Alkanes are used as fuels because they release a large amount of energy when they undergo combustion.

They also store a large amount of energy for a small amount of weight.

Combustion is the reaction between a fuel and oxygen, resulting in the release of energy as heat.

In complete combustion, there is a plentiful supply of oxygen so the fuel reacts completely to form carbon dioxide and water vapour.

In incomplete combustion, there is an insufficient supply of oxygen so the fuel doesn't react completely, producing carbon monoxide and water vapour

Incomplete combustion can also produce solid carbon (soot).

Incomplete combustion occurs when there is insufficient oxygen for the fuel to react completely.

This happens with longer chain alkanes which need more oxygen compared to shorter chains.

Hydrocarbon + oxygen ➔ carbon dioxide + water

The symbol equation for the complete combustion of propane is: C₃H₈ + 5O₂ ➔ 3CO₂ + 4H₂O.

Carbon monoxide, water vapour, carbon and unburnt hydrocarbons can be formed.

Carbon monoxide is poisonous and can prevent oxygen from binding to hemoglobin in the blood.

Some unburnt hydrocarbons are carcinogens and some can contribute to the formation of photochemical smog.

Oxides of nitrogen are produced when nitrogen and oxygen in the air combine at the high temperatures and pressures present in internal combustion engines.

In the atmosphere, nitrogen oxides can react to form nitric acid which contributes to acid rain. They are also a cause of photochemical smog.

A catalytic converter is a device fitted to a vehicle's exhaust system which uses transition metal catalysts (such as platinumum, palladium or rhodium) to convert polluting gases into less harmful products.

Sulfur dioxide pollution originates from sulfur-containing impurities in fuels like coal and crude oil. When burned, these impurities are oxidised to form sulfur dioxide.

In the atmosphere, sulfur dioxide reacts with oxygen and water to form sulfuric acid, contributing to acid rain.

Sulfur dioxide (SO₂), an acidic gas, can be removed from flue gases by reacting it with an alkali. A slurry of calcium oxide (CaO) or calcium carbonate (CaCO₃) is sprayed onto the flue gases, neutralising the SO₂ and producing a solid waste product, calcium sulfite (CaSO₃):

The respective equations for the neutralisation of SO₂ by CaO and CaCO₃ are:

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

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

Carbon dioxide is a greenhouse gas which absorbs infrared radiation (heat) in the atmosphere and emits it backs towards Earth in a process known as the greenhouse effect. This causes the Earth's temperature to rise, which is the process we call global warming. 

Carbon monoxide + nitrogen monoxide ➔ nitrogen + carbon dioxide

The symbol equation is: 2CO + 2NO ➔ N₂ + 2CO₂.

The low reactivity of alkanes is due to:

1. The strong C-C and C-H covalent bonds require a lot of energy to break.
2. The non-polar nature of C-C bonds make it difficult for alkanes to attract nucleophiles or electrophiles.