c) Group 7 elements - chlorine, bromine and iodine

c) Group 7 elements - chlorine, bromine and iodine 

2.8 recall the colours and physical states of the elements at room temperature. 

Group 7 Element	Colour	Physical state (at room temperature)
Chlorine	Green	Gas
Bromine	Red/brown	Liquid
Iodine	Dark grey	Solid

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

As you travel down Group 7, the reactivity of the elements decreases. The elements will therefore have a darker colour and higher boiling point as you travel down the group, so fluorine will be the most reactive as it is the highest element in the group. It will have the darkest colour and the highest  boiling point. Then, the element furthest down in the group, Astatine, will have the lightest colour and the lowest  boiling point as it has the largest atomic number of all the elements in the group (the shell with the missing electron is furthest from the nucleus, so the pull from the positive nucleus is weaker.) 

2.10 understand the difference between hydrogen chloride gas and hydrochloric acid 

 Hydrogen chloride (HCI) is a gas at room temperature. Hydrochloric acid is hydrogen chloride dissolved in water. When dissolved in water, the HCI molecules split up into H+ ions and CI- ions in a process known as dissociation. The solution that is formed as a result of this is hydrochloric acid. Therefore, hydrogen chloride dissociates in water to form a solution of hydrochloric acid (it is an acid because it has H+ ions.) 

2.11 explain, in terms of disassociation, why hydrogen chloride is acidic in water but not in methyl benzene

Hydrogen chloride is acidic in water because it's molecules split into H+ and CI- ions in water, making it an acidic solution due to the presence of H+ ions. However, hydrogen chloride is not acidic in methyl benzene because it does not disassociate into H+ and CI- ions. With no H+ ions present, the hydrogen chloride is not an acid in methyl benzene. 

2.12 describe the relative reactivites of the elements in group 7

Group 7 elements become less reactive as you go down the group. The higher up Group 7 an element is, the closer the shell with the missing electron is to the nucleus, therefore the stronger the pull from the positive nucleus. The further down the element in Group 7, the further the electron shell with the missing electron and therefore the weaker the pull from the nucleus. 

2.13 describe 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 one in which a more reactive element displaces a less reactive element from a compound. The elements in group 7 take part in these, following the rule that the elements are more reactive the higher up in the group they are. A more reactive halogen will displace a less reactive one that is bonded as a salt, but only if the salt is dissolved in water or a gas.

So 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 the more reactive chlorine will react with the potassium in the potassium chloride to displace the iodide, forming potassium chlorine. 

2.14 understand these displacement reactions as redox reactions

A redox reaction is a reaction in which both reduction (gain of electrons) and oxidation (loss of electrons) happens simultaneously. Displacement reactions between halogens and salt solutions are redox reactions because both oxidation and reduction occurs. 

So, in the reaction between chlorine water and potassium iodide, chlorine is reduced because it gains electrons and iodine is oxidised because it loses electrons. The displacement is therefore a redox reaction.