3.3.2.3 - Combustion of Alkanes

Why alkanes are useful fuels

When an alkane burns, it reacts with oxygen in an exothermic reaction. The large energy release is why methane, propane, butane, petrol, and similar hydrocarbon mixtures are practical fuels for heating, transport, and electricity generation.

If oxygen is plentiful, the combustion is complete and the products are carbon dioxide and water. For methane, the balanced equation is:

For complete combustion of a hydrocarbon, all carbon atoms end up in $\mathrm{CO_2}$ and all hydrogen atoms end up in $\mathrm{H_2O}$.

The same idea applies to other organic compounds as well: with enough oxygen, carbon is fully oxidised to carbon dioxide and hydrogen to water.

Worked example: balance the complete combustion of propane, $\mathrm{C_3H_8}$.

1. Put 3 in front of $\mathrm{CO_2}$ to balance carbon.
2. Put 4 in front of $\mathrm{H_2O}$ to balance hydrogen.
3. Count oxygen atoms on the right: $3 \times 2 + 4 \times 1 = 10$, so you need 5 $\mathrm{O_2}$ molecules.

If oxygen is limited, the combustion is incomplete. Then the carbon is not fully oxidised, so carbon monoxide and sometimes carbon, as soot, are formed instead of all the carbon becoming carbon dioxide.

For example:

With an even poorer oxygen supply, solid carbon can also appear. That is why a yellow smoky flame is a warning sign that combustion is incomplete.

Where the pollutants come from

The useful energy release is only part of the story. In real engines and furnaces, the combustion conditions are not perfect, so several pollutants can form.

Carbon monoxide, $\mathrm{CO}$, comes from incomplete combustion. It is especially dangerous because it is poisonous and colourless. In the body, it binds strongly to haemoglobin, reducing the blood's ability to carry oxygen.

Carbon particulates, often called soot, are tiny solid carbon particles formed when combustion is very incomplete. These particles can irritate the lungs and worsen respiratory disease. Keep this separate from unburned hydrocarbons: soot is solid carbon, while unburned hydrocarbons are fuel molecules that escaped without reacting fully.

Unburned hydrocarbons are also pollutants from internal combustion engines. They contribute to photochemical smog and are harmful because they add reactive organic material to the atmosphere.

Nitrogen oxides, written as $\mathrm{NO_x}$, are formed in car engines because the temperature is high enough for nitrogen and oxygen from the air to react:

The nitrogen oxides are mainly formed from the air entering the engine, not from the alkane fuel itself. Nitrogen oxides contribute to acid rain and photochemical smog.

Sulfur dioxide, $\mathrm{SO_2}$, is formed when sulfur-containing impurities in fuels are burned. It is another major air pollutant and a cause of acid rain.

How catalytic converters clean exhaust gases

Keep the boundary clear here: catalytic converters remove gaseous pollutants from internal combustion engines. That means gases such as carbon monoxide, nitrogen oxides, and unburned hydrocarbons. They do not remove solid carbon particles.

Inside, there is a ceramic honeycomb coated with catalysts such as platinum and rhodium. The honeycomb design gives a very large surface area, so many gas molecules can contact the catalyst as they flow through the exhaust.

The diagram below shows the basic layout of a catalytic converter, with polluted exhaust gases entering and less harmful gases leaving after reaction on the honeycomb catalyst surface.