4B: The elements of Group 7 (halogens)

Question 1

State and explain the trend in melting and boiling temperatures of Group 7 elements.

Answer 1

Down the group,
there are more electrons,
so London forces are stronger,
so melting and boiling points increase.

Question 2

What is the physical state of chlorine at room temperature?

Answer 2

Green gas

Question 3

What is the physical state of bromine at room temperature?

Answer 3

Red-brown liquid

Question 4

What is the physical state of iodine at room temperature?

Answer 4

Grey-black solid

Question 5

State and explain the trend in electronegativity for Group 7 elements.

Answer 5

Down the group,
same nuclear charge,
atomic radius and shielding increases,
so the attraction between the nucleus and the shared electron pair weakens,
so electronegativity decreases.

Question 6

State and explain the trend in reactivity of Group 7 elements.

Answer 6

Down the group,
same nuclear charge,
atomic radius and shielding increases,
so the attraction between the nucleus and the outer electrons weakens,
so oxidising strength decreases

Question 7

What colour is chlorine in aqueous and organic solutions?

Answer 7

aq: very pale green
org: very pale green

Question 8

What colour is bromine in aqueous and organic solutions?

Answer 8

aq: yellow-orange
org: red

Question 9

What colour is iodine in aqueous and organic solutions?

Answer 9

aq: brown
org: purple

Question 10

Explain why chlorine is used in water purification.

Answer 10

The reaction between chlorine and water produces chloric acid.
Dissolved chlorine and chloric acid can oxidise harmful bacteria.

Question 11

What are the products when chlorine disproportionates in cold, dilute aqueous NaOH?

Answer 11

NaCl + NaClO (Bleach) + H₂O

Question 12

What are the products when chlorine disproportionates in hot, aqueous NaOH?

Answer 12

NaCl + NaClO₃ + H₂O

Question 13

Explain how the reactions of solid Group 1 halides with concentrated sulfuric acid illustrate the difference in reducing ability of hydrogen chloride compared to other hydrogen halides.

Answer 13

First reactions produce hydrogen halides.
The hydrogen halides react again with the sulfuric acid only with bromide and iodide
- not chloride; reducing strength is too low.

Question 14

Explain how the reactions of solid Group 1 halides with concentrated sulfuric acid illustrate the difference in reducing ability of hydrogen bromide and hydrogen iodide.

Answer 14

HBr produces SO₂
HI produces H₂S

The sulfur in H₂S is reduced more than in SO₂, showing that hydrogen iodide has more reducing strength.

Question 15

Describe the test for halides.

Answer 15

Add nitric acid (reduces false positives from carbonate ions)
Add silver nitrate - observe colour of precipitate.
Add dilute then concentrated ammonia - observe whether the precipitate redissolves

Question 16

Describe the observations from the halide test if chloride ions are present.

Answer 16

White precipitate.
Redissolves in dilute and concentrated ammonia.

Question 17

Describe the observations from the halide test if bromide ions are present.

Answer 17

Cream precipitate.
Redissolves only in concentrated ammonia.

Question 18

Describe the observations from the halide test if iodide ions are present.

Answer 18

Yellow precipitate.
Does not redissolve in ammonia solution.

Question 19

Describe the observation when hydrogen halides react with ammonia.

Answer 19

Dense white solid / smoke
(Ammonium halide formed)

Question 20

Describe the observation when hydrogen halides react with water.

Answer 20

Steamy white fumes
- turns damp blue litmus red
(acid)

Question 21

Predict the state and colour of fluorine and astatine at room temperature.

Answer 21

Fluorine: Pale yellow gas

Astatine: Dark black solid