1.3.3 - Group 7

Question 1

Small amounts of chlorine are added to water makes it safer to drink.

• Write an equation for reaction between water and chlorine.
• Use oxidation numbers to show where the chlorine is oxidised and where the chlorine is reduced.

Answer 1

• Cl₂ (aq) + H₂O (l) → HCl (aq) + HClO (aq)
• 0 -1 +1
• The chlorine atom in the hydrochloric acid is oxidised.
• The chlorine atom in chloric(I) acid is reduced.

Question 2

What is the difference between a halogen and a halide?

Answer 2

• A halogen - Elements of Group 7 that exist in a diatomic form eg. Br₂.
• A halide - Singular ions from Group 7 that have a negative charge eg. Cl^-.

Question 2

Silver nitrate is added to a dissolved, unknown halide. A precipitate is formed however the colour is hard to define. Describe a test that could indicate what the halide ion is present.

Answer 2

• The precipitate should be added to dilute and concentrated ammonia.
• Difference halide precipitates have different solubilities in ammonia:
• AqCl is soluable in dilute NH_3.
• AqBr is soluable in concentrated NH_3.
• AgI is insoluble in concentrated NH_3.

Question 3

Chlorine and dilute aqueous sodium hydroxide react together to make bleach at room temperature.

1. Write an equation to show this reaction.
2. Use oxidation numbers to show what is being oxidised and what is being reduced.
3. What type of reaction is this?

Answer 3

1. Cl2 (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq)
2. 0 -1 +1
3. This is a disproportionation reaction.

Question 4

Define the term ‘disproportionation’.

Answer 4

Disproportionation - is the oxidation and reduction of the same element in a redox reaction.

Question 5

Describe a test that allows you to distinguish between NaCl, NaBr and NaI.

Answer 5

• The unknown halide must be dissolved in water.
• An aqueous solution of silver nitrate (AgNO₃) is added.
• The silver ions react with the halide ions to form a silver halide precipitate, AgX (s).
• If it is a white precipitate - the halide is chlorine.
• If it is a cream precipitate - the halide is bromine.
• If it is a yellow precipitate - the halide is iodine.
• This reaction is a precipitation reaction.

Question 7

1. Explain why chlorine oxidises both Br^- and I^- ions but Iodine does not oxidise Cl^- ions.
2. Write two equations that show these reactions.

Answer 7

1. Chlorine is more reactive so it will oxidise and displace Br^- and I^- ions.
2. Cl₂ (aq) + 2Br^- (aq) → 2Cl^- (aq) + Br₂ (aq)
3. Cl₂ (aq) + I₂^- (aq) → 2Cl^- (aq) + I^₂ (aq)

Question 8

List some of the dangers of using chlorine to treat drinking water.

Answer 8

• Chlorine is toxic.
• Chlorine can react with organic matter in the water to form traces of chlorinated hydrocarbons.
• This may cause cancer.

Question 9

Explain the trend seen in the boiling points of Cl₂, Br₂ and I₂ (Group 7 elements).

Answer 9

• As you down the Group 7 elements, the boiling point increases.
• This is because the number of electrons are increasing so the van der Waals’ forces are stronger between the molecules as you go down the group.
• This means more energy is required to break these forces apart.

Question 10

Describe and explain the trend of reactivity in Group 7.

Answer 10

• Describe - As you go down the group, the reactivity decreases.
• Explain - This is because their oxidising power (the power to attract and capture an electron to form a halide ion) decreases.
• This happens because:
• the atomic radius increases.
• the electron shielding increases.
• therefore the nuclear attraction decreases.
• …so the ability to gain an electron into the p sub-shell decreases.