OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 14

1 mark · Medium difficulty · Multiple Choice

Determine which statements are correct for an electrochemical cell constructed from copper and silver redox systems.

Practise this question

Question

Multiple-choice question 14 featuring two half-equations with standard electrode potentials: Cu2+(aq) + 2e- = Cu(s) with E-theta = +0.34 V, and Ag+(aq) + e- = Ag(s) with E-theta = +0.80 V. Three numbered statements follow regarding cell potential, electrode reactions, and electrode mass changes, with options A, B, C, and D.
Question text

14 A cell is constructed from the two redox systems below.

Cu2+(aq) + 2e– Cu(s) E ө = +0.34 V

Ag+(aq) + e– Ag(s) E ө = +0.80 V

Which statement(s) is/are correct for the cell?

1 The cell potential is 1.14 V.

2 The reaction at the copper electrode is Cu(s) Cu2+(aq) + 2e–.

3 The silver electrode increases in mass.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C.

14 C 1

How to answer it

Electrochemical Cell Reactions and Potentials

What this question tests

This question assesses your understanding of electrochemical cells, specifically identifying reduction and oxidation half-equations, calculating standard cell potentials using electrode potentials (E-theta values), predicting feasibility, and determining physical changes at electrodes (such as mass increases due to metal deposition).

Question 14 Analysis

Evaluating Statements 1, 2, and 3

✅ Correct Answer: Option C

Statements 2 and 3 are correct. Statement 1 contains a common calculation trap!

💡 Key Knowledge: Identifying the Poles

  • Silver system has a more positive E-theta value (+0.80 V) than copper (+0.34 V).
  • Therefore, silver acts as the positive electrode (reduction occurs: Ag⁺ + e⁻ → Ag).
  • Copper acts as the negative electrode (oxidation occurs: Cu → Cu²⁺ + 2e⁻).

🧠 Exam Technique: Statement Breakdown

  • Statement 1 check: Cell potential = E-theta(positive) - E-theta(negative) = (+0.80) - (+0.34) = +0.46 V. (Statement says 1.14 V, which incorrectly adds them together: 0.80 + 0.34). False.
  • Statement 2 check: Copper undergoes oxidation: Cu(s) → Cu²⁺(aq) + 2e⁻ . True.
  • Statement 3 check: Ag⁺ ions are reduced at the silver electrode to form solid silver ( Ag(s) ), causing the electrode to increase in mass. True.

❌ Common Errors

  • Adding potentials: Students frequently add +0.80 V and +0.34 V together instead of subtracting them, leading to the false value of 1.14 V. Always remember: Cell Potential = Right - Left (or Positive - Negative).
  • Electrode mass confusion: Forgetting that reduction produces solid metal on the electrode surface, increasing its mass, while oxidation strips metal atoms into ions, decreasing mass.
Mark Scheme Allocation: Correct answer is C (Only 2 and 3). Worth 1 mark.

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 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.