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

Start by pinning each E° value to its redox couple, then build the physical 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 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. Reverse the other half-equation for the cell reaction. E° stays unchanged when you scale a half-equation to balance electrons.

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

What can happen in a cellZn → Zn²⁺ + 2e⁻The reaction runs as oxidation here; keep the tabulated E° notation as E°(Zn²⁺/Zn).

Exam precision

Name the couple, then name the reacting species

When I check an electrode-potential explanation, I want to see the redox couple and the reacting species named. That keeps the direction of oxidation and reduction clear on either board.

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° applies to standard conditions

If concentration or gas pressure changes, treat the half-equation as an equilibrium. A shift towards reduction makes the actual electrode potential more positive; a shift towards oxidation 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”; name the couple 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 conventional E° couple notation when the reaction runs as oxidation. Leaving identical species on both sides of the overall reaction. Using “higher/lower” vaguely where “more positive/more negative” says exactly what is meant. Putting electron flow through the salt bridge; electrons travel through the external wire. Using a reactive metal electrode where an aqueous/aquous redox couple needs inert Pt. Calling a non-standard cell standard; check the relevant ion concentrations and gas pressure. 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