Which is more reactive, an alkane or an alkene and why?

Alkenes are more reactive due to the presence of a carbon carbon double bond (always important to state what the double bond is between or may not get the marks!). This is because the carbon carbon double bond is a centre for high electron density and so can be attacked by an electrophile (an ion or molecule that attacks regions of high electron density) which will break the bond. The carbon carbon double bond is made up of a pi bond and a sigma bond. The pi bond is weak and can be relatively easily broken meaning new single bonds can be formed.