Edexcel A-Level Chemistry Paper 3, June 2019: Question 2

7 marks · Medium difficulty · Calculations

State standard conditions for electrode potentials, identify errors in an electrochemical cell diagram, and calculate silver ion concentration using the Nernst equation.

Practise this question

Question

Exam question about an Ag+(aq)|Ag(s) half-cell consisting of three parts: (a)(i) asks for standard temperature and pressure conditions (1 mark); (a)(ii) shows an experimental diagram of a hydrogen-platinum half-cell connected via a salt bridge containing potassium chloride to a silver half-cell containing 1 mol dm-3 silver nitrate and an ammeter, requiring three mistakes and their corrections to be identified in a table (3 marks); (b) provides the Nernst equation E = E_theta + (RT/96500) ln[Ag+(aq)], giving E_theta = +0.80V, measured E = +0.72V at 20 degrees Celsius, and asks to calculate the concentration of silver ions (3 marks).
Question text

2 This question is about the Ag+(aq)|Ag(s) half-cell.

(a) A student was asked to plan an experiment to measure the standard electrode potential

of the Ag+(aq)|Ag(s) half-cell.

(i) State the conditions of temperature and pressure under which

standard electrode potentials are measured.

(1)

(ii) The student drew the diagram shown.

A

hydrogen gas

platinum electrode silver electrode

1 mol dm–3 1 mol dm–3

ethanoic acid salt bridge silver nitrate

containing aqueous

potassium chloride

Identify three mistakes in this diagram and the modifications that should be

made to correct them.

(3)

Mistake in diagram Modification needed to correct mistake

(b) The standard electrode potential,*P58308A0432*E, of the Ag+(aq)|Ag(s) half-cell is +0.80V.

The effect of changing the concentration of the ions on the value of the electrode

potential, E, in this half-cell is calculated using the equation

RT +

E = E + × ln[Ag (aq)]

96500

where T is the temperature in kelvin and R is the gas constant.

The electrode potential of a Ag+(aq)|Ag(s) half-cell was measured at 20 °C and

found to be +0.72V.

Calculate the concentration of silver ions, in mol dm–3, in this half-cell.

(3)

(Total for Question 2 = 7 marks)

Mark scheme

Show the mark scheme Mark scheme providing acceptable answers and additional guidance for parts (a)(i), (a)(ii), and (b). Part (a)(i) requires 298 K / 25 °C and 1 atm / 1 bar / 100 kPa. Part (a)(ii) lists three errors: ammeter instead of high-resistance voltmeter, ethanoic acid instead of a 1.0 mol dm-3 strong acid for H+(aq), and potassium chloride in the salt bridge instead of potassium nitrate or sodium nitrate. Part (b) outlines the step-by-step calculation using the Nernst equation, leading to a silver ion concentration of approximately 4.19 x 10^-2 mol dm-3.

How to answer it

Study Guide: The Ag+(aq)|Ag(s) Half-Cell

What this question tests

This question assesses your understanding of electrochemical cells, standard conditions for measuring electrode potentials, the ability to spot experimental errors in electrochemical setups, and how to apply the Nernst-style logarithmic equation to calculate ion concentrations under non-standard conditions.

Part (a)(i) — Standard Conditions

State the conditions of temperature and pressure

✅ Correct Answer

  • Temperature: 298 K (or 25 °C )
  • Pressure: 100 kPa (or 1 atm / 1 bar / 101 kPa )

💡 Key Knowledge

Both value and correct units must be stated to secure the mark. Memorise standard measurement conditions thoroughly as examiners are strictly intolerant of missing units.

Marks available: 1
Part (a)(ii) — Electrochemical Cell Diagram Analysis

Identify three mistakes in the diagram and their corrections

✅ Correct Answers (Mistake & Fix Pairs)

  • Mistake 1: An ammeter ( A ) is used.
    Fix: Replace with a (high resistance) voltmeter.
  • Mistake 2: Ethanoic acid ( CH₃COOH ) is used in the standard hydrogen electrode.
    Fix: Replace with a strong acid of 1.0 mol dm⁻³ H⁺(aq) (e.g., 1.0 mol dm⁻³ HCl or 0.5 mol dm⁻³ H₂SO₄ ).
  • Mistake 3: Potassium chloride ( KCl ) is used in the salt bridge.
    Fix: Replace with potassium nitrate ( KNO₃ ) or sodium nitrate ( NaNO₃ ).

🧠 Exam Technique & Guidance

Why KCl is wrong: Chloride ions ( Cl⁻ ) react with silver ions ( Ag⁺ ) to form an insoluble white precipitate of silver chloride ( AgCl ), disrupting the silver half-cell. Nitrates are always safe because soluble silver nitrate is used anyway.

Marks available: 3 (1 mark per correct row)
Part (b) — Non-Standard Electrode Potential Calculation

Calculate the concentration of silver ions

📐 Step-by-Step Calculation

  1. Identify known values:
    E = +0.72 V , Eᵩ = +0.80 V , R = 8.31 J mol⁻¹ K⁻¹ , T = 20 °C = 293 K , F = 96500 C mol⁻¹
  2. Substitute values into the expression:
    0.72 = 0.80 + ((8.31 × 293) / 96500) × ln[Ag⁺(aq)]
  3. Rearrange to solve for ln[Ag⁺(aq)]:
    0.72 - 0.80 = 0.02523 × ln[Ag⁺(aq)]
    -0.08 = 0.02523 × ln[Ag⁺(aq)]
    ln[Ag⁺(aq)] = -0.08 / 0.02523 = -3.1707
  4. Exponentiate (inverse ln) to find concentration:
    [Ag⁺(aq)] = e⁻³·¹⁷⁰⁷ = 0.041976
    Final Answer: 0.042 mol dm⁻³ (to 2 significant figures, or 4.2 × 10⁻² mol dm⁻³ )

❌ Common Calculation Traps

  • Temperature conversion error: Forgetting to convert Celsius to Kelvin ( 20 + 273 = 293 K ).
  • Logarithm confusion: Getting stuck after finding ln([Ag⁺]) because of forgetting how to undo a natural logarithm using eˣ or shift ln on the calculator.

🧠 Marking Breakdown

  • Mark 1: Correct substitution of values into the formula.
  • Mark 2: Correct rearrangement and calculation of ln[Ag⁺(aq)] .
  • Mark 3: Correct evaluation of final concentration value with appropriate units.
Marks available: 3

Topics

Physical Chemistry · Core Practicals · Topic 14: Redox II · Core Practical 10: Construct electrochemical cells and measure electrode potentials

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.