Hydrogen Halides

• Hydrogen halides can be made by adding a concentrated acid to a solid ionic halide.

• To make hydrogen chloride, concentrated phosphoric acid can be added to sodium chloride.

NaCl + H₃PO₄ → HCl + NaH₂PO₄

Ionic equation: Cl^- + H₃PO₄ → HCl + H₂PO₄

• All hydrogen halide can be made using concentrated phosphoric acid.

• Only hydrogen chloride can be made using concentrated sulfuric acid.

NaCl + H₂SO₄ → HCl + NaHSO₄

Ionic equation: Cl^- + H₂SO₄ → HCl + HSO₄^-

• Hydrogen bromide and hydrogen iodide cannot be made using concentrated sulfuric acid.

• When NaBr and NaI react with sulfuric acid, Br₂ and I₂ are made.

• This is because sulfuric acid is an oxidising agent and iodide and bromide ions are strong enough reducing agents to reduce sulfuric acid.

• Bromide ions reduce sulfuric acid to sulfur dioxide

2NaBr + 2H₂SO₄ → Na₂SO₄ + Br₂ + SO₂ + H₂O

Ionic equation: H₂SO₄ + 2H⁺ + 2Br^- → Br₂ + SO₂ + 2H₂O

• Iodide ions are strong enough reducing agents that they fully reduce sulfuric acid to sulfur dioxide.

8NaI + 5H₂SO₄ → 4Na₂SO₄ + 4I₂ + H₂S + 4H₂O

Ionic equation: H₂SO₄ + 8H⁺ + 8I^- → 4I₂ + H₂S + 4H₂O

Thermal stability of hydrogen halides

• When heated, hydrogen fluoride and hydrogen chloride are stable and won’t break down into hydrogen and halide ions.

• Hydrogen bromide will break down slightly, while hydrogen iodide will break down more.

• This is because, going down group 7, the strength of the hydrogen-halide bond gets weaker.

• The halogen atoms get bigger down the group, so the bonding electrons are further away from the nucleus and are shielded by more inner shells of electrons.

Acidic nature of hydrogen halides

• Hydrogen chloride, hydrogen bromide, and hydrogen iodide dissolve in water to create strong acids.

• When they dissolve in water, they dissociate to produce hydrogen ions and halide ions.

• e.g. HCl_(aq) → H⁺_(aq) + Cl^-_(aq)

• Hydrogen fluoride is still acidic however it does not fully dissociate in water, so it is a weak acid.

Reaction of hydrogen halides with ammonia

• Ammonia is a base, so it can accept a proton to form the positively charged ammonium ions.

• The ammonium ion can bond with a negative halide ion to produce an ammonium halide.

• e.g. HCl_(aq) + NH_3(aq) → NH₄Cl_(aq)

Reaction of hydrogen halides with sulfuric acid

• Hydrogen fluoride and hydrogen chloride are not strong enough reducing agents to react with sulfuric acid.

• Hydrogen bromide can reduce sulfuric acid to sulfur dioxide

2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

• Hydrogen iodide can reduce sulfuric acid to hydrogen sulfide

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O