Combustion reactions of alkanes

Alkanes serve as efficient fuels, releasing a significant amount of energy when burnt. They are used in various applications such as power generation, heating, and transportation due to this property.

Alkanes can undergo two types of combustion reactions: complete and incomplete, depending on the oxygen availability.

Complete combustion of alkanes

When there is an ample supply of oxygen, alkanes combust completely, forming carbon dioxide and water vapour as products.

For instance, the complete combustion of methane (CH₄) is represented by the following equation:

CH_4(g) + 2O_2(g) ➔ CO_2(g) + 2H₂O_(g)

Key points:

• Alkanes in their liquid state must be vaporised before combustion.
• Smaller alkanes, due to their lower boiling points, vaporise and thus combust more readily.
• Larger alkanes have more chemical bonds, hence when combusted, they release more energy per mole, making them better fuels.

Incomplete combustion of alkanes

When the oxygen supply is limited, alkanes undergo incomplete combustion, leading to the formation of carbon monoxide and water vapour.

For example, incomplete combustion of methane (CH₄) can be represented by the following equation:

CH_4(g) + 3⁄2O_2(g) ➔ CO_(g) + 2H₂O_(g)

Incomplete combustion may also lead to the production of solid carbon (soot) and the release of unburnt hydrocarbons into the atmosphere.

Environmental impacts of burning fuels

The process of burning alkanes releases several pollutants into the environment.

Carbon particulates:

• Created during incomplete combustion as soot.
• Consist of tiny black particles that cause respiratory issues and breathing problems.
• Soot can accumulate in engines, affecting their efficiency and potentially causing long-term damage.

Carbon monoxide:

• Produced during incomplete combustion.
• Binds with haemoglobin more effectively than oxygen, inhibiting oxygen transportation within the body.
• High concentrations lead to asphyxiation, while lower levels may cause blurred vision, poor coordination, and headaches.

Carbon dioxide:

• Produced during complete combustion.
• As a greenhouse gas, carbon dioxide absorbs and emits infrared radiation back towards Earth, contributing to the greenhouse effect.
• The greenhouse effect is a process by which greenhouse gases trap heat in the Earth's atmosphere, leading to rising surface temperatures (i.e., global warming) and climate change.

Unburnt hydrocarbons:

• A by-product of incomplete combustion.
• Some unburnt hydrocarbons are carcinogenic.
• Unburnt hydrocarbons react with nitrogen oxides in sunlight to produce photochemical smog. This causes respiratory issues.

Nitrogen oxides:

• Generated when atmospheric nitrogen and oxygen react at high temperatures and pressures within engines, as represented by the equation: N_2(g) + O_2(g) ➔ 2NO_(g)

• Nitrogen oxides contribute to the formation of photochemical smog when they react with unburnt hydrocarbons in the presence of sunlight.

• Nitrogen oxides also contribute to the formation of acid rain. When nitrogen dioxide dissolves in water, it forms nitric acid (HNO₃), which lowers the pH of rainwater: 2NO_2(g) + H₂O_(l) + 1⁄2O_2(g) ➔ 2HNO_3(aq)

Sulfur dioxide:

• Formed by oxidising sulfur impurities in some fossil fuels, as represented by the equation: S_(s) + O_2(g) ➔ SO_2(g)

• Sulfur dioxide dissolves into clouds and oxidises into sulfuric acid, the main component of acid rain. This damages plants, animals and infrastructure.

Catalytic converters reduce emissions

To mitigate the harmful pollutants produced by burning alkanes, catalytic converters have been developed as an effective solution.

How catalytic converters work:

• Installed on vehicle exhaust systems to remove pollutants.
• Contain a honeycomb structure coated with catalyst metals such as platinum, rhodium, and palladium.
• These metals catalyse the conversion of harmful compounds into less harmful substances as exhaust gases pass through.
• They oxidise carbon, carbon monoxide, and unburnt hydrocarbons into carbon dioxide and water, while reducing nitrogen oxides to nitrogen and oxygen gases.

An example reaction in a catalytic converter is:

2CO_(g) + 2NO_(g) ➔ 2CO_2(g) + N_2(g)