Halogens (Group 7)

Question 1

What is chlorine widely used for?

Answer 1

Treating water, making it fit for drinking.

Question 2

What do the halogens exist as in nature?

Answer 2

Diatomic molecules

Question 3

Describe the trend in boiling point down group 7.

Answer 3

As you go down group 7, the size of molecule increases.
London forces increase, as there are more electrons.
Need more energy to break

Question 4

How can chlorine be used to make bleach?

Answer 4

Bleach is made by reacting chlorine with sodium hydroxide to make sodium chlorate (I) solution.

Cl2 + 2NaOH -> NaCl + NaOCl

This is a disproportionation reaction, as the same element (chlorine) is both oxidised (Cl2 -> NaOCl) and reduced (NaCl)

Question 5

Describe the advantages and disadvantages of using chlorine in water treatment plants

Answer 5

+ - the chlorine kills bacteria in the water.

• • chlorine is toxic, so care has to be taken in the plant.
• • chlorine can react with organic matter to form chlorinated hydrocarbons which may be carcinogenic.

Question 6

Write an equation for the equilibrium when a chlorine molecule reacts with a water molecule to form chloric (I) acid, HClO

Answer 6

Cl2 + H2O ⇌ HClO + HCl

Question 7

What happens when the halogens form an ionic compound?

Answer 7

Halogen atoms each gain one electron when forming an ionic compound.

Question 8

Describe the trend in reactivity as you go down group 7

Answer 8

Reactivity reduces down group 7.
Further down the group, less energy is released when the halide ion is formed.

Question 9

Explain the trend in reactivity as you go down group 7

Answer 9

The reactivity reduces because, further down the group:
- the added electron is further from the nucleus
- the added electron is shielded more by electrons in inner shells
- less energy is released by forming the ion.

Question 10

What is the rule of displacement reactions of halogens with halides?

Answer 10

A more reactive halogen will displace a less reactive halogen from its salt.

Question 11

Result of Cl2 added to Cl- (aq)

Answer 11

No change

Question 12

Result of Br2 added to Cl- (aq)

Answer 12

No reaction - chlorine more reactive

Question 13

Result of I2 added to Cl- (aq)

Answer 13

No reaction - chlorine more reactive

Question 14

Result of Cl2 added to Br- (aq)

Answer 14

Solution turns orange

Question 15

Result of Br2 added to Br- (aq)

Answer 15

No change

Question 16

Result of I2 added to Br- (aq)

Answer 16

No reaction - bromine more reactive

Question 17

Result of Cl2 added to I- (aq)

Answer 17

Solution turns brown

Question 18

Result of Br2 added to I- (aq)

Answer 18

Solution turns brown

Question 19

Describe the displacement reactions of the halogens

Answer 19

Question 20

Going down the group, oxidising ability…

Answer 20

decreases

Question 21

Why do flouride ions and chloride ions not do redox reactions with H2SO4?

Answer 21

They are not strong enough reducing agents to reduce S in H2SO4.

Question 22

Redox reaction between NaBr and H2SO4

Answer 22

2Br- + H2SO4 + 2H+ -> SO2 + 2H2O + Br2

In this reaction, the oxidation state of
sulfur is reduced from +6 to +4.

Question 23

Which reactions take place between bromide ions and concentrated sulfuric acid, and what would we observe?

Answer 23

Acid-base reactions and redox reactions.
We would observe brown vapour.

Question 24

When concentrated sulfuric acid is added to sodium iodide salt, we can make several observations.

State the chemical formula of the substances which is observed as:
- steamy fumes
- black solid
- smell of rotten eggs

Answer 24

steamy fumes = HI (g)
black solid = I2 (s)
rotten eggs = H2S (G)

Question 25

When concentrated sulfuric acid is added to sodium iodide salt, what type of reactions take place?

Answer 25

• Acid-base reaction
• Redox reaction

Question 26

What is the name given to a halogen that has gained an electron?

Answer 26

Halide

Question 27

What happens to reducing power down group 7?

Answer 27

Down the group of halides, reducing power increases.
This trend is explained by the increasing number of electron shells.

This causes the distance between the outer electrons and the nucleus to increase, so that attraction decreases.

Therefore, the electron is more easily lost and donated to another atom which reduces that atom.

Question 28

Why do we acidify the aqueous solution of silver nitrate?

Answer 28

Soluble carbonates and hydroxides would react with silver nitrate to form insoluble precipitates. This would invalidate using silver nitrate as a test to identify halide ions.

In order to prevent these precipitates forming, we acidify the aqueous solution using dilute
nitric acid.

This reacts with hydroxides to form
water.

It also reacts with carbonates to form
water and carbon dioxide.