Combustion of Alkanes (AQA A-Level Chemistry): Revision Notes

3.2.3 Combustion of Alkanes

Alkanes, which are hydrocarbons, are commonly used as fuels because they undergo combustion reactions that release significant amounts of energy. Combustion can be either complete or incomplete, depending on the availability of oxygen. However, the combustion of alkanes also produces pollutants, some of which are harmful to the environment and human health.

Complete Combustion

In the presence of excess oxygen, alkanes undergo complete combustion, producing carbon dioxide ($CO_2$) and water ($H_2O$). Complete combustion is desirable because it releases the maximum amount of energy and avoids the production of harmful by-products like carbon monoxide ($CO$).

General Equation:

$$\text{Alkane} + O_2 \rightarrow CO_2 + H_2O$$

Incomplete Combustion

When oxygen is limited, alkanes undergo incomplete combustion, leading to the production of carbon monoxide ($CO$) or carbon (soot) in addition to carbon dioxide and water.

General Equation (for carbon monoxide formation):

$$\text{Alkane} + O_2 \rightarrow CO + H_2O$$

Incomplete combustion is less efficient, producing less energy and several harmful by-products, including:

• Carbon monoxide ($CO$): A colourless, odourless, and poisonous gas that can be fatal if inhaled in large quantities.
• Carbon (soot): Particles that can cause respiratory issues and dirty engine parts.

Pollutants Produced by Combustion

The combustion of alkanes, especially in internal combustion engines, generates several pollutants:

• Carbon dioxide ($CO_2$): A greenhouse gas that contributes to global warming.
• Carbon monoxide ($CO$): Formed during incomplete combustion and is toxic when inhaled.
• Unburnt hydrocarbons: These can react with nitrogen oxides to form photochemical smog.
• Nitrogen oxides ($NO_x$): Produced at the high temperatures in car engines when nitrogen and oxygen from the air react. These gases contribute to the formation of acid rain and smog.
• Sulfur dioxide ($SO_2$): Formed when hydrocarbons containing sulphur are combusted. Sulphur dioxide reacts with water in the atmosphere, forming acid rain.

Catalytic Converters

Catalytic converters are devices installed in cars to reduce harmful emissions. They contain a ceramic honeycomb structure coated with a thin layer of catalyst metals such as platinum, palladium, or rhodium. The honeycomb provides a large surface area for reactions.

Catalytic converters facilitate the following reactions:

• $CO$ is converted to $CO_2$:

$$2CO + O_2 \rightarrow 2CO_2$$

• Unburnt hydrocarbons are converted to $CO_2$ and $H_2O$.
• Nitrogen oxides are reduced to nitrogen and oxygen:

$$2NO \rightarrow N_2 + O_2$$

Flue-Gas Desulfurisation

Sulfur dioxide ($SO_2$) from coal-fired power stations is a major contributor to acid rain.

To combat this, flue gases can be treated by flue-gas desulfurisation:

1. The waste gases pass through a scrubber containing an alkaline slurry of calcium oxide ($CaO$) or calcium carbonate ($CaCO_3$) mixed with water.
2. The sulphur dioxide reacts with the calcium compounds to form calcium sulphate ($CaSO_4$), a harmless salt:

$$SO_2 + CaO \rightarrow CaSO_3$$