Colourful Solutions > Energy from fuels > Combustion

Combustion is probably the first chemical reaction that mankind knew and although it is the oldest, it is also one of the most interesting.

We live on the earth in an oxidising atmosphere that contains up to 20% oxygen. Oxygen is a very reactive gas which can oxidise even relatively unreactive substances.

Syllabus reference R1.3.1

Reactivity 1.3.1 - Reactive metals, non-metals and organic compounds undergo combustion reactions when heated in oxygen.

  • Deduce equations for reactions of combustion, including hydrocarbons and alcohols.

Guidance

Tools and links

  • Reactivity 2.2 - Why is high activation energy often considered to be a useful property of a fuel?
  • Reactivity 3.2 - Which species are the oxidizing and reducing agents in a combustion reaction?

Combustion of metals

Reactive metals will burn readily in oxygen. The group 1 metals all burn easily in air.

Oxygen is able to remove electrons from the active metals leaving positive metal ions and creating oxides.

Sodium forms a simple oxide

4Na + O2 → 2Na2O

But in excess oxygen

Sodium forms a peroxide

2Na + O2 → Na2O2

Group 2 metals will also burn, but require a little more activation to start the reaction.

Calcium, strontium and magnesium form simple oxides. Most students are familiar with the blinding light produced when magnesium burns.

Magnesium reacts in air

2Mg + O2 → 2MgO

Aluminium, from group 13 will burn in air when powdered, and its a similar story for zinc and iron.

The oxides formed by group 1 metals are all highly basic, while those of group 2 metals are weakly basic. The peridic trend is clear.

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Combustion of non-metals

Some, but not all, non-metallic elements burn in oxygen. Those that do produce acidic oxides. The strength of acidity increases with increasing oxidation state and the periodic trend towards the right hand side.

Carbon burns in air to produce carbon dioxide if the supply of air is adequate, or carbon monoxide if the air supply is restricted.

Carbon forms carbon dioxide in excess air

C(s) + O2(g) → CO2(g)

Carbon forms carbon monoxide in limited air

2C(s) + O2(g) → 2CO(g)

Carbon dioxide is weakly acidic, while carbon monoxide is neutral.

Both phosphorus and sulfur burn readily in air forming acidic oxides.

Phosphorus forms phosphorus(V) oxide in excess air

4P(s) + 5O2(g) → P4O10(s)

Sulfur forms sulfur(IV) oxide in excess air

S(s) + O2(g) → SO2(s)

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Combustion of hydrocarbons

Since the 19th century hydrocarbons have been extracted from the earth to use as fuels for light and heat, and in the 20th century to power internal combustion engines.

These hydrocarbons, known as fossil fuels, burn easily in sufficient air, making carbon dioxide and water.

Generating combustion equations

In complete combustion, all of the carbon atoms in the fuel molecule ends up as carbon dioxide and all of the hydrogen atoms ends up in water molecules.

Combustion of propane, C3H8

There are 3 carbon atoms so 3 molecules of carbon dioxide must be made. There are 8 hydrogen atoms, enough to make 4 water molecules. So far we have:

C3H8 + oxygen → 3CO2(g) + 4H2O(l)

Now we must count up the oxygen atoms in the product to find how many molecules of oxygen are needed on the left hand side.

3CO2(g) + 4H2O(l) contains 10 oxygen atoms = 5O2

So the final equation is:

Complete combustion of propane

C3H8 + 5O2(g) → 3CO2(g) + 4H2O(l)

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Combustion of other organic compounds

Most organic compounds burn in air due to their high carbon and hydrogen content. Probably the most well known example of non-flammable organics are the chloro- and fluorocarbons. Tetrachloromethane finds use in fire extinguishers.

The alcohols are flammable and ethanol is used as a 50% fuel additive to petrol to make a mixture that is suitable for internal combustion, "gasahol".

Complete combustion of ethanol

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)

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