OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 12

1 mark · Medium difficulty · Multiple Choice

Determine the ratio of MnO2(s) to OH-(aq) in the balanced redox equation for the reaction between I-(aq) and MnO4-(aq).

Practise this question

Question

Multiple choice question 12 showing an unbalanced redox equation between iodide ions and manganate(VII) ions: I-(aq) + MnO4-(aq) + H2O(l) -> IO-(aq) + MnO2(s) + OH-(aq). Four multiple-choice options for the ratio of MnO2(s) to OH-(aq) are given: A (1:3), B (1:2), C (1:1), D (3:2), with a box for the answer and 1 mark indicated.
Question text

12 Iodide ions, I−(aq), react with MnO −(aq). The unbalanced equation is shown below.

I−(aq) + MnO −(aq) + H O(l) IO−(aq) + MnO (s) + OH−(aq)

42 2

What is the ratio of MnO (s) to OH−(aq) in the balanced equation?

A 1 : 3

B 1 : 2

C 1 : 1

D 3 : 2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 12 with the correct answer option C.

12 C 1 2.6

How to answer it

Balancing Redox Equations in Alkaline Conditions

What this question tests

This question assesses your ability to balance complex redox equations in alkaline aqueous solutions using oxidation states or ion-electron half-equations, and to correctly interpret stoichiometric ratios from a fully balanced equation.

Question 12 Overview

Balancing a Redox Reaction to Find Stoichiometric Ratios

✅ Correct Answer

C (1 : 1)

In the fully balanced equation, the stoichiometric coefficient for both MnO₂(s) and OH⁻(aq) is 2, giving a simplification ratio of 1 : 1 .

💡 Key Knowledge

  • Oxidation States: Tracking changes in oxidation numbers to identify reduction and oxidation.
  • Half-Equation Method: Balancing atoms, oxygen using H₂O , and hydrogen/alkalinity using OH⁻ ions in alkaline conditions.
  • Electron Transfer: Equating the number of gained and lost electrons between oxidizing and reducing agents.

🧠 Exam Technique

  • Do not attempt to guess or balance by trial and error inspection alone for complex redox systems.
  • Systematically split the process into reduction and oxidation half-equations to ensure charge and mass balance are absolute.

❌ Common Errors

  • Forgetting to simplify the final ratio (e.g., leaving it as 2:2 instead of reducing to 1:1).
  • Confusing acidic and alkaline balancing protocols (adding H⁺ instead of OH⁻ ).

📐 Step-by-Step Calculation Guide

  1. Identify oxidation number changes:
    • Iodine in I⁻ is oxidized to IO⁻ (I changes from -1 to +1 , a loss of 2 electrons).
    • Manganese in MnO₄⁻ is reduced to MnO₂ (Mn changes from +7 to +4 , a gain of 3 electrons).
  2. Balance the electron transfer:
    • Multiply the iodine half-reaction by 3 and the manganese half-reaction by 2 to balance total electrons exchanged (6 electrons total).
  3. Set up balanced coefficients before water/hydroxide adjustment:
    • 3I⁻ + 2MnO₄⁻ + ... → 3IO⁻ + 2MnO₂ + ...
  4. Balance oxygen and charge using water and hydroxide ions:
    • Balancing charges and remaining atoms leads to the full stoichiometric equation:
      3I⁻(aq) + 2MnO₄⁻(aq) + H₂O(l) → 3IO⁻(aq) + 2MnO₂(s) + 2OH⁻(aq)
  5. Extract the target ratio:
    • Coefficient of MnO₂ = 2
    • Coefficient of OH⁻ = 2
    • Ratio MnO₂ : OH⁻ = 2 : 2 simplifies to 1 : 1 .
Examiner Note: Top-level candidates quickly recognized that balancing the half-equations directly leads to the integer coefficients without requiring full trial-and-error inspection of every species. Common incorrect choices like A or B typically arose from arithmetic slips in balancing the hydrogen/oxygen atoms for alkaline solutions.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.