THE HALOGENS

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

Halogen:	Colour:	State at Room Temperature:
Fluorine	Pale yellow	Gas
Chlorine	Pale green	Gas
Bromine	Dark red	Liquid
Iodine	Dark grey	Solid
Astatine	Dark coloured	Solid

2.11    understand the difference between hydrogen chloride and hydrochloric acid

Both hydrogen chloride and hydrochloric acid have the formula HCl. Hydrogen chloride is a gas and hydrochloric acid is a solution in water. When hydrogen chloride dissolves 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 dissolves in water it dissociates (splits up). When hydrogen chloride dissolves in methylbenzene it only dissolves (it does not split up into H⁺ ions) so it is not acidic.

2.13    recall the relative reactivities of the elements within Group 7

The halogens become less reactive as you go down the group, this means that the oxidising ability of the elements falls as you go down the group. (The halogens are good oxidising agents. This means they take electrons away. If it takes electrons away from something it must gain them). So when a halogen oxidises something it removes electrons from it.

Oxidation
IsLossReductionIsGain

(of electrons)

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

An example experiment to show how a reactive halogen will replace a less reactive halogen is adding chlorine to potassium iodide – the chlorine will displace the iodide from its salt forming potassium chloride and iodine.

e.g. Cl₂ + 2KI  è 2KCl + I₂   

(Group 7 elements are diatomic - it must be 2KI so when the iodine is displaced it forms I₂.)

2.15    understand these displacement reactions as redox reactions

Redox reactions are reactions were one thing is oxidised and the other reduced (think O.I.L.R.I.G). The more reactive halogen will remove the electrons and be reduced (gain electrons). The less reactive halogen will be oxidised (lose electrons).

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

As atoms get bigger down groups their intermolecular forces grow stronger. In group 7’s case this means the halogens will go from gas at room temperature to a solid at room temperature (see 2.9). As group 7 elements get darker down the group fluorine must be the lightest and astatine the darkest.

2.39i   recall simple tests for the anions Cl^-, Br^- and I^- using dilute nitric acid and silver nitrate solution

To test for Cl^-, Br^- and I^- anions add nitric acid followed by silver nitrate solution.

If it is Cl^- a white precipitate form (insoluble AgCl).

If it is Br^- a cream precipitate will form  (insoluble AgBr).

If it is I^- a yellow precipitate will form (insoluble AgI).

2.40    describe a simple test for chlorine gas

Chlorine gas will put out a lighted splint.

Chlorine gas will turn moist universal indicator paper red.