4 - Reactions of Halide Ions

Are halide ions reducing or oxidising agents?

Reducing agents

Make another species gain electrons

Are oxidised

2F⁻ → F₂ + 2e⁻

How does strength as a reducing agent change down Group 7?

Increases

Why does strength as a reducing agent change down Group 7?

Outer energy level further away from nucleus

Less attraction between nucleus and electron

More readily lost

What is the reaction of sodium chloride with sulfuric acid?

NaCl(s) + H₂SO₄(aq) → NaHSO₄(s) + HCl(g)

-1 +6 +6 -1

No change in oxidation state - not a redox reaction

What kind of reaction is sodium chloride and sulfuric acid?

Acid-base reaction

Acid is proton donor

What is the first reaction of sodium bromide with sulfuric acid?

NaBr(s) + H₂SO₄(aq) → NaHSO₄(s) + HBr(g)

-1 +6 +6 -1

No change in oxidation state - not a redox reaction

What is the second reaction of sodium bromide with sulfuric acid?

2H⁺ + 2Br⁻ + H₂SO₄(aq) → SO₂(g) + 2H₂O(l) + Br₂(g)

2(-1) +6 +4 0

Change in oxidation state - redox reaction

What is the role of H₂SO₄ in the second reaction of sodium bromide with sulfuric acid?

Oxidising agent

What is the overall reaction of sodium bromide with sulfuric acid?

2NaBr(s) + 2H₂SO₄(aq) → Na₂SO₄(s) + SO₂(g) + 2H₂O(l) + Br₂(g)

What are the observations of the overall reaction of sodium bromide with sulfuric acid?

HBr(g) - steamy fumes

Br₂(g) - brown fumes

SO₂(g) - colourless gas

Why does sodium bromide react twice when sodium chloride reacts once?

Br⁻ is a stronger reducing agent than Cl⁻

Br⁻ gets oxidised to Br₂

What is the first reaction of sodium iodide with sulfuric acid?

NaI(s) + H₂SO₄(aq) → NaHSO₄(s) + HI(g)

-1 +6 +6 -1

No change in oxidation state - not a redox reaction

What is the second reaction of sodium iodide with sulfuric acid?

8H⁺ + 8I⁻ + H₂SO₄(aq) → H₂S(g) + 4H₂O(l) + 4I₂(g)

8(-1) +6 -2 0

Change in oxidation state - redox reaction

What are the observations of the overall reaction of sodium iodide with sulfuric acid?

HI(g) - steamy fumes

I₂(s) - black solid

H₂S(g) - bad egg smell

S(s) - yellow solid

How do these reactions show the strength of halide ions as reducing agents?

Cl⁻ did not make S change oxidation state - weak reducing agent

Br⁻ reduced S from +6 to +4 - stronger reducing agent

I⁻ reduced S from +6 to -2 (or 0 if S is formed) - strongest reducing agent

What is the method for the chemical tests for halides?

* Put 1 pipette of each halide solution into test tubes
* Add a few drops of nitric acid to each
* Add half a pipette of silver nitrate solution to each test tube and record observations

Why is nitric acid added to each halide sample?

Remove carbonate or hydroxide ions

Could also form a precipitate with silver ions

What is the equation for the formation of a silver halide as the positive result of the test for halides?

Ag⁺(aq) + X⁻(aq) → AgX(s)

What are the two tests to distinguish between silver halides?

Add dilute ammonia

Add concentrated ammonia

What are the observations of the test for halides?

Silver fluoride - no precipitate

Silver chloride - white precipitate, dissolves in dilute ammonia

Silver bromide - cream precipitate, dissolves in concentrated ammonia

Silver iodide - pale yellow precipitate, insoluble in concentrated ammonia