OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 11

1 mark · Easy difficulty · Multiple Choice

Identify the strongest reducing agent given the standard electrode potentials of two redox systems.

Practise this question

Question

Question 11 presents two redox equilibria: Mg2+(aq) + 2e- reversible reaction with Mg(s) with standard electrode potential of -2.37 V, and Pb2+(aq) + 2e- reversible reaction with Pb(s) with standard electrode potential of -0.13 V. It asks 'Which species is the strongest reducing agent?' with options A: Mg2+(aq), B: Mg(s), C: Pb2+(aq), and D: Pb(s).

Mark scheme

Show the mark scheme Mark scheme for question 11 indicates the correct answer is B, awarded 1 mark.

How to answer it

Standard Electrode Potentials & Identifying Reducing Agents

📋 What this question tests
  • Understanding standard electrode potentials (E⦵) written as reduction equilibria.
  • Distinguishing between oxidizing agents (electron acceptors) and reducing agents (electron donors).
  • Applying the rule that more negative E⦵ values correspond to equilibria shifting to the left, releasing electrons readily.
  • Selecting the correct chemical species, including proper oxidation states and physical forms.

Question 11 (Multiple Choice)

Identifying the strongest reducing agent from standard reduction potentials

✅ Correct Answer

B: Mg(s)

Award [1] mark for option B only.

💡 Key Knowledge

  • Standard potentials are always written as reductions:
    Oxidised form + ne⁻ ⇌ Reduced form
  • More negative E⦵: Equilibrium lies further to the left. The species on the right releases electrons most readily (it is easily oxidized and acts as a strong reducing agent).
  • More positive E⦵: Equilibrium lies further to the right. The species on the left gains electrons most readily (acts as a strong oxidizing agent).

📐 Step-by-Step Logic Breakdown

  1. Compare the E⦵ values:
    • Mg2+(aq) + 2e⁻ ⇌ Mg(s)    E⦵ = −2.37 V
    • Pb2+(aq) + 2e⁻ ⇌ Pb(s)    E⦵ = −0.13 V
  2. Find the more negative value:
    −2.37 V is significantly more negative than −0.13 V.
  3. Determine the direction of release:
    The more negative system releases electrons:
    Mg(s) → Mg2+(aq) + 2e⁻
  4. Identify the agent:
    A reducing agent donates electrons. Here, solid magnesium, Mg(s), donates electrons and is oxidized. Therefore, Mg(s) is the strongest reducing agent.

❌ Common Errors & Pitfalls

  • Selecting the ion instead of the metal (Mg2+ instead of Mg): Mg2+ has already lost electrons; it cannot donate electrons to act as a reducing agent.
  • Confusing reducing agent with oxidising agent:
    • Strongest oxidising agent = Pb2+(aq) (species on LHS with most positive E⦵)
    • Strongest reducing agent = Mg(s) (species on RHS with most negative E⦵)
  • Misreading negative signs: Thinking −0.13 V is "more negative" or smaller than −2.37 V.

🧠 Exam Technique & Examiner Insight

A reliable mnemonic tool for redox equilibria arranged from most negative to most positive:

  • Top Right = Best Reducer: Write the most negative half-cell at the top. The species on the right-hand side (the reduced metal/species) is the strongest reducing agent.
  • Bottom Left = Best Oxidiser: The half-cell with the most positive potential sits at the bottom; the species on the left-hand side (the ion) is the strongest oxidising agent.
  • Always double-check whether the question asks for the reducing agent or oxidising agent before circling your answer.

Topics

Module 5: Physical chemistry and transition elements · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.