Redox reactions of the halogens

Chlorine is prepared in the laboratory by oxidising concentrated hydrochloric acid (containing Cl-) with potassium permanganate

2Cl-(aq) → Cl2(g) + 2e- The reaction must be done in a fume cupboard. Even this volume of chlorine gas in a school laboratory is dangerous.

Chlorine is a pale green poisonous gas with a pungent (sharp) smell.

Like potassium permanganate, chlorine is an oxidising agent – it removes electrons from other reagents.

Chlorine gas oxidises potassium bromide solution. 2Br–(aq) → Br2(aq) + 2e– Cl2(g) + 2e– → 2Cl–(aq) Cl2(g) + 2Br–(aq) → 2Cl–(aq) + Br2(aq)

Chlorine gas oxidises potassium iodide solution. 2I–(aq) → I2(aq) + 2e– Cl2(g) + 2e– → 2Cl–(aq) Cl2(g) + 2I–(aq) → 2Cl–(aq) + I2(aq) Remember that I2 forms brown I3– with excess I–(aq).

Bromine water and potassium iodide solution: Bromine water plus potassium iodide solution. 2I–(aq) → I2(aq) + 2e– Br2(g) + 2e– → 2Br–(aq) Br2(g) + 2I–(aq) → 2Br–(aq) + I2(aq)

Like chlorine, bromine is able to oxidise iodide to iodine. When chloroform is added to the brown solution, the characteristic purple colour of iodine in chloroform is visible.

When sodium chloride solution is added to bromine water there is no reaction. Bromine is not able to remove electrons from chloride ions. Similarly, there are no reactions between iodine water and chloride or bromide ions.

The halogens are oxidising agents: they will remove electrons from other species. Chlorine gas will oxidise Br– or I–. Bromine will oxidise I– but not Cl–. Iodine will not oxidise Cl– or Br–. Chlorine is the strongest oxidising agent of the three, and iodine is the weakest.

The halogens also remove electrons from metals. We will observe the reactions of the halogens with the iron in steel wool.

When heated steel wool is plunged into a gas jar of chlorine, it immediately reacts, producing clouds of iron(II) and iron(III) chloride.

After 10 minutes most of the solid chlorides have fallen to the bottom of the gas jar.

A few drops of liquid bromine are put in the bottom of a gas jar and left to vaporise.

Heated steel wool is plunged in, as before. There is little reaction, though the steel wool does darken.

If you’re lucky – and the steel wool is hot enough – you will see a faint glow. We tried 4 times to get this photo!

The iron bromide formed is a different colour from the unreacted steel wool.

When iodine vapour passes over heated steel wool there is little reaction.

Unreacted steel wool After reaction with iodine.

Chlorine reacts vigorously with heated steel wool to form iron(III) chloride. Bromine reacts slightly with heated steel wool to form iron bromides. Iodine does not appear to react with steel wool.