A-Level Chemistry - Group 7: The Halogens (AQA)

Describe and explain the trend in electronegativity down the group

It decreases down the group.

The size of the molecule increases down the group so the bonding electrons are further away from the positive nucleus and there is more shielding.

Describe and explain the trend in boiling point down the group

It increases down the group.

As the size of the molecule increases, so do the Van der Waals' forces and there are more electrons attracted to the positive nucleus. More energy is needed to overcome these forces.

Describe and explain the trend in oxidising ability down the group.

It decreases down the group.

Oxidising agents gain electrons.

As they are less reactive down the group because the atoms become larger and there is more shielding (so there is less attraction between the nucleus and the outer electrons).

Therefore down the group, it's harder for them to gain electrons.

What ions will chlorine displace in a reaction?

Bromide - Cl2 + 2Br- --> 2Cl- +Br2 (orange solution)

Iodide - Cl2 + 2I- --> 2Cl- + I2 (brown solution)

What ions will bromine displace in a reaction?

Iodide - Br2 + 2I- --> 2Br- + I2 (brown solution)

What ions will iodine displace in a reaction?

None

Describe and explain the trend in reducing ability down the group.

It increases down the group.

The halide ions are reducing agents as they will lose electrons to become halogen molecules.

Down the group, the ions get bigger, so the electrons are further from the positive nucleus and there's greater shielding.

What is the equation for chlorine and fluorine attempting to reduce sulfuric acid?

NaF + H2SO4 --> NaHSO4 + HF

NaCl + H2SO4 --> NaHSO4 + HCl

The oxidation states remain the same.

What are the equations for bromine reducing sulfuric acid?

NaBr + H2SO4 --> NaHSO4 + HBr

2HBr + H2SO4 --> Br2 + SO2 + 2H2O

Sulfur is reduced from +6 to +4

Bromine is oxidised from -1 to 0

What are the equations for iodine reducing sulfuric acid?

NaI + H2SO4 --> NaHSO4 + HI

2HI + H2SO4 --> I2 + SO2 + 2H2O

6HI + SO2 --> H2S + 3I2 + 2H2O

In the second equation:

Sulfur is reduced from +6 to +4

Iodine is oxidised from -1 to 0

In the third equation:

Sulfur is reduced from +4 to -2

Iodine is oxidised from -1 to 0

What is the test for a halide?

Add dilute nitric acid to remove other ions.

Add silver nitrate solution which forms a silver halide precipitate

Cl- = White precipitate

Br- = Cream precipitate

I- = Yellow precipitate

What is the trend of solubility of silver halides in ammonia?

It decreases down the group.

Cl- = Dissolves in dilute NH3

Br- = Dissolves in concentrated NH3

I- = Dissolves in concentrated NH3

What is the equation for the reaction of chlorine with cold water?

Cl2 + H2O --> ClO- + Cl- + 2H+

It is a disproportionation recation as the chlorine is both reduced from 0 to -1 and oxidised from 0 to +1

What is the equation for the reaction of chlorine and water in the presence of UV light?

2Cl2 + 2H2O --> 4HCL + O2

What is are the advantages of adding chlorine to drinking water?

- It kills disease-causing microorganisms

- Some chlorine persists in the water to prevent reinfection further down the supply.

- It prevents the growth of algae, removes bad tastes and smells, and removes discolouration from organic compounds.

What is are the disadvantages of adding chlorine to drinking water?

- Chlorine gas is harmful as it irritates the respiratory system.

- Chlorine in a liquid form causes chemical burns.

- Water contains many organic compunds which could react with chlorine to form chlorinated hydorcarbons which are carcinogenic.

Why is chlorine used in drinking water despite the disadvantages?

The benefits outweight the risks.

What is the equation for the reaction of chlorine with sodium hydroxide?

2NaOH + Cl2 --> NaClO + NaCl + H2O

It is a disproportionation reaction as chlorine is both reduced from 0 to -1 and oxidised from 0 to 1+

What are the products used for from the reaction between Cl2 and NaOH?

NaClO is a key ingredient in the production of bleach which has a number of uses.