposes. Questions so far have been trivial, amounting just to you recognising carbon dioxide as a greenhouse gas.



Statement 15.3(c)

This statement is about using infra-red spectroscopy in monitoring air pollution. Unless you have already met IR spectroscopy, it won't make much sense to you.

I will cover this as a part of Section 22.2.


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© Jim Clark 2010 (last modified June 2014)