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A2 · Practice questions · Standard electrode potentials

Measure against zero.

Six original Cambridge-style questions on standard electrode potentials.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Reduction, standard conditions, platinum.

01
Recall
[3 marks]

Define the term standard electrode potential and state the standard conditions.

  • The potential difference (e.m.f.) of a half-cell compared with the standard hydrogen electrode. ✓
  • Measured at 298 K, with all ions at 1.00 mol dm−3, and any gases at 100 kPa (or 101 kPa). ✓
  • With a high-resistance voltmeter, so no current flows. ✓
02
Recall
[4 marks]

Draw and label (or describe) the apparatus used to measure E⊖ for the Fe3+/Fe2+ couple.

  • Standard hydrogen electrode: H2 at 100 kPa bubbling over platinum in 1.00 mol dm−3 H+. ✓
  • Second half-cell: platinum electrode in a solution containing 1.00 mol dm−3 Fe3+ and 1.00 mol dm−3 Fe2+. ✓
  • A salt bridge of KNO3(aq) joins the two solutions. ✓
  • A high-resistance voltmeter connects the two electrodes; temperature 298 K. ✓
03
Explain
[2 marks]

Explain why a platinum electrode is used in the Cl2/Cl− half-cell rather than a chlorine electrode.

  • Chlorine is a gas and a non-conductor, so it cannot be an electrode. ✓
  • Platinum is inert and conducts electrons between the external circuit and the Cl2/Cl− equilibrium at its surface. ✓
04
Analysis
[3 marks]

Use E⊖ values to arrange Ag+, Fe3+, I2 and Cu2+ in order of increasing oxidising power. Explain your reasoning.

  • E⊖: Cu2+/Cu +0.34, I2/I− +0.54, Fe3+/Fe2+ +0.77, Ag+/Ag +0.80 V. ✓
  • The more positive E⊖, the greater the tendency of the left-hand species to gain electrons. ✓
  • Increasing oxidising power: Cu2+ < I2 < Fe3+ < Ag+. ✓
05
Explain
[2 marks]

Explain why a salt bridge made with potassium chloride is not suitable for a cell containing an Ag+/Ag half-cell, and give a suitable alternative.

  • Cl− ions would react with Ag+ to precipitate AgCl, changing [Ag+] and the potential. ✓
  • Use potassium nitrate, KNO3(aq), whose ions do not react. ✓
06
Analysis
[2 marks]

The E⊖ for Zn2+/Zn is −0.76 V. State the polarity of the zinc electrode when connected to the SHE and the direction of electron flow in the external circuit.

  • Zinc is the negative electrode (it is oxidised: Zn → Zn2+ + 2e−). ✓
  • Electrons flow through the wire from the zinc to the platinum of the SHE. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.