Higher and foundation tier
There is a family of hydrocarbons called the alkenes, they are much more reactive than the alkanes. Alkenes are unsaturated hydrocarbons, this means that they contain carbon atoms which have a double covalent bond between them e.g The diagram below shows the first three alkenes.
If you study the image you will notice that all the alkene have one C=C bond present. This double covalent bond is present in all alkenes and is responsible for their characteristic reactions. The first 3 alkenes are shown below:
The alkenes form a new homologous series of hydrocarbons. They are unsaturated, meaning they contain C=C, carbon carbon double covalent bonds. The number of hydrogens in an alkene is simply double the number of carbon atoms. This gives the general formula CnH2n. Like alkanes they combust to form carbon dioxide and water vapour but they burn with a more sooty and dirty flame than the alkanes. They will also release less energy than alkanes when they burn. However alkenes are mentioned are much more reactive than alkanes due to the presence of the C=C, this "functional group" (reactive part of a molecule which gives the molecule its characteristic properties) makes the alkene molecule react readily with other substances.