Electrode Potentials — A-Level Chemistry Revision

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What is Electrode Potentials?

Electrode potentials quantify the tendency of a species to be reduced and gain electrons. Standard electrode potentials (E°) are measured relative to the standard hydrogen electrode (SHE) under standard conditions. These values are used to calculate the electromotive force (e.m.f.) of an electrochemical cell and to predict the feasibility of redox reactions. A more positive E° value indicates a greater tendency for a species to be reduced.

Board notes: All boards cover the calculation of cell e.m.f. and the prediction of redox reaction feasibility. AQA often includes questions on different types of electrochemical cells, including non-rechargeable, rechargeable, and fuel cells. Edexcel may focus on the industrial applications of electrolysis, linking it to electrode potentials. OCR frequently tests the construction of conventional cell diagrams and the role of the salt bridge.

Worked example

Common mistakes

• 1Getting the formula for cell e.m.f. the wrong way around. The correct formula is E°cell = E°(positive terminal) - E°(negative terminal), which is equivalent to E°(reduced species) - E°(oxidised species). The result must be positive for a feasible reaction.
• 2Confusing the direction of electron flow. In an electrochemical cell, electrons always flow from the more negative half-cell to the more positive half-cell, i.e., from the site of oxidation to the site of reduction.
• 3Forgetting that standard conditions are required for E° values to be valid. These conditions are 298K temperature, 1.00 mol dm-3 concentration of ions, and 100 kPa pressure for any gases.

Electrode Potentials exam questions

Exam-style questions for Electrode Potentials with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel and OCR specifications.

Frequently asked questions

• What is the standard hydrogen electrode (SHE)?

  The SHE is the reference standard for all other electrode potentials. It consists of hydrogen gas bubbling over a platinum electrode in a solution of H+ ions at 1 mol dm-3 concentration. Its electrode potential is defined as exactly 0.00V under standard conditions.

• Can a reaction with a positive E°cell still not happen?

  Yes. Standard electrode potentials only predict thermodynamic feasibility. They do not give any information about the rate of reaction. A reaction may have a very high activation energy, making it kinetically stable and proceed immeasurably slowly, even if it is thermodynamically feasible.