Alkanes
Alkanes are a homologous series of saturated hydrocarbons. The first five are methane, ethane, propane, butane and pentane.
The general formula for alkanes is: CnH2n + 2
For example, ethane:
Ethane has two carbons, so n=2.
The formula of ethane must be C2H2(2) + 2 = C2H6.
Complete Combustion of Alkanes
If there is enough oxygen, alkanes will burn in oxygen completely to give carbon dioxide and water. The general equation for combustion:
Hydrocarbon + Oxygen à Carbon dioxide + Water
The combustion of methane would be: CH4(g) + 202(g) -> C02(g) + 2H20(l)
Note: Balancing combustion equations can be annoying. An easy way would be to balance them in the order of carbon, hydrogen then oxygen.
Incomplete Combustion
If there isn’t enough oxygen, you get incomplete combustion, in which carbon monoxide and water are produced instead. Carbon monoxide is a colourless, odourless and poisonous gas. It is dangerous because it can combine to our haemoglobin and stop it from carrying oxygen. As a result, you get ill or even die because oxygen cannot travel to all parts of your body.
Reaction with Bromine
Alkanes react with bromine under the presence of ultra-violet light. One hydrogen from the hydrocarbon would be replaced by a bromine atom. This is known as a substitution reaction. Bromine can be used as an indicator for alkanes and alkenes without UV light. Adding bromine water to alkanes produces no colour change. Reacting bromine water to alkenes make it turn from brown to colourless.
However, if the mixture of bromine and methane is reacted under UV light, it loses its colour, a mixture of bromomethane and hydrogen bromide gases is formed.
CH4(g) + Br2(g) -> CH3Br (g) + HBr(g)