Edexcel IGCSE Chemistry · Spec 4.19-4.22
Alkanes and Combustion
Covers use the alkane general formula, Draw and name alkanes up to five carbon atoms and Distinguish complete and incomplete combustion.
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Alkanes and Combustion
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Explained
Alkanes and combustion
An alkane is a saturated hydrocarbon. Both words are doing work, and dropping either one is the most common way marks are lost on this topic. Hydrocarbon means the molecule contains hydrogen and carbon only. Saturated means every bond between carbon atoms is a single bond, so there is no room for anything else to be added.
The general formula, and what it is for
Alkanes follow the general formula CnH2n+2. Count the carbons, double it, add two, and you have the number of hydrogens. Methane has one carbon and four hydrogens. Ethane has two and six. Propane has three and eight. Butane has four and ten. Pentane has five and twelve.
The formula is not there to be memorised for its own sake. It lets you check a structure you have drawn. If your drawing of pentane has eleven hydrogens, you have missed one, and the formula tells you so before the examiner does.
Complete combustion
Given plenty of oxygen, an alkane burns completely. The carbon ends up as carbon dioxide and the hydrogen ends up as water. Nothing else is produced, because there is nothing else in the molecule to produce.
That last point is worth holding on to. Because a hydrocarbon contains only carbon and hydrogen, complete combustion cannot give you anything other than carbon dioxide and water.
Incomplete combustion, and why it matters
When the oxygen supply is limited, there is not enough to convert all the carbon to carbon dioxide. Carbon monoxide forms instead, and often carbon itself, which appears as soot.
Carbon monoxide is the reason this is a safety topic rather than a bookkeeping one. It is toxic, colourless and has no smell, and it binds to haemoglobin in place of oxygen. A blocked flue or a poorly maintained boiler limits the oxygen supply, which is exactly the condition that produces it.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry record that on a three mark question about alkanes, incomplete answers addressing only the term hydrocarbon and not saturated were common, and scored two out of three.
Reports also note that when comparing alkanes and alkenes, candidates often remembered that alkenes make polymers but far fewer mentioned that alkanes are more flammable, or connected that to their use as fuels such as petrol. If a question asks why short chain hydrocarbons are more useful, say why: they are more flammable and burn more easily.
Naming and drawing the first five
The prefixes tell you the number of carbon atoms: meth for one, eth for two, prop for three, but for four, pent for five. The ending "ane" tells you it is saturated. That is the whole naming system for this part of the course.
When you draw a displayed formula, every bond must be shown, including every carbon to hydrogen bond. A carbon atom with only three bonds drawn is wrong even if the shape looks right, because carbon always forms four.
Spec 4.19-4.22
What you need to know
- Use the alkane general formula
- Draw and name alkanes up to five carbon atoms
- Distinguish complete and incomplete combustion
Active recall
Quick check
Answer each question before opening the answer.
What is the general formula of an alkane?
CnH2n+2
What are the products of complete combustion?
Carbon dioxide and water
Why is carbon monoxide dangerous?
It reduces the blood's ability to carry oxygen
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