Alkenes are produced during the cracking of crude oil fractions. Describe the differences between alkanes and alkenes, and explain why alkenes are considered more useful products of cracking than the long-chain alkane molecules they are made from.

Alkanes have the general formula CₙH₂ₙ₊₂ and are saturated hydrocarbons – all C–C bonds are single.
Alkenes have the general formula CₙH₂ₙ and contain at least one C=C double bond – they are unsaturated.
Alkenes are useful because the double bond can be reacted to give a wide range of products; they are the main monomers for polymers such as polyethylene and PVC, so there is a high industrial demand.
Long‑chain alkanes are less useful because they are very viscous and have high boiling points, making them poor fuels and difficult to transport; the smaller alkenes produced by cracking are easier to use as fuels or chemical feedstocks.