2.3 Group 7 (17), the halogens

AS Paper 1 A Level Papers 1 and 3

Explain how the electronegativity changes down group 7.

The electronegativity decreases down group 7.
The atoms get larger, as there is an extra shell.
The electrons are further from the nucleus and there is more shielding, so there is less attraction of the bonding pair of electrons.

Explain how the boiling point of the group 7 elements changes in terms of their structure and bonding.

The boiling point increases down group 7.
The size and relative mass of the molecules increases.
So there are more electrons and stronger van der Waals forces between the molecules.

Give the colours and standard states of the first four halogens.

Fluorine: pale yellow, gas
Chlorine: green, gas
Bromine: red-brown, liquid
Iodine: black, solid

Explain how the oxidising ability of the halogens changes down the group.

The oxidising ability of the halogens decreases down the group because they become less reactive.
The atoms become larger, so the outer shell is further from the nucleus, and there is more shielding, so electrons are less strongly attracted to it.

Under what conditions will displacement reactions of halide ions in aqueous solution occur?

A halogen will displace a halid from the solution if the halide is below it in the periodic table.
This is because the halogen has a higher oxidising ability and is more reactive.

Explain how the reducing ability of the halide ions changes down the group.

The reducing ability of the halide ions increases down the group.
To reduce something, the halide ion must lose an electron.
As the ions get bigger down the group, the electrons are further from the positive nucleus, and there is more shielding.
There is less attraction between the nucleus and outer electron.

Give the chemical equations for:
sodium fluoride and concentrated sulfuric acid.
sodium chloride and concentrated sulfuric acid.

NaF_(s) + H₂SO_4(l) → NaHSO_4(s) + HF_(g)
NaCl_(s) + H₂SO_4(l) → NaHSO_4(s) + HCl_(g)

Describe the observations in the reactions between:
sodium fluoride and concentrated sulfuric acid.
sodium chloride and concentrated sulfuric acid.

In both reactions, white steamy fumes of HF or HCl are evolved.

Why is there only one reaction when sodium fluoride or sodium chloride reacts with concentrated sulfuric acid?

There is only one reaction because the reducing abilities of the fluoride and chloride ions are not strong enough.

What happens when sodium bromide reacts with concentrated sulfuric acid?

When sodium bromide reacts with concentrated sulfuric acid, hydrogen bromide is produced.
The hydrogen bromide then reacts with the sulfuric acid in a redox reaction, where the HBr reduces the SO₄^2- ions.
This is because Br- is a stronger reducing agent than Cl- and F-.

Give the chemical equations for the reactions between sodium bromide and concentrated sulfuric acid.

NaBr_(s) + H₂SO_4(l) → NaHSO_4(s) + HBr_(g)
2HBr_(g) + H₂SO_4(l) → Br_2(g) + SO_2(g) + 2H₂O_(l)

Give the overall equation for the reaction between sodium bromide and concentrated sulfuric acid.

2NaBr_(s) + 2H₂SO_4(l) → Na₂SO_4(s) + Br_2(g) + SO_2(g) + 2H₂O_(l)

Describe the observations in the reaction between sodium bromide and concentrated sulfuric acid.

In the reaction between sodium bromide and concentrated sulfuric acid, white steamy fumes of HBr and orange fumes of bromine are evolved.

What happens when sodium iodide reacts with concentrated sulfuric acid?

When sodium iodide reacts with concentrated sulfuric acid, hydrogen iodide is produced.
The hydrogen iodide then reacts with the sulfuric acid in a redox reaction, where the HI reduces the SO₄^2- ions.
The hydrogen iodine finally reduces the SO₂ to H₂S.

Give the chemical equations for the reactions between sodium iodide and concentrated sulfuric acid.

NaI_(s) + H₂SO_4(l) → Na₂SO_4(s) + HI_(g)
2HI_(g) + H₂SO_4(l) → I_2(g) + SO_2(g) + 2H₂O_(l)
6HI_(g) + H₂SO_4(l) → 3I_2(s) + S_(s) + 4H₂O_(l)
8HI_(g) + H₂SO_4(l) → 4I_2(s) + H₂S_(g) + 4H₂O_(l)

Describe the observations in the reaction between sodium iodide and concentrated sulfuric acid.

In the reaction between sodium iodide and concentrated sulfuric acid, white steamy fumes of hydrogen iodide are evolved. A black solid of iodine is made. A yellow solid of sulfur and a gas with a bad egg smell is made.

What is used to identify and distinguish between halide ions?

Acidified silver nitrate solution is used to identify and distinguish between halide ions.

Give the colours of the precipitate, if any, for each halide ion.

Fluoride: no precipitate
Chloride: white precipitate
Bromide: cream precipitate
Iodide: yellow precipitate

Explain why silver nitrate solution is used to identify halide ions.

Silver nitrate solution is used to identify halide ions because of the distinct precipitates made.

Explain why the silver nitrate solution is acidified when identifying halide ions.

The silver nitrate solution is acidified to prevent the precipitation of ions which would interfere with the test.

How does the solubility of the silver halides in ammonia change down the group?

The solubility of the silver halides in ammonia decreases down the group. AgCl is the most soluble, and AgI is the least soluble.

Explain why ammonia solution is added when identifying halide ions.

Ammonia solution is added when identifying halide ions because the halide ions have distinct solubilities in ammonia.

What happens during the disproportionation reaction of chlorine with water?

The disproportionation reaction of chlorine with water forms chloride ions and chlorate(I) ions.

Give an equation for the disproportionation reaction of chlorine in water.

Cl_2(g) + H₂O_(l) ⇌ 2H⁺_(aq) + Cl^-_(aq) + ClO^-_(aq)

What happens during the decomposition reaction of chlorine with water?

The decomposition reaction of chlorine with water forms chloride ions and oxygen.

Give an equation for the decomposition reaction of chlorine with water.

2Cl_2(g) + 2H₂O_(l) ⇌ 4H⁺_(aq) + 4Cl^-_(aq) + O_2(g)

Give two reasons why chlorine is used in water treatment.

Chlorine is used in water treatment because it kills disease-causing microorganisms. The chlorine that remains in the water prevents reinfection.
It prevents the growth of algae, eliminating bad tastes and smells, and removes discolouration caused by organic compounds.

Give three risks associated with chlorine being used in water treatment.

One risk with chlorine is that chlorine gas is very harmful if it is breathed in as it irritates the respiratory system.
Liquid chlorine causes severe chemical burns on the eyes and skin.
Chlorinated hydrocarbons may form, which are carcinogenic.

Give one reason why the health benefits of water treatment by chlorine outweigh its toxic effects.

One reason why the health benefits of water treatment by chlorine outweigh its toxic effects surrounds the increased cancer risk being smaller than the risk of a cholera epidemic.

What is the main product of the reaction of chlorine with cold, dilute, aqueous NaOH?

The main product of the reaction of chlorine with cold, dilute, aqueous NaOH is sodium chlorate(I) solution.

Write an equation for the reaction of chlorine with cold, dilute, aqueous NaOH.

2NaOH_(aq) + Cl_2(g) → NaClO_(aq) + NaCl_(aq) + H₂O_(l)

What are four uses of sodium chlorate(I) solution?

Sodium chlorate(I) solution (bleach) is used in water treatment, to bleach paper and textiles, and in cleaning toilets.