AQA A-Level Chemistry Paper 3, June 2025: Question 3

11 marks · Medium difficulty · Practical Techniques & Data Analysis

Identify apparatus errors in a standard electrode potential setup, state the voltmeter property required to measure cell EMF, deduce overall cell reactions, and use electrode potential data to explain redox observations and calculate cell EMF.

Practise this question

Question

Question text

03 A student does an experiment to measure the standard electrode potential of an

Fe3+/Fe2+ electrode at 298 K

Figure 1 shows the apparatus the student uses.

Figure 1

03.1 Figure 1 shows three mistakes made by the student.

Identify each mistake and the change needed to correct it.

[3 marks]

Mistake 1

Change 1

Mistake 2

Change 2

Mistake 3

Change 3

03.2 What feature of the voltmeter ensures that the voltmeter measures the cell EMF?

[1 mark]

Tick ( ) one box.

Low resistance

Variable resistance

High resistance

03.3 The Fe3+/Fe2+ electrode is the positive electrode when the cell is set up

correctly at 298 K

Give the redox equation for the overall reaction that occurs in the cell when the

voltmeter is replaced with a lamp.

[1 mark]

03.4 Table 4 shows some standard electrode potentials.

Table 4

Half-equation Eo / V

Fe2+(aq) + 2 e– → Fe(s) –0.44

Cu2+(aq) + e– → Cu+(aq) +0.15

Cu2+(aq) + 2 e– → Cu(s) +0.34

Cu+(aq) + e– → Cu(s) +0.52

Fe3+(aq) + e– → Fe2+(aq) +0.77

Finely powdered iron filings are added to a solution of copper(II) sulfate and the

mixture is stirred.

The solution gradually changes from blue to pale green and a brown solid forms.

Use the data in Table 4 to explain these observations.

Give an ionic equation for the reaction that occurs.

Give the conventional representation of the cell that has the same overall reaction as

in your equation.

Calculate the EMF, in volts, of this cell.

[6 marks]

Explanation

Equation

Conventional representation

*12* EMF V

Mark scheme

Show the mark scheme

Question Answers Additional comments/Guidelines Mark

ALLOW 1 mark for 3 mistakes if no other mark

M1 mistake the concentration of H SO OR [H+] ≠ 1 mol dm–3 It must be clear that it is the concentration that is

change use 0.5 mol dm–3 H SO OR 1 mol dm–3 HCl / HNO wrong, not the identity of the acid

24 3

ALLOW 1 mol dm–3 H+ for the change

M2 mistake NaOH / wrong substance on filter paper/salt bridge ALLOW if NaOH not mentioned e.g. salt bridge 3

03.1

change use KNO3 would react with H2SO4 (3 x AO3)

ALLOW suitable alternative (name or formula)

e.g. KCl

M3 mistake Fe / wrong electrode/metal (on RHS) ALLOW graphite OR different named unreactive

change Pt metal e.g. Au, Ag (NOT Cu)

03.2 High resistance

(1 x AO1)

ALLOW multiples/fractions

3+ 2+ + 1

03.3 2 Fe + H2 → 2 Fe + 2 H IGNORE state symbols

(1 x AO2)

NOT ⇌

– A-LEVEL CHEMISTRY – 7405/3 –

M1 Cu is the brown (solid) IGNORE Fe3+ is brown

M2 Fe2+ is the (pale) green (solution)

M3 because Eϴ Fe2+/Fe more negative than Eϴ Cu2+/Cu OR ALLOW reference to +ve EMF of the correct

because Eϴ Cu2+/Cu more positive than Eϴ Fe2+/Fe cell/reaction

ALLOW Eϴ Fe2+/Fe < Eϴ Cu2+/Cu OR

Eϴ Cu2+/Cu > Eϴ Fe2+/Fe

References to the half-cells / half-equations

must refer to both Fe and Fe2+ (in either order)

AND to Cu and Cu2+ (in either order) to identify

the half-equations.

IGNORE Eϴ Fe2+/Fe greater than or less than 6

03.4 Eϴ Cu2+/Cu

(2 x AO2,

2+ 2+ 4 x AO3)

M4 Cu + Fe ⟶ Cu + Fe IGNORE state symbols

NOT ⇌

M5 Fe(s) | Fe2+(aq) || Cu2+(aq) | Cu(s) M5 ECF from M4 where clear which two half

(State symbols required) equations are being used; but LHS and RHS of

cell representation must match direction of

reaction in M4 for incorrect half equations

ALLOW Cu(s) | Cu2+(aq) || Fe2+(aq) | Fe(s) only

if Fe2+ + Cu ⟶ Fe + Cu2+ in M4

M6 (+)0.78 (V) M6 ECF from M4 or M5 where clear which two

half equations are being used

NOT negative EMF

How to answer it

Electrochemical Cells: SHE Setup, Cell Notation & EMF

WHAT THIS QUESTION TESTS

This question assesses practical apparatus understanding, theoretical principles, and mathematical problem-solving in AQA A-Level Electrochemistry:

Experimental cell setup: Standard conditions for the Standard Hydrogen Electrode (SHE), inert electrode selection, and salt bridge compatibility.
Apparatus function: Why a high-resistance voltmeter is required to measure electromotive force (EMF).
Redox stoichiometry: Combining half-equations to deduce feasible full ionic redox equations.
Electrochemical predictions: Using standard electrode potentials (E°) to justify physical observations, construct conventional cell notations, and calculate standard cell EMF.

PART 03.1 • 3 MARKS

Identifying Setup Errors in an Electrochemical Cell

Detecting three mistakes in the experimental measurement of E°(Fe³⁺/Fe²⁺)

✅ Mark Scheme Correct Answers

#	Mistake Identified	Change Needed
1	Concentration of H₂SO₄ is 1.00 mol dm⁻³ / [H⁺] ≠ 1.00 mol dm⁻³	Use 0.50 mol dm⁻³ H₂SO₄ (or 1.00 mol dm⁻³ HCl / HNO₃)
2	Filter paper soaked in NaOH(aq) / wrong salt bridge solution	Soak in KNO₃(aq) (or KCl(aq))
3	Fe metal electrode used on RHS	Replace with an inert Pt (platinum) electrode (or graphite)

Award 1 mark for each fully corrected pair [Mistake + Correction]. If a student correctly identifies all 3 mistakes but provides no valid corrections, award 1 mark overall.

💡 Key Knowledge

Diprotic Acids: 1.00 mol dm⁻³ H₂SO₄ dissociates to give 2.00 mol dm⁻³ H⁺(aq). Standard conditions require exactly 1.00 mol dm⁻³ H⁺(aq).
Salt Bridge Function: It must supply mobile ions without reacting with either half-cell. NaOH contains OH⁻, which neutralises H⁺ in the SHE and precipitates Fe(OH)₂ / Fe(OH)₃ in the RHS beaker!
Inert Electrodes: When both members of a redox couple are in the same phase (Fe³⁺(aq) and Fe²⁺(aq)), there is no solid conducting metal. An inert conductor (Pt) must be used to transfer electrons. An iron electrode would oxidise directly (Fe → Fe²⁺ + 2e⁻).

🧠 Exam Technique

Always scan electrochemical diagrams systematically in four steps:

Concentrations: Look for diprotic acids like H₂SO₄ where [H⁺] is doubled.
Salt bridge: Ensure the salt won't form precipitates or acid-base reactions.
Electrodes: Verify if a reactive metal is mistakenly placed into a solution containing two ions of the same metal in different oxidation states.
Gas conditions: Check temperature (298 K) and pressure (100 kPa).

❌ Common Student Errors

Stating "H₂SO₄ is the wrong acid" — H₂SO₄ is acceptable, but its concentration is wrong.
Replacing Fe with another reactive metal such as Cu, Zn, or Mg instead of an unreactive conductor (Pt or graphite).
Suggesting distilled water for the salt bridge — pure water does not contain sufficient mobile ions to conduct current.

PART 03.2 • 1 MARK

Voltmeter Characteristics in Cell EMF Measurement

Selecting the property that allows measurement of maximum potential difference

✅ Correct Answer

Tick: High resistance

1 Mark: AO1 recall. Exactly one box must be ticked.

💡 Key Knowledge

An ideal voltmeter has infinite (very high) resistance.
This ensures virtually zero current flows through the external circuit.
Preventing current flow keeps the half-cell reactions at equilibrium and avoids changes in ion concentrations or potential drops across internal cell resistance.

❌ Common Errors

Ticking low resistance by confusing voltmeters with ammeters. An ammeter requires low resistance to avoid impeding flow, whereas a voltmeter requires high resistance so no flow occurs.

PART 03.3 • 1 MARK

Overall Cell Redox Equation

Fe³⁺/Fe²⁺ electrode as the positive terminal

✅ Correct Equation

2Fe³⁺ + H₂ → 2Fe²⁺ + 2H⁺

1 Mark (AO2). Multiples or fractions allowed. State symbols not required. Must be a single forward arrow (NOT ⇌).

🧠 How to Derive the Equation

The question specifies that the Fe³⁺/Fe²⁺ electrode is positive.
Electrons flow towards the positive electrode, meaning reduction happens here:
Fe³⁺ + e⁻ → Fe²⁺
The standard hydrogen electrode must therefore undergo oxidation:
H₂ → 2H⁺ + 2e⁻
Multiply the reduction half-equation by 2 to balance electrons and add them together.

PART 03.4 • 6 MARKS

Predicting Observations, Equation, Representation & EMF

Reaction of finely powdered iron with aqueous copper(II) sulfate

✅ Mark Scheme Breakdown (6 Marks Total)

M1 (Observation): The brown solid is copper / Cu.
M2 (Observation): The pale green solution contains iron(II) ions / Fe²⁺.
M3 (Explanation): Because E°(Fe²⁺/Fe) is more negative than E°(Cu²⁺/Cu)
(OR E°(Cu²⁺/Cu) is more positive than E°(Fe²⁺/Fe)).
M4 (Ionic Equation): Cu²⁺ + Fe → Cu + Fe²⁺
M5 (Cell Representation):
Fe(s) | Fe²⁺(aq) || Cu²⁺(aq) | Cu(s)
Note: State symbols are compulsory for this mark!
M6 (EMF): +0.78 V

📐 Calculation & Data Selection

Step 1: Identify the relevant half-equations from Table 4:

Fe²⁺(aq) + 2e⁻ ⇌ Fe(s) E° = -0.44 V
Cu²⁺(aq) + 2e⁻ ⇌ Cu(s) E° = +0.34 V

(Be careful to pick the half-equation yielding solid copper, Cu(s), and not Cu⁺(aq)!)

Step 2: Calculate standard EMF:

E°_cell = E°(RHS / reduction) - E°(LHS / oxidation)
E°_cell = (+0.34 V) - (-0.44 V) = +0.78 V

A positive EMF (+0.78 V > 0) proves the reaction is feasible.

🧠 IUPAC Conventional Representation Rules

For M5, remember the golden rule: ROOR (Reduced | Oxidised || Oxidised | Reduced):

Left-hand side is the oxidation half-cell: Fe(s) | Fe²⁺(aq)
Double vertical line represents the salt bridge: ||
Right-hand side is the reduction half-cell: Cu²⁺(aq) | Cu(s)
Single vertical line ( | ) denotes a phase boundary between a solid and an aqueous solution.
State symbols: Specifically demanded by examiners for M5. Omitting (s) or (aq) loses the mark immediately!

❌ Critical Examiner Pitfalls for Part 03.4

Vague E° comparisons (M3): Writing "E° of iron is lower" is rejected. You must specify both species in the couple: "E° of Fe²⁺/Fe is more negative than E° of Cu²⁺/Cu". Do not compare single elements.
Attributing brown solid to Fe³⁺ (M1): Fe³⁺ solutions can appear orange-brown, but the solid precipitated here is metallic Cu. Saying "brown solid is Fe³⁺" scores zero for M1.
Negative EMF (M6): Cell EMF must always be quoted as a positive value (+0.78 V) for a spontaneous cell reaction.

Topics

Physical Chemistry · Required Practicals · 3.1.11 Electrode Potentials · Required Practical 8: Measuring the EMF of an electrochemical cell

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.