Which half-equation runs as reduction in an electrochemical cell?

The conventional reduction half-equation with the more positive standard electrode potential has the greater tendency to run in the reduction direction. The other half-equation is reversed for oxidation.

Do you multiply E° when you multiply a half-equation?

No. Standard electrode potential is an intensive potential. Scale the half-equation to balance electrons but keep the same E° value.

Should I write the E° of zinc in an exam?

It is safer and chemically more precise to name the redox couple, for example E°(Zn2+/Zn), then separately identify Zn as the species being oxidised if the reaction runs backwards.

AQA 3.1.11.1 · OCR A 5.2.3 · Year 13

Electrode potentials and electrochemical cells

Learn what an E° value actually refers to, build the physical electrochemical cell, and practise the reasoning and wording that AQA and OCR A expect.

Start a generated exam set Open Cell Lab

more positive E°

MnO₄⁻/Mn²⁺+1.51 V

Ag⁺/Ag+0.80 V

Fe³⁺/Fe²⁺+0.77 V

Cu²⁺/Cu+0.34 V

H⁺/H₂0.00 V

Zn²⁺/Zn−0.76 V

more negative E°

Choose a route

Get the chemistry straight, then make it exam-safe

1Generated exam setsOriginal multi-part questions with linked calculations, explanations, unfamiliar data and explicit mark points.Start a set → 2Single-skill exam practiceE°cell, direction, oxidising and reducing agents, half-equations and precise explanations.Practise → 3E° ExplorerChoose any two redox couples and see which reduction runs forwards, what is oxidised and the resulting overall equation.Compare couples → 4E versus E° labChange concentration or gas pressure, move the half-cell equilibrium and predict the actual electrode potential.Change the conditions → 5Electrochemical Cell LabBuild half-cells, choose metal or platinum electrodes, add the salt bridge and work out polarity and electron flow.Build a cell → 6Standard hydrogen electrodeBuild the 0.00 V reference correctly: Pt, H₂, H⁺ and every standard condition.Build SHE → 7AQA conventional cell notationConstruct representations with the correct order, phase boundaries, salt bridge and Pt electrodes.Build notation → 8Feasibility & advanced redoxE versus E°, concentration, kinetic limits, selective reductions, disproportionation and catalyst cycles.Practise advanced redox → 9Fuel cells & rechargeable cellsDischarge, recharging, commercial-cell data, AQA lithium chemistry and board-aware fuel-cell practice.Practise commercial cells → 10Hard mixed questionsUnfamiliar couples, missing data and mixed reasoning across the whole electrode-potential topic.Try harder →

Start with the convention

Every tabulated E° half-equation is written as a reduction

For E°(Zn²⁺/Zn) = −0.76 V, the table is referring to the reduction half-equation Zn²⁺ + 2e⁻ ⇌ Zn. In an actual cell, zinc metal may run in the opposite direction and be oxidised.

The more positive E° couple has the greater tendency to run in the reduction direction. The other half-equation is reversed for the cell reaction. Multiplying a half-equation to balance electrons does not multiply E°.

Tabulated reductionZn²⁺ + 2e⁻ ⇌ ZnE°(Zn²⁺/Zn) = −0.76 V

What can happen in a cellZn → Zn²⁺ + 2e⁻The reaction reverses; the tabulated E° notation does not become E°(Zn/Zn²⁺).

Examiner-report lesson

Name the couple, then name the reacting species

AQA examiner reports repeatedly flag vague phrases such as “the E° of zinc”. OCR is usually less fussy, but precise chemistry is safe on both boards.

Too vague

“Zinc has a lower electrode potential, so it reacts.”

Exam-safe

E°(Zn²⁺/Zn) is more negative than E°(V³⁺/V²⁺), so Zn is oxidised and V³⁺ is reduced to V²⁺.

Away from standard conditions

E° is not the voltage under every set of conditions

If concentration or gas pressure changes, treat the half-equation as an equilibrium. A shift that increases the tendency for the reduction to occur makes the actual electrode potential more positive; a shift away from reduction makes E less positive.

The tabulated E° value stays fixed. At A-level you can reason this out without using the Nernst equation.

Try the E versus E° lab →

Standard valueE°Defined for standard conditions

Actual half-cellEDepends on the conditions now present

Common mistakes to avoid

Calling an E° value “the E° of Zn” rather than naming Zn²⁺/Zn. Adding E° values or multiplying E° when a half-equation is scaled. Choosing the wrong species as oxidising or reducing agent. Reversing the chemistry but also reversing the conventional couple notation. Getting the overall reaction right but leaving identical species on both sides. Using “higher/lower” vaguely where “more positive/more negative” says exactly what is meant. Sending electrons through the salt bridge instead of the external wire. Using a reactive metal electrode where an aqueous/aquous redox couple needs inert Pt. Calling a cell standard when the relevant ion concentrations or gas pressure are not standard. Confusing a kinetic change such as electrode surface area with a change in equilibrium electrode potential.

From half-cells to real devices

Build the physical cell for the practical side, or carry the same redox reasoning into storage cells, recharging and fuel cells. The commercial-cell route changes what it expects you to recall depending on AQA or OCR A.

Open Cell Lab Commercial cells