AQA A-Level Chemistry Paper 3, 2019: Question 3

17 marks · Hard difficulty · Practical Techniques & Data Analysis

Analyse the setup and data for electrochemical cells, including identifying components, plotting an Ecell graph against ln([Zn2+]/[Cu2+]), and calculating temperature and electrode potentials.

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Question text

03 Figure 1 represents the cell used to measure the standard electrode potential for the

Fe3+/Fe2+ electrode.

Figure 1

03.1 Name the piece of apparatus labelled A.

[1 mark]

03.2 State the purpose of A.

[1 mark]

03.3 Name the substance used as electrode B in Figure 1.

[1 mark]

03.4 Complete Table 1 to identify C, D and E from Figure 1.

Include the essential conditions for each.

[4 marks]

Table 1

Identity Conditions

*10* D

03.5 The standard electrode potential, Eo, for the Fe3+/Fe2+ electrode is +0.77 V

Give the ionic equation for the overall reaction in the cell in Figure 1.

State the change that needs to be made to the apparatus in Figure 1 to allow the cell

reaction to go to completion.

[2 marks]

Ionic equation

Change

03.6 A student sets up a cell as shown in the cell representation.

Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)

The student measures the cell EMF, Ecell, with several different concentrations of

Cu2+ ions and Zn2+ ions.

The results are shown in Table 2.

Table 2

Complete Table 2 to show the value missing from experiment 4.

Plot a graph of E against ln ([Zn2+]/[Cu2+]) on the grid.

cell

[3 marks]

03.7 This equation shows how Ecell varies with concentration for this reaction.

This equation is in the form of the equation for a straight line, y = mx + c

Calculate the gradient of your plotted line on the graph in question 03.6.

You must show your working.

Use your gradient to calculate the temperature, T, at which the measurements of Ecell

were taken.

(If you were unable to calculate a gradient you should use the value −0.016 V

This is not the correct value.)

[3 marks]

Gradient V

T K

03.8 In experiment 2 in Table 2 the electrode potential of the Cu2+/Cu electrode is +0.33 V

Use data from Table 2 in question 03.6 to calculate the electrode potential for the

Zn2+/Zn electrode in experiment 2.

Give one reason why your calculated value is different from the standard electrode

potential for Zn2+/Zn electrode.

[2 marks]

Electrode potential V

Reason

Mark scheme

Show the mark scheme

Question Answers Additional Comments/Guidelines Mark

salt bridge ALLOW description of salt bridge, e.g.

filter paper / string / wick soaked in suitable

solution

03.1 1

U tube (NOT YouTube) filled with suitable

solution / gel

NOT U tube alone

complete the circuit ALLOW ions to flow / move / transfer

ALLOW to balance charge / to maintain electrical

03.2 neutrality 1

IGNORE current / charge to flow

NOT electrons to flow

03.3 B = platinum ALLOW Pt / platinum black 1

– – –

Identity Conditions NOT incorrect state symbols

ALLOW M or molar or mol/dm3 for mol dm–3 1

M1 C HCl 1 mol dm–3

M1 ALLOW 1 mol dm–3 H+ 1

M2 D H2 / hydrogen 100 kPa –3

ALLOW 0.5 mol dm H2SO4 1

M3 E FeCl and FeCl 1 mol dm–3 ALLOW 1 mol dm–3 HNO

23 3

IGNORE 100 kPa

M4 298 K (any mention) M2 ALLOW 1 bar 1

03.4 NOT 1 atm / 101 kPa

NOT H for hydrogen 17

NOT 1 mol dm–3

M3 ALLOW 1 mol dm–3 Fe2+ and Fe3+

ALLOW other identified Fe(II) and Fe(III)

compounds with appropriate concentrations,

e.g. 1 mol dm–3 FeSO and 0.5 mol dm–3

Fe2(SO4)3

IGNORE 100 kPa

M1 H + 2 Fe3+ 2 H+ + 2 Fe2+ M1 IGNORE state symbols

ALLOW multiples / fractions 1

03.5 ALLOW equation with equilibrium sign if

forward reaction shown is in this direction

M2 ALLOW remove voltmeter– – –

M2 replace voltmeter with lamp/wire/ammeter owtte

M1 missing value (+) 2.3(0) 1

1.16

1.15

1.14

1.13

1.12

1.11

Ecell / V

1.1

1.09

03.6 1.08

1.07

1.06

1.05

1.04

-5 -4 -3 -2 -1 0 1 2 3 4 5

ln ([Zn2+]/[Cu2+])

M2 suitable scales (plotted points use at least half of grid) M2 ALLOW scales which use half the grid for 1

plotted points

M3 points plotted correctly (± ½ small square per point) and best fit 1

line drawn (within one small square of each point) M3 If M1 incorrect, should be plotted accordingly

and best fit line ignore if anomalous

– – –

M1 gradient = 0.013 (must be negative) M1 ALLOW –0.0125 to –0.0136 1

ALLOW ECF from graph if outside this range

–5 𝐌𝟏 M3 temperature must match gradient unless 1

M2 M1 = ( ) 4.3 x 10 T or T = 19

(−)4.3 𝑥 10−5

0.016 used (ALLOW positive temperature if 1

M3 T = 302 or 303 (K) positive gradient used)

03.7

at least 2sf

Correct M3 also scores M2

NOT negative temperature

M3 (Alternate gradient = 0.016 gives) T = 372 (K)

M1 E = 0.8(0) V 1

M2 non standard conditions or M2 ALLOW temperature is not 298K 1

03.8 2+ –3 NOT concentration (of Zn2+) greater than

concentration (of Zn ) not 1 (mol dm ) or

1 (mol dm–3)

concentration (of Zn2+) less than 1 (mol dm–3)

NOT concentration (of Zn2+) is different

How to answer it

Electrochemical Cells, Standard Electrodes & EMF Graphical Analysis

📋 What This Question Tests

This question assesses comprehensive mastery of AQA A-Level Physical Chemistry (Electrode Potentials and Electrochemical Cells):

Standard Hydrogen Electrode (SHE): Components, standard operating conditions (temperature, pressure, concentrations), and experimental setup.
Salt Bridge & Cell Function: Role of the salt bridge in completing the circuit via ion movement; modifying high-resistance setups to allow current to flow.
Redox Equilibria & Cell Reactions: Writing overall balanced ionic equations from half-equations and standard reduction potentials.
Mathematical & Graphical Skills: Calculating logarithmic concentration ratios, plotting precision linear graphs, determining negative gradients, and rearranging straight-line equations ( y = mx + c ) to extract physical quantities (temperature, T).
Non-Standard Conditions: Calculating non-standard half-cell potentials using E_cell = E_RHS - E_LHS and applying Le Chatelier's principle to electrode equilibria.

Part 03.1, 03.2 & 03.3 — Apparatus, Purpose of Salt Bridge & Inert Electrodes

Identifying standard cell components

✅ Mark Scheme Answers

03.1: Salt bridge (ALLOW filter paper / wick soaked in suitable solution or U-tube filled with suitable solution/gel). [1 mark]
03.2: Complete the circuit (OR allow ions to flow / move; balance charge / maintain electrical neutrality). [1 mark]
03.3: Platinum / Pt (ALLOW platinum black). [1 mark]

❌ Common Errors & Pitfalls

03.1: Writing simply "U-tube" or "glass tube" scores 0. It must specify a salt bridge or gel/solution-filled tube.
03.2: Saying "allows electrons to flow" loses the mark immediately. Electrons only flow through external metallic wiring; ions move through the salt bridge.
03.3: Giving an active metal (like iron) rather than an inert conductor ( Pt ) that can catalyze electron transfer without reacting.

🧠 Exam Technique: The Fe³⁺/Fe²⁺ Half-Cell

Because both Fe³⁺ and Fe²⁺ are aqueous ions in solution, there is no solid metal phase present to act as the conducting terminal. You must supply an unreactive solid conductor: an inert platinum (Pt) electrode.

Part 03.4 — Standard Hydrogen Electrode (SHE) Conditions

Completing Table 1 for reagents and standard conditions

✅ Correct Table 1 Completion [4 marks]

Label	Identity	Conditions
C (Solution in SHE)	HCl (or any strong monoprotic acid / H⁺(aq) )	1.0 mol dm⁻³ (if H₂SO₄ , must be 0.5 mol dm⁻³ )
D (Gas into SHE)	H₂ / hydrogen gas	100 kPa (or 1 bar )
E (Solution in Fe half-cell)	FeCl₂ and FeCl₃ (or equimolar Fe²⁺ and Fe³⁺ salts)	1.0 mol dm⁻³ with respect to both ions
Temperature (M4)	298 K (or 25 °C) stated anywhere in the conditions column.

💡 Key Knowledge: IUPAC Standard Conditions

Standard conditions are strictly defined as:

Temperature: 298 K (25 °C).
Pressure: 100 kPa (1 bar). Note: 1 atm / 101 kPa is not modern IUPAC standard!
Concentration: 1.0 mol dm⁻³ for all dissolved ionic species.

❌ Examiner Misconceptions in 03.4

Gas D: Writing "H" instead of H₂ . Hydrogen exists as diatomic molecules.
Solution E: Naming only one iron salt (e.g. just FeCl₃ ). Both oxidation states ( Fe²⁺ and Fe³⁺ ) must be simultaneously present in solution at unit concentration.
Sulfuric acid trap: If giving H₂SO₄ , stating 1.0 mol dm⁻³ loses the mark because it provides 2.0 mol dm⁻³ H⁺ . It must be 0.5 mol dm⁻³ .

Part 03.5 — Overall Cell Reaction & Allowing It to Run to Completion

Cell EMF, direction of spontaneous change, and external circuit

✅ Mark Scheme Answers

Ionic equation [1 mark]:

H₂ + 2Fe³⁺ → 2H⁺ + 2Fe²⁺

(ALLOW reversible arrow if forward reaction matches this direction; ALLOW multiples or fractions; IGNORE state symbols.)

Apparatus Change [1 mark]:

Replace the voltmeter with an ammeter / bulb / lamp / wire (or remove the voltmeter / connect external circuit with a wire).

🧠 Exam Technique: Deducing the Spontaneous Reaction

E°(Fe³⁺/Fe²⁺) = +0.77 V
E°(2H⁺/H₂) = 0.00 V
The more positive electrode ( Fe³⁺/Fe²⁺ ) undergoes reduction:
Fe³⁺ + e⁻ → Fe²⁺
The more negative electrode ( SHE ) undergoes oxidation:
H₂ → 2H⁺ + 2e⁻
Multiply iron reduction by 2 to balance electrons and sum together.

💡 Why Must the Voltmeter be Replaced?

A voltmeter has near-infinite electrical resistance so that practically zero current flows, enabling the measurement of true electromotive force (EMF). To allow the reaction to proceed spontaneously to completion, electrons must flow freely, requiring a low-resistance path (wire, ammeter, or load).

Part 03.6 — Data Completion & Graph Plotting

Logarithmic calculation and precision plotting

📐 Calculation: Missing Value for Experiment 4 [1 mark]

For Experiment 4: [Zn²⁺] = 1.0 mol dm⁻³ , [Cu²⁺] = 0.10 mol dm⁻³

ln([Zn²⁺] / [Cu²⁺]) = ln(1.0 / 0.10) = ln(10) = +2.302585... ≈ +2.30 (or 2.3)

Table 2 gives values to 2 decimal places / 3 significant figures, matching -2.30 from Experiment 2.

📊 Graph Plotting Rules [2 marks]

M2 (Scale): Plot E_cell / V on the y-axis (range approx. 1.00 to 1.20 V, with 1 large square = 0.02 V) and ln([Zn²⁺]/[Cu²⁺]) on the x-axis (range -5 to +5, with 1 large square = 1 unit). The points must occupy at least half the graph grid in both directions.
M3 (Points & Best-Fit Line): Plot all 5 points accurately within ±0.5 small square:
• (-4.61, 1.16)
• (-2.30, 1.13)
• (0.00, 1.10)
• (+2.30, 1.07)
• (+4.61, 1.04)
Draw a straight, continuous line of best fit through all plotted points using a ruler (within 1 small square of each point).

Part 03.7 — Calculating Gradient and Determining Temperature T

Connecting straight-line mechanics to physical equations

📐 Step-by-Step Gradient and Temperature Calculation [3 marks]

The equation provided is:
E_cell = (-4.3 × 10⁻⁵ × T) × ln([Zn²⁺]/[Cu²⁺]) + E°_cell

Comparing with y = mx + c :

y = E_cell
x = ln([Zn²⁺]/[Cu²⁺])
gradient ( m ) = -4.3 × 10⁻⁵ × T
intercept ( c ) = E°_cell

Step 1: Calculate the Gradient (M1)
Pick two widely separated points on the line of best fit, e.g. at x₁ = -4.61, y₁ = 1.16 and x₂ = +4.61, y₂ = 1.04 :
gradient = Δy / Δx = (1.04 - 1.16) / (4.61 - (-4.61)) = -0.12 / 9.22 = -0.0130 V
(Mark scheme allows: -0.0125 to -0.0136. Must include negative sign!)

Step 2: Rearrange for T (M2)
gradient = -4.3 × 10⁻⁵ × T
T = gradient / (-4.3 × 10⁻⁵) = (-0.013) / (-4.3 × 10⁻⁵)

Step 3: Evaluate Temperature T (M3)
T = 302 K (or 303 K depending on gradient).
(If student was unable to calculate gradient and used the substitute -0.016 V: T = -0.016 / (-4.3 × 10⁻⁵) = 372 K .)

❌ Critical Calculation Traps

Omission of the negative sign: The slope clearly slopes downwards from left to right; the gradient must be negative. Omitting the sign loses M1.
Negative Temperature: Absolute temperature in Kelvin cannot be negative. The negative sign of the gradient cancels the negative in -4.3 × 10⁻⁵ .

Part 03.8 — Non-Standard Electrode Potential Calculation & Explanation

Cell potential arithmetic and equilibrium shifts

📐 Step-by-Step Calculation [1 mark]

From Table 2, Experiment 2: E_cell = +1.13 V .

Cell convention: Zn(s)|Zn²⁺(aq)||Cu²⁺(aq)|Cu(s)

E_cell = E_RHS - E_LHS

1.13 = E(Cu²⁺/Cu) - E(Zn²⁺/Zn)

1.13 = (+0.33) - E(Zn²⁺/Zn)

E(Zn²⁺/Zn) = +0.33 - 1.13 = -0.80 V (or -0.8 V)

💡 Explanation for Difference [1 mark]

Standard E°(Zn²⁺/Zn) = -0.76 V . Why is the calculated value -0.80 V ?

Accepted Reason: Non-standard conditions were used — specifically, the concentration of Zn²⁺ is 0.10 mol dm⁻³ , which is less than 1.0 mol dm⁻³ (or temperature is not 298 K).

🧠 Chemical Deep-Dive: Le Chatelier's Principle at the Electrode

Consider the reduction half-cell equilibrium:

Zn²⁺(aq) + 2e⁻ ⇌ Zn(s)

In Experiment 2, [Zn²⁺] = 0.10 mol dm⁻³ (lower than standard 1.0 mol dm⁻³). By Le Chatelier's principle, decreasing [Zn²⁺] shifts equilibrium to the left, releasing more electrons and making the electrode potential more negative (-0.80 V compared to -0.76 V).

❌ Examiner Warning for 03.8 Reason

Do NOT simply write "the concentration of Zn²⁺ is different". You must specify either non-standard conditions, that concentration is not 1 mol dm⁻³, or that it is less than 1 mol dm⁻³. Vague answers lose the mark.

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, 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.