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

Question 3

Why are halides reducing agents?

Answer 3

They donate an electron

Question 4

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

Answer 4

Reducing ability increases

Question 5

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

Answer 5

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

Question 6

Which halide is the strongest reducing agent?

Answer 6

Iodide

Question 7

Give an equation for the reaction between solid sodium fluoride / chloride and concentrated sulfuric acid where HF / HCl is produced
State an observation of this reaction

Answer 7

NaF _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HF _(g)
NaCl _(s) + H₂SO_{4 (aq)} → NaHSO_{4 (aq)} + HCl _(g)
- White misty fumes of HF or HCl are produced

Question 8

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

Answer 8

Acid-base

Question 9

What is the role of the chloride ions when reacted with concentrated sulfuric acid?

Answer 9

They are a proton acceptor

Question 10

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

Answer 10

Fluoride and chloride both have low reducing abilities

Question 11

Give an equation for the reaction between solid sodium bromide and concentrated sulfuric acid where HBr is produced
State an observation of this reaction

Answer 11

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

Question 12

Give an equation for the reaction between hydrogen bromide and concentrated sulfuric acid where SO₂ is produced and state what occurs in terms of reduction
State the observations of this reaction

Answer 12

2HBr _(g) + H₂SO_{4 (aq)} → Br_{2 g} + SO_{2 (g)} + 2H₂O _(l)
- Bromide ions then reduce the S in H₂SO₄ from +6 to +4
- Orange bromine vapour and choking SO₂ gas are produced

Question 13

Give the overall equation of the reaction between solid sodium bromide and concentrated sulfuric acid

Answer 13

2NaBr + 3H₂SO₄ → 2NaHSO₄ + SO₂ + 2H₂O

Question 14

Give an equation for the reaction between solid sodium iodide and concentrated sulfuric acid where HI is produced
State an observation of this reaction

Answer 14

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

Question 15

Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where SO₂ is produced and state what occurs in terms of reduction
State the observations of this reaction

Answer 15

2HI _(g) + H₂SO_{4 (aq)} → I_{2 g} + SO_{2 (g)} + 2H₂O _(l)
- Iodide ions reduce the S in H₂SO₄ from +6 to +4
- Violet iodine vapour and choking SO₂ gas is produced

Question 16

Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where solid sulfur is produced and state what occurs in terms of reduction
State the observations of this reaction

Answer 16

6HI _(g) + H₂SO_{4 (aq)} → 3I_{2 (s)} + S _(s) + 4H₂O _(l)
- Iodide ions reduce the S in H₂SO₄ from +6 to 0
- Solid yellow sulfur is produced

Question 17

Give an equation for the reaction between hydrogen iodide and concentrated sulfuric acid where H₂S gas is produced and state what occurs in terms of reduction
State the observations of this reaction

Answer 17

8HI _(g) + H₂SO_{4 (aq)} → 4I_{2 (s)} + H₂S _(g)+ 4H₂O _(l)
- Iodide ions reduce the S from H₂SO₄ from +6 to -2
- H₂S gas is produced
- Rotten egg smell of H₂S gas is observed

Question 18

State the half equation for the reduction of H₂SO₄ to SO₂

Answer 18

H₂SO₄ + 2H⁺ + 2e^- → SO₂ + 2H₂O

Question 19

State the half equation for the reduction of H₂SO₄ to S

Answer 19

H₂SO₄ + 6H⁺ + 6e^- → S + 4H₂O

Question 20

State the half equation for the reduction of H₂SO₄ to H₂S

Answer 20

H₂SO₄ + 8H⁺ + 8e^- → H₂S + 4H₂O

Question 21

State the general half equation of the oxidation of halide ions

Answer 21

2X^- → X₂ + 2e^-

X = halogen

Question 22

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

Answer 22

Aqueous silver nitrate (AgNO₃)

Question 23

Why must the silver nitrate (AgNO₃) be acidified?

Answer 23

To remove any ions like carbonates that may form precipitates
If impurities are not removed it can show misleading results

Question 24

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

Answer 24

White

Ag⁺ _(aq) + Cl^- _(aq) → AgCl _(s)

Question 25

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

Answer 25

Cream Ag⁺ _(aq) + Br^- _(aq) → AgBr _(s)

Question 26

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

Answer 26

Pale yellow Ag⁺ _(aq) + I^- _(aq) → AgI _(s)

Question 27

Silver bromide and silver iodide have similar colours and can be hard to distinguish between. To confirm the identity of the halide present we can add excess...

Answer 27

ammonia solution

Question 28

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

Answer 28

The silver chloride will dissolve, confirming that chloride ions are present

Question 29

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

Answer 29

The silver bromide will dissolve, confirming that bromide ions are present

Question 30

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

Answer 30

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