Edexcel A-Level Chemistry Paper 1, June 2019: Question 2

✅ Correct Answer

Cl₂ + 2KBr → Br₂ + 2KCl

(Ionic equation accepted: Cl₂ + 2Br⁻ → Br₂ + 2Cl⁻ )

💡 Key Knowledge

• Chlorine is a stronger oxidising agent than bromine and displaces bromide ions.
• Bromine is formed, which gives the solution its characteristic orange colour.

❌ Common Errors

• Writing halogen atoms instead of diatomic molecules ( Cl instead of Cl₂ ).
• Failing to balance the equation properly with coefficients.

✅ Correct Answer

Must make reference to three distinct marking points:

• M1: The precipitate is a mixture of silver chloride and silver bromide (or bromide ions were in excess / not all oxidised).
• M2: Silver chloride ( AgCl ) dissolves in dilute ammonia.
• M3: Silver bromide ( AgBr ) does not dissolve in dilute ammonia.

🧠 Exam Technique

When a question asks why some of a precipitate dissolves, systematically break down your answer: state what solids are present in the mixture, state the solubility behavior of the first, and contrast it with the solubility behavior of the second in the added reagent.

❌ Common Errors

Students frequently lose marks by stating that chloride ions dissolve, rather than specifying that silver chloride precipitate dissolves. Remember that AgBr requires concentrated ammonia to dissolve, not dilute.

✅ Correct Answer

Iodine is a weaker oxidising agent than chlorine (or iodine cannot oxidise bromide ions / iodide/iodine is a stronger reducing agent).

💡 Key Knowledge

Oxidising ability decreases down Group 7: Cl₂ > Br₂ > I₂ . A halogen can only displace a halide that sits below it in the periodic table.

❌ Common Errors

Using vague terminology such as "iodine is less reactive" or "iodine cannot displace it" without linking the observation to oxidising/reducing strength. Edexcel mark schemes often penalise generic reactivity statements.

✅ Correct Answer

3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O

🧠 Exam Technique

Balance atoms and charges carefully. Remember that hot alkali produces the chlorate(V) ion ( ClO₃⁻ ) alongside chloride ions, whereas cold alkali produces chlorate(I) ( ClO⁻ ).

✅ Correct Answer

• The oxidation number of chlorine changes from 0 (in Cl₂ ) to -1 (in Cl⁻ ), so it is reduced.
• The oxidation number of chlorine changes from 0 (in Cl₂ ) to +5 (in ClO₃⁻ ), so it is oxidised.

💡 Key Knowledge

Disproportionation is a redox reaction in which the same element is simultaneously oxidized and reduced.

❌ Common Errors

Giving general definitions of disproportionation without explicitly stating both the starting and ending oxidation numbers for chlorine, or omitting whether it was oxidised or reduced.