The standard reduction potentials are Cu2+/Cu = +0.34 V and Zn2+/Zn = −0.76 V. Calculate E⦵cell for the spontaneous zinc–copper cell and identify the positive electrode.
[3 marks]
Total for this question: 3
A-level Chemistry required practicals
Measuring the EMF of an electrochemical cell
AQA 3.1.11.1 · RP8
A-level Chemistry (7405) · Required practical 8 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Construct an electrochemical cell, measure its EMF and relate the polarity to feasible redox reactions and cell notation.
Apparatus
- Two half-cell solutions in separate beakers
- Clean metal electrodes or an inert platinum electrode where no conducting solid is present
- Salt bridge containing an inert electrolyte
- High-resistance digital voltmeter and connecting leads
- Volumetric glassware, thermometer, abrasive paper and eye protection
Apparatus & techniques (AT)
AT j · electrochemical cells
Builds two half-cells with a salt bridge and measures their cell EMF and polarity.
AT k · safe handling
Handles metal-ion solutions, electrodes and glassware safely while preventing cross-contamination.
Safety
Hazard
Some metal salts are harmful and environmentally hazardous.
Control
Wear eye protection and gloves where required, use small quantities and collect metal-containing waste in the labelled container.
Hazard
Glass salt bridges and electrodes can break during insertion.
Control
Support each component securely and do not force glassware through bungs.
Method
- 1Prepare each half-cell at the stated concentration and temperature, using clean glassware and enough solution to immerse the electrode consistently.
- 2Polish solid metal electrodes to remove surface contamination; use an inert platinum electrode for a half-cell containing only dissolved species.
- 3Connect the half-cells with a salt bridge whose electrolyte does not react with either half-cell, then connect the electrodes to a high-resistance voltmeter.
- 4Allow the reading to settle, record the sign and EMF, and identify which electrode is connected to the positive terminal.
- 5If investigating concentration or temperature, change only that condition in one half-cell and rebuild or rinse the apparatus between readings.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 2: Editorial focus: select compatible electrodes, bridge electrolyte and conditions for a valid cell.
- CPAC 3: Editorial focus: identify solution hazards and avoid direct contact with metal-ion solutions.
- CPAC 4: Editorial focus: record stable signed readings and all conditions needed to interpret them.
- CPAC 5: Editorial focus: connect polarity, cell notation and electrode-potential calculations.
Variables
Independent
A selected half-cell concentration or temperature
Dependent
Measured cell EMF
Control
- The other half-cell concentration and temperature
- Electrode identity, exposed area and cleanliness
- Salt-bridge composition and voltmeter
Results & processing
- Record both the magnitude and sign of each stable voltmeter reading together with half-cell conditions.
- Write cell notation with the oxidation half-cell on the left and the reduction half-cell on the right, using phase boundaries correctly.
- Under standard conditions, calculate Eocell = Eoright − Eoleft and compare it with the measurement.
Analysis skills
- Calculate standard cell EMF from two standard reduction potentials.
- Deduce oxidation and reduction directions from the sign of the reading.
- Write cell notation including inert electrodes and phase boundaries.
Uncertainty
Sources
- Voltmeter resolution and reading drift
- Liquid-junction potential at the salt bridge
- Electrode contamination and non-standard concentration or temperature
Calculations
- Quote the digital resolution as an absolute reading uncertainty where appropriate.
- For a difference from a predicted value, calculate measured minus predicted EMF and state the sign.
Interpretation
- A stable high-resistance reading reduces loading but does not remove junction potential.
- Agreement with a standard calculation is expected only when the experimental conditions are standard.
Exam angles
- Correct a cell diagram containing the wrong inert electrode, salt bridge or polarity.
- Calculate Eocell, write the spontaneous overall equation and explain a measured discrepancy.
- Describe a valid preparation of a half-cell solution before assembly.
Where students lose marks
Making a half-cell solution up to an approximate volume before assembling the cell.
Fix: Complete the stated quantitative preparation with appropriate volumetric glassware, mixing thoroughly before transferring solution to the half-cell.
- Examiner report: C3-20 · PDF p. 6
Using a reactive electrode where the half-cell contains only dissolved oxidised and reduced species.
Fix: Use an inert platinum electrode to transfer electrons without adding another redox couple.
- Examiner report: C3-25 · PDF p. 5
Choosing a salt-bridge electrolyte that precipitates or reacts with a half-cell ion.
Fix: Select an inert electrolyte and check both ions against both half-cells before assembly.
- Examiner report: C3-25 · PDF p. 5
Comparing a non-standard measurement directly with a standard electrode-potential calculation.
Fix: State that the tabulated calculation assumes standard concentration, pressure and temperature conditions.
Reversing the overall reaction after correctly identifying the positive electrode.
Fix: Write reduction at the more positive electrode, reverse the other half-equation for oxidation, then add and cancel electrons.
Improve the method
- Use freshly polished electrodes, clean glassware and a new salt bridge to reduce contamination and junction drift.
- Use a high-resistance voltmeter so negligible current flows and the cell composition changes as little as possible.
- Thermostat both half-cells and record readings only after the value stabilises.
Source references
- Specification: CSpec 3.1.11.1 · PDF p. 34
Try it — exam-style
Easy
ORIGINAL
Medium
ORIGINAL
A Fe3+/Fe2+ half-cell is connected to a metal/metal-ion half-cell. State the electrode needed in the iron half-cell, one requirement for the salt bridge and why the voltmeter should have high resistance.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
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