6.1.5Unit 6. Practical Skills in Chemistry II
Electrochemical Cells — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 9 of 14 Unit quiz
Part 1 of 6Building a Cell
Two half-cells joined by a salt bridge make an electrochemical cell.
Half-cells
- Each half-cell contains a metal in a solution of its own ions, such as zinc in zinc sulfate or copper in copper(II) sulfate.
Salt bridge
- A strip of filter paper soaked in aqueous potassium nitrate completes the circuit by allowing ions to move.
Voltmeter
- A high-resistance voltmeter measures the e.m.f. without letting appreciable current flow.
Inert electrode
- For ions such as Fe³⁺/Fe²⁺ a platinum electrode carries electrons into the solution.
Part 2 of 6Measuring e.m.f.
1
Clean the metal strips with emery paper, then place each in 1.00 mol dm⁻³ solution of its ions.
2
Connect the two solutions with a salt bridge made from filter paper soaked in saturated potassium nitrate solution.
3
Connect the voltmeter, with the positive terminal to the more positive electrode, and read the voltage.
4
For Zn²⁺/Zn and Cu²⁺/Cu the standard value is E° = (+0.34) – (–0.76) = +1.10 V.
5
Replace one half-cell with another metal and compare the measured values with predictions from electrode potentials.
Part 3 of 6Predicting Cell Behaviour
Predicting Cell Behaviour
Positive electrode
- The half-cell with the more positive electrode potential is reduced, so electrons flow towards it.
Negative electrode
- The less positive half-cell is oxidised, losing electrons from the metal.
Cell e.m.f.
- The cell e.m.f. is found by subtracting: E°cell = E°(positive) – E°(negative).
Concentration effect
- Lowering [Cu²⁺] makes the copper electrode less positive, so the cell voltage falls slightly.
| Half-cell | E° / V |
|---|---|
| Zn²⁺ + 2e⁻ ⇌ Zn | –0.76 |
| Cu²⁺ + 2e⁻ ⇌ Cu | +0.34 |
| Fe³⁺ + e⁻ ⇌ Fe²⁺ | +0.77 |
Part 4 of 6Why Readings Differ
Non-standard conditions
- Concentrations other than 1.00 mol dm⁻³ or temperatures other than 298 K shift the e.m.f. from the standard value.
Surface layers
- Oxide coatings on the metal strips change the electrode potential until they are removed.
Salt bridge resistance
- A dry or narrow salt bridge raises resistance and lowers the voltage reading.
Rate effect
- Some half-reactions are slow, so the predicted potential may not be seen.
Part 5 of 6Worked Example
Question Calculate E°cell for a cell with Zn²⁺/Zn (–0.76 V) and Fe³⁺/Fe²⁺ (+0.77 V) half-cells.
- The Fe³⁺/Fe²⁺ half-cell has the more positive potential, so it is the positive electrode.
- E°cell = (+0.77) – (–0.76) = +1.53 V.
Answer E°cell = +1.53 V
Part 6 of 6Exam Checklist & Tips
6.1.5 – Key Points for the Exam
- Salt bridge A connection that allows ions to move and complete the circuit without mixing the solutions.
- Electrode potential The e.m.f. of a half-cell measured against the standard hydrogen electrode.
- E°cell The standard e.m.f. of a cell, equal to E°(positive) – E°(negative).
- Half-cell One electrode in contact with its ions.
Common Mistakes
- Using a wire instead of a salt bridge, which does not carry ions between solutions.
- Adding both electrode potentials rather than subtracting.
- Using a low-resistance meter that draws current and gives a low reading.
- Using potassium chloride in a salt bridge with silver ions, as it would precipitate silver chloride.
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Magnification Triangle & Unit Conversion
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