Reactions of Alkanes
1. Combustion.
The combustion of alkanes is of major economic importance. Methane is burned in large quantities across europe as "natural gas". It is used in domestic and comercial heating. Methane is also used as a source of hydrogen for the chemical industry with procucts like ammonia (fertilisers) and in margarine manufacture. Propane and Butane are used as products such as "calor gas". Longer chain alkanes like Octane are burned as petrol in cars and other engines.Complete combustion
Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.
Equations
It is quite important that you can write properly balanced equations for these reactions, because they often come up as a part of thermochemistry calculations. Learn to balance the equations don't spend time learning them there are too many for that. For example, with propane (C3H8), you can balance the carbons and hydrogens as you write the equation down. Your first draft would be:
Counting the oxygens leads directly to the final version:
With butane (C4H10), you can again balance the carbons and hydrogens as you write the equation down.
Counting the oxygens leads to a slight problem - with 13 on the right-hand side. The simple trick is to allow yourself to have "six-and-a-half" O2 molecules on the left.
If that offends you, double everything:
Note: You might well come across either version of these equations. The ones with the halves left in are often used in calculation work.
Trends
The hydrocarbons become harder to ignite as the molecules get bigger. This is because the bigger molecules don't vaporise so easily - the reaction is much better if the oxygen and the hydrocarbon are well mixed as gases. If the liquid isn't very volatile, only those molecules on the surface can react with the oxygen.
Bigger molecules have greater Van der Waals attractions which makes it more difficult for them to break away from their neighbours and turn to a gas.
Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. However, combustion tends to be less complete as the number of carbon atoms in the molecules rises. That means that the bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame. Incomplete combustion
Incomplete combustion
(where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.
As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over! The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colourless poisonous gas.
Why carbon monoxide is poisonous? Oxygen is carried around the blood by haemoglobin. Unfortunately carbon monoxide binds to exactly the same site on the haemoglobin that oxygen does. The difference is that carbon monoxide binds irreversibly - making that particular molecule of haemoglobin useless for carrying oxygen. If you breath in enough carbon monoxide you will die from a sort of internal suffocation.
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