One electrode dipping into a solution containing the oxidised and reduced forms of a species in equilibrium.

What does a salt bridge do?

Completes the circuit by letting ions flow, and keeps each half-cell electrically neutral.

Why use a platinum electrode in an Fe³⁺(aq)/Fe²⁺(aq) half-cell?

There is no metal to act as an electrode; platinum is inert and conducts electrons to and from the ions.

What is the standard hydrogen electrode (SHE)?

H₂(g) at 100 kPa over a platinum electrode in 1.00 mol dm⁻³ H⁺(aq) at 298 K; E° = 0.00 V.

Define standard electrode potential.

The emf of a half-cell under standard conditions, measured relative to the standard hydrogen electrode.

State the standard conditions for measuring E°.

Ion concentrations 1.00 mol dm⁻³, temperature 298 K, gas pressure 100 kPa.

How is E°cell calculated?

E°cell = E°(positive electrode) − E°(negative electrode), i.e. E°(right) − E°(left) in a cell diagram.

Are E° values multiplied by the number of electrons in the balanced equation?

No. Electrode potentials do not depend on the amount of substance.

Which electrode is written on the left of a cell diagram?

The negative electrode (more negative E°), where oxidation occurs.

What do | and || mean in a cell diagram?

| is a phase boundary; || is the salt bridge.

Write the cell diagram for a cell of Zn²⁺/Zn and Cu²⁺/Cu.

Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), E°cell = +1.10 V.

Why must a high-resistance voltmeter be used?

So almost no current flows, concentrations do not change and the reading is the true emf.

Why does lowering [Cu²⁺] make the Cu²⁺/Cu electrode less positive?

Cu²⁺ + 2e⁻ ⇌ Cu shifts left, so the electrode has less tendency to accept electrons.

Electrochemical cells and cell diagramsEdexcel A-Level Chemistry: Flashcards

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What is a half-cell?

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What is a half-cell?: One electrode dipping into a solution containing the oxidised and reduced forms of a species in equilibrium.
What does a salt bridge do?: Completes the circuit by letting ions flow, and keeps each half-cell electrically neutral.
Why use a platinum electrode in an Fe³⁺(aq)/Fe²⁺(aq) half-cell?: There is no metal to act as an electrode; platinum is inert and conducts electrons to and from the ions.
What is the standard hydrogen electrode (SHE)?: H₂(g) at 100 kPa over a platinum electrode in 1.00 mol dm⁻³ H⁺(aq) at 298 K; E° = 0.00 V.
Define standard electrode potential.: The emf of a half-cell under standard conditions, measured relative to the standard hydrogen electrode.
State the standard conditions for measuring E°.: Ion concentrations 1.00 mol dm⁻³, temperature 298 K, gas pressure 100 kPa.
How is E°cell calculated?: E°cell = E°(positive electrode) − E°(negative electrode), i.e. E°(right) − E°(left) in a cell diagram.
Are E° values multiplied by the number of electrons in the balanced equation?: No. Electrode potentials do not depend on the amount of substance.
Which electrode is written on the left of a cell diagram?: The negative electrode (more negative E°), where oxidation occurs.
What do | and || mean in a cell diagram?: | is a phase boundary; || is the salt bridge.
Write the cell diagram for a cell of Zn²⁺/Zn and Cu²⁺/Cu.: Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), E°cell = +1.10 V.
Why must a high-resistance voltmeter be used?: So almost no current flows, concentrations do not change and the reading is the true emf.
Why does lowering [Cu²⁺] make the Cu²⁺/Cu electrode less positive?: Cu²⁺ + 2e⁻ ⇌ Cu shifts left, so the electrode has less tendency to accept electrons.

Exam questions on Electrochemical cells and cell diagrams

A teacher builds a cell from a zinc half-cell and a copper half-cell. Each metal strip dips into a 1.00 mol dm⁻³ solution of its own ions at 298 K, the two solutions are joined by a salt bridge, and a high-resistance voltmeter connects the metals. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V.
Explain the function of the salt bridge in this cell.2 marks
A student investigates a cell made from an Fe³⁺(aq)/Fe²⁺(aq) half-cell and an Ag⁺(aq)/Ag(s) half-cell. Standard electrode potentials: Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V; Ag⁺(aq) + e⁻ ⇌ Ag(s), E° = +0.80 V.
Write the conventional cell diagram for this cell, including state symbols.2 marks
A student wants to measure the standard electrode potential of the Cu²⁺(aq)/Cu(s) half-cell by connecting it to a standard hydrogen electrode (SHE).
Describe the standard hydrogen electrode, including the conditions needed.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).