Group 7

Question 0

What group 7 element is typically known for going through sublimation?

Answer 0

Iodine.

Question 1

Define sublimation.

Answer 1

When a substance goes directly from a solid to a gaseous form.

Question 2

State and explain the trend in melting and boiling points of group 7 halogens.

Answer 2

As we go down the group, the melting and boiling points decrease. This is because they are larger molecules, so there are more van der Waals forces, so more energy is required to overcome them.

Question 3

What are van der Waals forces?

Answer 3

A temporary positive and negative inducement in molecules which provide a temporary attraction between molecules. This is due to the movement of electrons “sloshing around”. See http://www.chemguide.co.uk/atoms/bonding/vdw.html

Question 4

Define Electronegativity.

Answer 4

Electronegativity is an atom/ion’s ability to attract electron mass, specifically bonding pairs of electrons.

Question 5

Explain the trend in electronegativity as we go down group 7.

Answer 5

It decreases. Fluorine is the most electronegative element. As a halogen atom gets bigger the bonding pairs of electrons get further away from the nucleus so there is weaker attraction. In addition, the shielding also causes there to be a weaker attraction.

Question 6

What effect does electronegativity have on an atom?

Answer 6

It makes it very reactive.

Question 7

Why does HCl become H+ and Cl-?

Answer 7

Cl is quite electronegative so it attracts the bonding pair of electrons, which means that when they ionise we get H+ which makes it acidic.

Question 8

What is oxidation?

Answer 8

The loss of electrons.

Question 9

What is reduction?

Answer 9

The gain of electrons.

Question 10

What is an oxidising agent?

Answer 10

A substance which gains electrons when it reacts (it is reduced) and therefore oxidises another substance.

Question 11

What is a reducing agent?

Answer 11

A reducing agent is a substance that gives electrons and is oxidised.

Question 12

What sort of agents are halogens?

Answer 12

As they become negatively charged, they are given electrons, so they are reduced. Therefore, they are oxidising agents.

Question 13

Give the test for halide ions.

Answer 13

Add nitric acid and silver nitrate. A white precipitate is silver chloride; a creamy precipitate is silver bromide and a yellow precipitate is silver iodide. We can add dilute ammonia solution to it. If it redissolves the precipitate it confirms AgCl. If we add strong ammonia solution and it redissolves, it confirms AgBr. AgI doesn’t redissolve.

Question 15

Which ion can we not test for using silver nitrate?

Answer 15

Fluoride.

Question 16

Give the word and symbol equation of chlorine dissolving in water to make acids. What happens to the Cl when it becomes part of HCl?

Answer 16

Cl2(aq) + H2O(l) ⇌ HCl(aq) + HClO(aq)
Chlorine + Water ⇌ Hydrochloric acid + Hypochlorous acid.
The Cl is reduced to Cl(-1).

Question 17

What sort of reaction is the reaction in which chlorine makes hypochlorous acid?

Answer 17

Disproportionation reaction: chlorine is both oxidised and reduced (oxidised to make HClO, reduced to make HCl).

Question 18

Give another name for hypochlorous acid.

Answer 18

Chloric (I) acid.

Question 19

Give the ionisation of chloric (I)/Hypochlorous acid to make a weak acid.

Answer 19

HClO ⇌ H+ (aq) + ClO- (aq)

Question 20

Give a use for the chlorate (I) ion (or hypochlorate ion).

Answer 20

A powerful disinfectant or bleach.

Question 21

What sort of agent is the chlorate (I) ion?

Answer 21

An oxidising agent.

Question 22

How can we make chlorine water? Give 2 ways.

Answer 22

Bubble chlorine gas through water until the solution is light green or mix equal volumes of 2M HCl and 0.15M sodium chlorate (I) - NaClO.

Question 23

How can we shift the equilibrium of this equation to the right in order to get a better bleach?
Cl2(aq) + H2O(l) ⇌ 2H+ (aq) + Cl-(aq) + ClO-(aq)

Answer 23

Add sodium hydroxide which makes sodium chlorate(I), NaClO, a key ingredient in chlorine-based bleach. Makes:
Cl2(aq) + 2NaOH(aq) ⇌ NaCl(aq) + NaClO(aq) + H2O(l)

Question 24

What happens when we warm sodium chlorate(I) above 75˚C?

Answer 24

The chlorate(I) ions disproportionate forming chloride ions (Cl-) and chlorate(V) ions (ClO₃⁻)
3ClO-(aq) → 2Cl-(aq) + ClO3-(aq)

Question 25

Give a use of sodium chlorate(V).

Answer 25

Weed killer.