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AS · Practice questions · Halogens as oxidising agents and the hydrogen halides

Displace and decompose.

Six original Cambridge-style questions on the halogens as oxidising agents and the hydrogen halides.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Higher halogen displaces lower halide.

01
Recall
[2 marks]

Write ionic equations for the reactions of (a) chlorine with iodide ions and (b) bromine with iodide ions.

  • (a) Cl2 + 2I− → 2Cl− + I2. ✓
  • (b) Br2 + 2I− → 2Br− + I2. ✓
02
Explain
[3 marks]

Describe how you would show that chlorine is a stronger oxidising agent than bromine, giving observations.

  • Add chlorine water to aqueous potassium bromide; it turns orange as Br2 forms. ✓
  • Add bromine water to aqueous potassium chloride; no reaction (stays the orange of the added bromine). ✓
  • Shaking with hexane confirms the halogen present (orange-red for Br2). ✓
03
Explain
[2 marks]

Explain why the oxidising power of the halogens decreases from chlorine to iodine.

  • Atomic radius and shielding increase down the group. ✓
  • The nucleus attracts an incoming electron less strongly, so the halogen gains electrons less readily. ✓
04
Explain
[3 marks]

Describe and explain the trend in thermal stability of HCl, HBr and HI.

  • Thermal stability decreases from HCl to HI. ✓
  • The H–X bond energy falls (431, 366, 299 kJ mol−1) because the halogen atom is larger and the bond longer. ✓
  • HI decomposes to H2 and purple I2 with a hot glass rod; HCl does not. ✓
05
Recall
[2 marks]

Compare the conditions needed for chlorine and for iodine to react with hydrogen.

  • Chlorine reacts explosively with hydrogen in sunlight (or a flame). ✓
  • Iodine reacts only slowly when heated, and the reaction is reversible. ✓
06
Analysis
[2 marks]

Use oxidation numbers to show that Br2 + 2I− → 2Br− + I2 is a redox reaction.

  • Br goes from 0 to −1: reduced. ✓
  • I goes from −1 to 0: oxidised. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.