The chemical properties of the halogen elements and the hydrogen halides

AS Level Chemistry · Group 17. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Halogens react by gaining electrons, so they are oxidising agents. A smaller atom attracts an extra electron more strongly, because the electron enters a shell closer to the nucleus with less shielding. Oxidising power therefore decreases down the group: chlorine is stronger than bromine, and bromine is stronger than iodine.

This explains displacement: a halogen oxidises the halide ions of any halogen below it in the group, but not those above it.

The same order appears with hydrogen. The reaction H2 + X2 → 2HX becomes less vigorous down the group. The hydrogen halides formed become less stable to heat, because the H–X bond gets longer and weaker.

Rules to remember

Common mistake

Students often say a halogen displaces the one above it. It is the other way round: a halogen higher in the group displaces a halide ion lower in the group, so iodine displaces nothing.

Worked example

Aqueous bromine is added to colourless aqueous sodium iodide. What is seen, and which species is oxidised?

  1. Bromine is above iodine in Group 17, so it is the stronger oxidising agent.
  2. It takes electrons from iodide ions: Br2 + 2I− → 2Br− + I2.
  3. Iodine goes from −1 to 0 (oxidised); bromine goes from 0 to −1 (reduced).
  4. The iodine formed colours the solution brown.

Answer: The solution turns brown as iodine forms; the iodide ion is oxidised.

Practice questions

Try each one, then open the answer.

1. Which halogen displaces iodine from aqueous potassium iodide but cannot displace bromine from aqueous potassium bromide?

  1. A
    bromine
  2. B
    chlorine
  3. C
    fluorine
  4. D
    iodine
Show answer

Answer: A. Oxidising power decreases down Group 17. Bromine can oxidise I− (Br2 + 2I− → 2Br− + I2) but cannot oxidise Br−. Chlorine would displace both.

2. Chlorine water is added to colourless aqueous potassium bromide. What is seen, and why?

  1. A
    the solution turns orange-yellow, because chlorine oxidises bromide ions to bromine
  2. B
    a cream precipitate forms, because potassium chloride is insoluble
  3. C
    no change, because chlorine is a weaker oxidising agent than bromine
  4. D
    the solution turns purple, because iodine is formed
Show answer

Answer: A. Cl2 + 2Br− → 2Cl− + Br2. Chlorine is the stronger oxidising agent, so it displaces bromine, which colours the solution orange-yellow.

3. Why is chlorine a stronger oxidising agent than bromine?

  1. A
    chlorine atoms are larger than bromine atoms
  2. B
    the Cl–Cl bond is weaker than the Br–Br bond
  3. C
    chlorine is a gas and bromine is a liquid
  4. D
    a chlorine atom has a smaller radius and less shielding, so it attracts an extra electron more strongly
Show answer

Answer: D. An oxidising agent gains electrons. The incoming electron enters the outer shell of the smaller chlorine atom closer to the nucleus, so the attraction is stronger and the reduction is more favourable.

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