3 - Halide ions

Question 1

State the four halides

Answer 1

Fluoride, chloride, bromide, iodide

Question 2

Are halides oxidising or reducing agents?

Answer 2

Reducing agents as they lose electrons

Question 3

What is the trend in reducing ability of the halide ions down group 7?

Answer 3

Reducing ability increases

Question 4

Why does reducing ability of halide ions down group 7 increase?

Answer 4

The electrostatic forces of attraction between outer electrons and the nucleus are weaker so they are easier to lose

Question 5

Reaction between solid sodium fluoride and concentrated sulfuric acid

Answer 5

NaF _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HF _(g)

Question 6

What is observed when solid sodium fluoride reacts with concentrated sulfuric acid?

Answer 6

White misty fumes of HF are given off

Question 7

Reaction between solid sodium chloride and concentrated sulfuric acid

Answer 7

NaCl _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HCl _(g)

Question 8

What is observed when solid sodium fluoride reacts with concentrated sulfuric acid?

Answer 8

White misty fumes of HCl are given off

Question 9

What kind of reaction occurs between fluoride / chloride and sulfuric acid?

Answer 9

Acid-base

Question 10

Why is there only an acid-base reaction and no further redox reaction when fluoride and chloride react with sulfuric acid?

Answer 10

Fluoride and chloride both have low reducing abilities

Question 11

Reaction between solid sodium bromide and concentrated sulfuric acid

Answer 11

Misty fumes of HBr gas are produced initially:
- NaBr _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HBr _(g)

Bromide ions then reduce H₂SO₄ to SO₂, orange bromine vapour and choking SO₂ gas are observed:
- 2HBr _(g) + H₂SO_{4 (aq)} → Br_{2 g} + SO_{2 (g)} + 2H₂O _(l)

Sulfur is reduced as its oxidation number decreases from +6 in H₂SO₄ to +4 in SO₂
Bromine is oxidised as its oxidation number increases from -1 in HBr to 0 in Br₂

Question 12

What were the oxidation number changes in the sulfur during the reaction between bromide and sulfuric acid?

Answer 12

+6 (H₂SO₄) to +4 (SO₂)

Question 13

What were the oxidation number changes in the bromide during the reaction between bromide and sulfuric acid?

Answer 13

-1 (HBr) to 0 (Br₂)

Question 14

Reaction between solid sodium iodide and concentrated sulfuric acid

Answer 14

Misty fumes of HI gas are produced initially:
- NaI _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HI _(g)

Iodide ions then reduce H₂SO₄ to SO₂, violet iodine vapour and choking SO₂ gas are observed:
- 2HI _(g) + H₂SO_{4 (aq)} → I_{2 g} + SO_{2 (g)} + 2H₂O _(l)

Iodide ions then reduce SO₂ further to prodce H₂S gas, violet iodine vapour and the rotten egg smell of H₂S gas are observed:
- 6HI _(g) + SO_{2 (g)} → H₂S _(g) + 3I_{2 (s)} + 2H₂O _(l)

Sulfur is reduced as its oxidation number decreases from +4 in SO₂ to -2 in H₂S
Iodide is oxidised as its oxidation number

Question 15

What were the oxidation number changes in the sulfur during the reaction between iodide and sulfuric acid?

Answer 15

2nd reaction: +6 (H₂SO₄) to +4 (SO₂)

3rd reaction: +4 (SO₂) to -2 (H₂S)

Question 16

What were the oxidation number changes in the iodide- during the reaction between iodide and sulfuric acid?

Answer 16

-1 (HI) to 0 (I₂)

Question 17

What solution can be used to test for the presence of halide ions?

Answer 17

Aqueous silver nitrate (AgNO₃)

Question 18

Describe how to test for halide ions in 3 steps

Answer 18

1) Add dilute nitric acid to the sample to remove any ions like carbonates that may form precipitates with silver nitrate
2) Add the silver nitrate solution where silver ions react with halide ions to form silver halide precipitates
3) Observe the precipitate colour to identify the halide

Question 19

When testing for halide ions, what colour precipitate appears if chloride is present?

Answer 19

White

Question 20

When testing for halide ions, what colour precipitate appears if bromide is present?

Answer 20

Cream

Question 21

When testing for halide ions, what colour precipitate appears if iodide is present?

Answer 21

Pale yellow

Question 22

Some silver halides have similar colours. To confirm the identity of the halide present we can add excess…

Answer 22

ammonia solution

Question 23

What is the effect of adding dilute ammonia solution to silver chloride?

Answer 23

Silver chloride dissolves in dilute ammonia to form a complex ion
AgCl _(s) + 2NH_{3 (aq)} → [Ag(NH₃)₂]⁺ _(aq) + Cl^- _(aq)

Question 24

What is the effect of adding concentrated ammonia solution to silver bromide?

Answer 24

Silver bromide dissolves in concentrated ammonia to form a complex ion

AgBr _(s) + 2NH_{3 (aq)} → [Ag(NH₃)₂]⁺ _(aq) + Br^- _(aq)

Does not dissolve in dilute ammonia

Question 25

What is the effect of adding concentrated ammonia solution to silver iodide?

Answer 25

Silver iodide does not react with ammonia as it is too insoluble