Halogens as oxidising agents and hydrogen halides

Who wins the electron?

Bubble chlorine through a colourless solution of sodium bromide and it turns red-brown: bromine has appeared. Where did it come from, and why does bromine never do the same to sodium chloride?

Oxidation is the loss of an electron; the substance that loses it is the reducing agent. Reduction is the gain of an electron; the substance that accepts it is the oxidising agent. The two always happen together, because an electron can only leave if something takes it.

Halogens accept electrons, so they are oxidising agents, and their oxidising power decreases down the group: F₂ > Cl₂ > Br₂ > I₂. A stronger halogen takes the electron from the ion of a weaker one and pushes it out of its compound. This is a displacement reaction: chlorine displaces bromine and iodine, and bromine displaces iodine.

Halogens join hydrogen to make hydrogen halides (H₂ + X₂ gives 2HX). All are gases at ordinary temperature except hydrogen fluoride, a liquid. Down the group the halogen atom is bigger, so the H–X bond is longer and weaker. It needs less energy to break, so thermal stability falls: HF > HCl > HBr > HI.

A halogen higher in the group displaces the halide of one lower down; bonds to hydrogen weaken down the group.

Key terms

Oxidising agent

Describing halogens.

Reducing agent

The other half of the reaction.

Displacement

Comparing oxidising power.

Thermal stability

Comparing hydrogen halides.

Short questions with model answers

1. Q1. In Cl₂ + 2NaBr → 2NaCl + Br₂, chlorine is the oxidising agent. Which is the reducing agent? (Short question iii, p. 127)

   • Who gains
   • Who loses

   Sodium bromide (the bromide ion) is the reducing agent.

2. Q2. Which hydrogen halide is least stable on heating, and why? (Exercise v, p. 127)

   • Bond length
   • Bond strength

   HI, because its H–I bond is the longest and weakest.

Common mistakes

• ✗ Bromine displaces chlorine from sodium chloride.

  ✓ Only a stronger oxidiser displaces a weaker one; Br₂ is weaker than Cl₂ (p. 122).

• ✗ The oxidising agent is the substance that loses electrons.

  ✓ It accepts electrons; the reducing agent loses them (p. 121).

• ✗ HI is the most stable hydrogen halide because iodine is the biggest.

  ✓ Bigger means a longer, weaker bond: HI is least stable (p. 122).

MCQs

1. 1. Which mixture will turn dark because of a reaction? (Exercise viii, p. 127)

   1. (a) Br₂ and NaCl
   2. (b) Br₂ and NaF
   3. (c) Cl₂ and NaF
   4. (d) Cl₂ and NaI
   Show answer

   (d) Chlorine displaces iodine, which colours the solution (p. 122).

2. 2. Which hydrogen halide is unstable at room temperature? (Exercise v, p. 127)

   1. (a) HBr
   2. (b) HI
   3. (c) HCl
   4. (d) HF
   Show answer

   (b) Longest, weakest H–X bond (p. 122).

3. 3. Reduction is:

   1. (a) Loss of an electron
   2. (b) Gain of an electron
   3. (c) Loss of a proton
   4. (d) Gain of a neutron
   Show answer

   (b) p. 121.

4. 4. Which is the strongest oxidising agent?

   1. (a) I₂
   2. (b) Br₂
   3. (c) Cl₂
   4. (d) F₂
   Show answer

   (d) Oxidising power decreases down the group (p. 122).

5. 5. Which hydrogen halide is a liquid at ordinary temperature, according to the book?

   1. (a) HF
   2. (b) HCl
   3. (c) HBr
   4. (d) HI
   Show answer

   (a) All others are gases (p. 122).

Quick revision

• Oxidation is electron loss (reducing agent); reduction is electron gain (oxidising agent).
• Halogens are oxidising agents; a stronger one displaces a weaker one from its salt.
• H–X bonds get longer and weaker down the group, so stability falls from HF to HI.