Combustion of Alkanes

This lesson covers: 

1. Reactivity of alkanes
2. Complete and incomplete combustion reactions of alkanes
3. Environmental impacts of burning alkanes
4. Methods to reduce emissions from burning alkanes

Alkanes are generally unreactive

Alkanes are relatively unreactive due to their non-polar nature and their strong covalent bonds. The electronegativity difference between carbon and hydrogen in alkanes is minimal, leading to an almost equal sharing of electrons and no significant partial charges. This makes it difficult for alkanes to attract nucleophiles or electrophiles.

However, alkanes do undergo certain reactions, including:

• Combustion - The reaction of alkanes with oxygen to release energy.
• Substitution - The replacing of H atoms with halogens in the presence of light.

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 need to be turned into gas (vaporised) before they can combust.
• 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:

• Created during incomplete combustion as soot, which consists 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.

Methods to reduce emissions

To mitigate the harmful pollutants produced by burning alkanes, technologies like catalytic converters and flue gas desulfurisation have been developed.

Catalytic converters

• 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)

Flue gas desulfurisation

• Used in power stations to remove sulfur dioxide from flue gases.
• Flue gases are mixed with an alkaline slurry containing calcium oxide or calcium carbonate.
• The acidic sulfur dioxide reacts with the calcium compounds to form solid calcium sulfite salts (CaCO₃), as represented by the equations:

SO_2(g) + CaO_(s) + 2H₂O_(l) ➔ CaSO_3(s) + 2H₂O_(l)

SO_2(g) + CaCO_3(s) + 2H₂O_(l) ➔ CaSO_3(s) + 2H₂O_(l) + CO_2(g)

• This harmless calcium sulfite salt is then removed from the flue gas stream before emission.
• This process prevents the emission of sulfur dioxide, thus reducing acid rain formation.