2.15 understand these displacement reaction as redox reactions

A loss of electrons is known as oxidation, and a gain of electrons is known as reduction. Displacement reactions involving halogens involve a transfer of electrons, and reduction and oxidation happen simultaneously. For example, in the example here, chorine is reduced and iodine is oxidised.

'redox' is short for 'reduction and oxidation' and, as displacement reactions involving halogens involve the transfer of electrons, they are redox reactions.

NOTE: a good way to remember which is oxidation and which is reduction with the anagram 'O.I.L.R.I.G' (Oxidation.Is.Loss.Reduction.Is.Gain)

2.14 describe the experiments to demonstrate that a more reactive halogen will displace a less reactive halogen from a solution of one of its salts

A displacement reaction is basically a reaction where the more reactive element displaces (pushes out) a less reactive element from a compound. For example...

Chlorine is more reactive than iodine (as it is higher up in group 7). Therefore, if you add chlorine water to potassium iodide solution, the chloride will react with the potassium to form potassium chloride (basically, it displaces the iodine). The iodine is displaced from the salt (potassium iodide) and just kind of gets left in the water solution (this turns the solution brown).

2.13 describe the relative reactivities of the elements in group 7

The elements of group 7 get less reactive as you go down. This is because, at the top of the group, the incomplete shell is near to the nucleus, so the pull from the positive nucleus is greater. Therefore, as you go down the pull from the nucleus gets weaker, so it is easier to 'get rid' of electrons.

2.12 explain, in terms of dissociation, why hydrogen chloride is acidic in water but not in methylbenzene

If HCL is dissolved in water, it dissociates (splits up) into H+ and Cl- ions. This solution (hydrochloric acid) is acidic because it contains H+ ions.

However, if HCL is dissolved in methylbenzene, it doesn't dissociate into H+ and CL- ions, therefore no H+ ions are present so the solution is not acidic.

2.11 understand the difference between hydrogen chlorine gas and hydrochloric acid

When hydrogen chloride gas (gas at room temperature) is dissolved in water the hydrogen chloride molecules split (into H+ ions and Cl- ions). This process is known as dissociation. The solution that is formed is hydrochloric acid, which is acidic as it contains H+ ions.

2.10 make predictions about the properties of other halogens in this group

The further down the group you go, the more reactive the elements become, the higher the boiling point, the darker the element will be and the 'more' solid (as it will have a higher boiling point).

2.9 recall the colours and physical states of the elements at room temperature

Chlorine - green - gas
Bromine - red-brown - liquid
Iodine - dark grey - solid