Group 7 elements- chlorine, bromine and iodine

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

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

Halogen	Colour	State at Room Temperature
F2	Yellow	Gas
Cl2	Green	Gas
Br2	Red-brown	Liquid
I2	Grey	Solid
At2	Dark coloured	Solid

The group 7 elements get darker down the group, so we can deduce that astatine is dark coloured, and is a solid too. As atoms get bigger down groups, their intermolecular forces grow stronger, so astatine can only be a solid. 

2.11 understand the difference between hydrogen chloride gas and hydrochloric acid

Both hydrogen chloride and hydrochloric acid have the formula HCl. Hydrogen chloride is a gas, and hydrochloric acid is its solution in water. When hydrogen chloride is dissolved in water, it forms H⁺ ions which makes it acidic. 

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

When hydrogen chloride is dissolved in water, it dissociates (basically, just splits up) to form H⁺ ions which are responsible for its acidic properties. But when hydrogen chloride is dissolved in methylbenzene, that's all that happens. It dissolves. It doesn't dissociate, so it doesn't form ions. This means that it just exists as HCl molecules, not H⁺ and Cl^- ions. So it is not acidic. 

2.13 recall the relative reactivities of the elements in Group 7

The halogens become less reactive as you go down the group, this means that its oxidising ability falls as you go down the group. (The halogens are good oxidising agents, this means it takes electrons away. If it takes electrons away from smth else, it means it itself gains electrons. Try to get your head around that. OILRIG-oxidation is loss, reduction is gain.)

So basically, when a halogen oxidises something, it does so by removing electrons from it. 

X₂ + 2e^- à 2X^- (halide ion) They gain an electron to have full outer shells, but that means they have a negative 1 charge. 

Each halogen has the ability to oxidise the ions of those underneath it in the Group, but not those above it. Chlorine can remove electrons from bromide or iodide ions, and bromine can remove electrons from iodide ions. 

Chlorine is a strong oxidising agent because its atoms readily attract an extra electron to make chloride ions. Bromine is less successful at attracting electrons, and iodine even less successful. Why? This is because the 'incoming electron' would be further away from the nucleus as you go down the group, as the atoms get larger. As there are more electron shells, the 'incoming electron' is further away, and so it doesn't feel the nucleus attraction as much-so it is less strongly attracted. So the ion is less readily formed.

I hope that makes sense. :S 

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

e.g. if you add chlorine to potassium bromide solution, chlorine would displace the bromide from its salt. 

Cl₂ + 2KI à 2KCl + I₂

Remember the Group 7 elements are diatomic, so it must be 2KI so that when iodine is displaced, it forms I_2. 

Remember: Each halogen has the ability to oxidise the ions of those underneath it in the Group, but not those above it. 

2.15 understand these displacement reactions as redox reactions

Redox reactions are basically reactions where one species is being oxidised and one is being reduced. So the more reactive halogen will remove the electrons, so in the above reaction, Chlorine oxidises iodine and gains an electron each (chlorine is diatomic) and so the iodide ions become iodine atoms again.

When I say chlorine oxidises iodine, it means that iodine is oxidised as it LOSES an electron (OILRIG!), but chlorine is reduced, as it gains an electron. Potassium forms K⁺ ions and chlorine forms Cl^- ions, so they can form KCl. 

It's all a bit confusing sometimes, but always refer to OILRIG. Even though chlorine may be reduced, it's called an oxidising agent because it oxidises other stuff--taking electrons away from them.