Reactions of Halides with Sulfuric Acid

Inorganic Chemistry of Group 7

What happens to the reducing power of halide ions down Group 7?

It increases down the group.

What is the order of reducing power of the halide ions F⁻, Cl⁻, Br⁻ and I⁻?

F⁻ < Cl⁻ < Br⁻ < I⁻.

Why does the reducing power of halide ions increase down Group 7?

The outer electrons are further from the nucleus and more shielded, so they are lost more easily.

What happens to halide ions when they act as reducing agents?

They lose electrons and are oxidised.

What is the general oxidation half-equation for a halide ion?

2X⁻ → X₂ + 2e⁻

Why can concentrated sulfuric acid act as both an acid and an oxidising agent?

It can donate H⁺ ions and can also gain electrons and be reduced.

What are the possible reduction products of concentrated sulfuric acid when reacting with halides?

SO₂, S and H₂S.

What is the oxidation number of sulfur in H₂SO₄?

+6.

What is the oxidation number of sulfur in SO₂?

+4.

What is the oxidation number of sulfur in elemental sulfur?

0.

What is the oxidation number of sulfur in H₂S?

−2.

What is the half-equation for reduction of sulfuric acid to sulfur dioxide?

H₂SO₄ + 2H⁺ + 2e⁻ → SO₂ + 2H₂O

What is the half-equation for reduction of sulfuric acid to sulfur?

H₂SO₄ + 6H⁺ + 6e⁻ → S + 4H₂O

What is the half-equation for reduction of sulfuric acid to hydrogen sulfide?

H₂SO₄ + 8H⁺ + 8e⁻ → H₂S + 4H₂O

How does sulfur’s oxidation number change when H₂SO₄ is reduced to SO₂?

It decreases from +6 to +4.

How does sulfur’s oxidation number change when H₂SO₄ is reduced to sulfur?

It decreases from +6 to 0.

How does sulfur’s oxidation number change when H₂SO₄ is reduced to H₂S?

It decreases from +6 to −2.

What happens when concentrated sulfuric acid is added to sodium chloride?

An acid-base reaction occurs, producing hydrogen chloride.

What are the observations when concentrated sulfuric acid is added to sodium chloride?

Misty fumes are produced.

What gas causes the misty fumes when concentrated sulfuric acid reacts with sodium chloride?

Hydrogen chloride, HCl.

What is the equation for sodium chloride reacting with concentrated sulfuric acid?

NaCl + H₂SO₄ → NaHSO₄ + HCl

Does a redox reaction occur between sodium chloride and concentrated sulfuric acid?

No.

Why does concentrated sulfuric acid not oxidise chloride ions?

Chloride ions are weak reducing agents.

What happens when concentrated sulfuric acid is added to sodium bromide?

An acid-base reaction first produces HBr, then HBr reduces sulfuric acid to SO₂ and is oxidised to Br₂.

What observations are seen when concentrated sulfuric acid is added to sodium bromide?

Misty fumes, brown fumes and a colourless choking gas.

What causes the misty fumes when concentrated sulfuric acid is added to sodium bromide?

Hydrogen bromide, HBr.

What causes the brown fumes when concentrated sulfuric acid is added to sodium bromide?

Bromine, Br₂.

What causes the colourless choking gas when concentrated sulfuric acid is added to sodium bromide?

Sulfur dioxide, SO₂.

What is the initial acid-base equation for sodium bromide with concentrated sulfuric acid?

NaBr + H₂SO₄ → NaHSO₄ + HBr

What is the oxidation half-equation for bromide ions?

2Br⁻ → Br₂ + 2e⁻

What is the reduction half-equation for sulfuric acid to sulfur dioxide?

H₂SO₄ + 2H⁺ + 2e⁻ → SO₂ + 2H₂O

What is the overall ionic redox equation between bromide ions and sulfuric acid?

2Br⁻ + H₂SO₄ + 2H⁺ → Br₂ + SO₂ + 2H₂O

What is the molecular redox equation for hydrogen bromide reducing sulfuric acid?

2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

What is oxidised in the reaction between HBr and concentrated sulfuric acid?

HBr, or Br⁻, is oxidised to Br₂.

What is reduced in the reaction between HBr and concentrated sulfuric acid?

H₂SO₄ is reduced to SO₂.

What happens when concentrated sulfuric acid is added to sodium iodide?

An acid-base reaction first forms HI, then iodide strongly reduces sulfuric acid to SO₂, S and H₂S while iodide is oxidised to I₂.

What observations are seen when concentrated sulfuric acid is added to sodium iodide?

Misty fumes, purple fumes or black solid, a colourless choking gas, a yellow solid and a colourless gas with a rotten-egg smell.

What causes the misty fumes when concentrated sulfuric acid is added to sodium iodide?

Hydrogen iodide, HI.

What causes the purple fumes or black solid when concentrated sulfuric acid is added to sodium iodide?

Iodine, I₂.

What causes the colourless choking gas when concentrated sulfuric acid is added to sodium iodide?

Sulfur dioxide, SO₂.

What causes the yellow solid when concentrated sulfuric acid is added to sodium iodide?

Sulfur, S.

What causes the colourless gas with a rotten-egg smell when concentrated sulfuric acid is added to sodium iodide?

Hydrogen sulfide, H₂S.

What is the initial acid-base equation for sodium iodide with concentrated sulfuric acid?

NaI + H₂SO₄ → NaHSO₄ + HI

What is the oxidation half-equation for iodide ions?

2I⁻ → I₂ + 2e⁻

Why does sodium iodide produce more reduction products than sodium bromide with concentrated sulfuric acid?

Iodide ions are stronger reducing agents than bromide ions.

What products can sulfuric acid be reduced to by iodide ions?

SO₂, S and H₂S.

What is the overall equation for HI reducing sulfuric acid to sulfur dioxide?

2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O

What is the overall equation for HI reducing sulfuric acid to sulfur?

6HI + H₂SO₄ → 3I₂ + S + 4H₂O

What is the overall equation for HI reducing sulfuric acid to hydrogen sulfide?

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O

What is oxidised when iodide ions react with concentrated sulfuric acid?

I⁻ is oxidised to I₂.

What is reduced when iodide ions react with concentrated sulfuric acid?

H₂SO₄ is reduced to SO₂, S or H₂S.

Which halide ion shows the greatest reducing power?

I⁻.

Which halide ion shows the least reducing power?

F⁻.

How does concentrated sulfuric acid behave with chloride ions?

It acts only as an acid.

How does concentrated sulfuric acid behave with bromide ions?

It acts as both an acid and an oxidising agent and is reduced mainly to SO₂.

How does concentrated sulfuric acid behave with iodide ions?

It acts as both an acid and a strong oxidising agent and can be reduced to SO₂, S and H₂S.

Why does the reaction with sodium chloride produce no chlorine?

Cl⁻ is not a strong enough reducing agent to reduce concentrated sulfuric acid.

Why is bromine produced when sodium bromide reacts with concentrated sulfuric acid?

Br⁻ reduces sulfuric acid and is oxidised to Br₂.

Why is iodine produced when sodium iodide reacts with concentrated sulfuric acid?

I⁻ is a strong reducing agent and is oxidised to I₂.

What trend do the reactions of NaCl, NaBr and NaI with concentrated sulfuric acid demonstrate?

The reducing power of the halide ions increases down Group 7.