3.8 describe the addition of alkenes with bromine, including the decolourising of bromine water as a test for alkenes

Halogens can react with alkenes to form haloalkenes (this does not need UV light, unlike the formation of haloalkanes). For example, bromine ad ethene react together, forming dibromoethane (as it is composed of two bromine atoms and an ethene molecule). This is known as an addition reaction as the carbon-carbon couple bond is split and a halogen atom (in the case, bromine) is added to each carbon.

ethene + bromine ---> dibromoethane

This reaction can also be used to determine whether a substance is an alkene or not. This is because if you add an ethene to bromine, the solution formed (in this case, dibromoethane) will be colourless. If the unknown solution does not contain an alkene, the solution will stay the colour of bromine (yellow-brown).

3.7 draw displayed formulae for alkenes with up to four carbon atoms in a molecule, and name the straight-chain isomers (knowledge of cis- and trans-isomers is not required

This one gets a bit more confusing than alkanes. Alkenes have one carbon-carbon double bond in their carbon chain (this means they are saturated).

alkene	formula	chemical structure	ball-and-stick model
ethene	C2H4
propene	C3H6
butene	C4H8

There are two possibilities for butene as the carbon-carbon double bond can go in two places.

NOTE: Ethene is the first alkene as 'methene' can not exist. This is because alkenes have carbon-carbon double bonds and 'methene' would only have 1 carbon, with no double bond.

3.6 recall that alkenes have the general formula CnH2n

Not much to explain here... alkenes have the general formula CnH2n