OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2021: Question 3

1 mark · Medium difficulty · Multiple Choice

Deduce the correct stoichiometric balancing numbers for silver and water in the redox reaction between silver and concentrated nitric acid.

Practise this question

Question

Multiple choice question 3 showing an unbalanced ionic equation for the reaction of silver with concentrated nitric acid: ... Ag(s) + ... NO3^-(aq) + ... H^+(aq) -> ... Ag^+(aq) + ... NO(g) + ... H2O(l). A table gives four options A to D for the balancing numbers of Ag(s) and H2O(l): A has 1 and 2, B has 2 and 3, C has 3 and 1, and D has 3 and 2. An answer box is provided at the bottom with a 1-mark allocation.
Question text

3 The unbalanced equation for the reaction of silver with concentrated nitric acid is shown below.

….. Ag(s) + ….. NO –(aq) + ….. H+(aq) → ….. Ag+(aq) + .…. NO(g) + ..… H O(l)

Which numbers for Ag and H2O will balance the equation?

Ag(s) H2O(l)

A 1 2

B 2 3

C 3 1

D 3 2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme displays a table indicating that the correct answer for question 3 is D, worth 1 mark, classified under assessment objective AO2.6.

3 D 1 AO2.6

How to answer it

Balancing Ionic Redox Equations

What this question tests

This question assesses your ability to determine oxidation numbers, construct half-equations, and use electron-transfer logic to balance a complex ionic redox equation in acidic conditions.

Question 3 • Multiple Choice

Balancing Silver with Concentrated Nitric Acid

The unbalanced equation is given as:

..... Ag(s) + ..... NO₃⁻(aq) + ..... H⁺(aq) → ..... Ag⁺(aq) + ..... NO(g) + ..... H₂O(l)

Which numbers for Ag(s) and H₂O(l) will balance the equation?

✅ Correct Answer: D (Ag = 3, H₂O = 2)

Option D correctly balances both the electron transfer (oxidation states) and the atomic/charge conservation, yielding 3 moles of silver and 2 moles of water.

💡 Key Knowledge

  • Oxidation state changes: Ag changes from 0 to +1 (loses 1 electron). Nitrogen in NO₃⁻ changes from +5 to +2 in NO (gains 3 electrons).
  • Electron balancing: To balance electrons, you need 3 Ag atoms for every 1 NO₃⁻ reduced to NO.
  • Acidic conditions: H⁺ ions and H₂O molecules are used to balance oxygen and hydrogen atoms after balancing the main redox elements.

🧠 Exam Technique

Instead of blindly guessing coefficients from the options, construct the balanced equation systematically using oxidation numbers or half-equations. Once fully balanced, read off the required coefficients for Ag and H₂O .

❌ Common Errors

  • Balancing only atoms by inspection without tracking oxidation number changes.
  • Confusing the oxidation state of nitrogen in nitrate ( +5 ) with nitrogen dioxide or other nitrogen species.
  • Failing to account for the stoichiometric ratio required for electron exchange (the 3:1 ratio between Ag and NO).

📐 Step-by-Step Derivation

  1. Identify oxidation numbers: Ag goes from 0 to +1. N in NO₃⁻ goes from +5 to +2 in NO .
  2. Set up electron transfer: Each Ag loses 1 e⁻. Each N gains 3 e⁻. Therefore, you need 3 Ag for every 1 NO .
  3. Place preliminary coefficients: Put a 3 in front of Ag , Ag⁺ , NO₃⁻ , and NO :

    3 Ag + 1 NO₃⁻ + ..... H⁺ → 3 Ag⁺ + 1 NO + ..... H₂O

  4. Balance Oxygen and Hydrogen:
    • Left side has 3 oxygens (from NO₃⁻). Right side has 1 oxygen (in NO). We need 2 more oxygens on the right, so add a coefficient of 2 in front of H₂O : 2 H₂O .
    • This introduces 4 hydrogens on the right ( 2 × H₂ ). Balance this by putting a 4 in front of H⁺ on the left.
  5. Final Balanced Equation:

    3 Ag(s) + 1 NO₃⁻(aq) + 4 H⁺(aq) → 3 Ag⁺(aq) + 1 NO(g) + 2 H₂O(l)

  6. Match with options: Coefficient for Ag = 3, coefficient for H₂O = 2. This matches option D.
Examiner Note: This question discriminates well between candidates who guess coefficients and those who understand systematic redox balancing via electron-transfer ratios.

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 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.