What is the oxidation number of Mn in MnO₄⁻?

+7 (four O at −2 total −8, and the ion is 1−).

Define standard electrode potential, E°.

The emf of a half-cell connected to a standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1.00 mol dm⁻³).

State the standard conditions for measuring E°.

Temperature 298 K, gas pressure 100 kPa, ion concentration 1.00 mol dm⁻³.

What is the half-equation of the standard hydrogen electrode?

2H⁺(aq) + 2e⁻ ⇌ H₂(g), with E° = 0.00 V.

Why is platinum used in the SHE?

It is inert and conducts electricity; platinum black has a large surface area that adsorbs H₂.

Why is a reference electrode needed?

Only a potential difference between two half-cells can be measured, not the potential of a single half-cell.

What is the role of the salt bridge?

It completes the circuit by letting ions move between the half-cells without the solutions mixing.

Why must the voltmeter have a high resistance?

So negligible current flows and the concentrations stay at their standard values.

How is E° measured for Cl₂|Cl⁻?

A platinum electrode in 1.00 mol dm⁻³ Cl⁻(aq) with Cl₂ gas at 100 kPa bubbled over it, connected to the SHE.

How is E° measured for Fe³⁺|Fe²⁺?

A platinum electrode in a solution of 1.00 mol dm⁻³ Fe³⁺ and 1.00 mol dm⁻³ Fe²⁺, connected to the SHE.

What does a very positive E° tell you?

The species on the left of the half-equation is a strong oxidising agent, as it gains electrons readily.

Standard electrode potentials and the hydrogen electrodeEdexcel International A Level Chemistry: Flashcards

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Define oxidation in terms of electrons and oxidation number.

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Define oxidation in terms of electrons and oxidation number.: Loss of electrons; an increase in oxidation number.
What is the oxidation number of Mn in MnO₄⁻?: +7 (four O at −2 total −8, and the ion is 1−).
Define standard electrode potential, E°.: The emf of a half-cell connected to a standard hydrogen electrode under standard conditions (298 K, 100 kPa, 1.00 mol dm⁻³).
State the standard conditions for measuring E°.: Temperature 298 K, gas pressure 100 kPa, ion concentration 1.00 mol dm⁻³.
What is the half-equation of the standard hydrogen electrode?: 2H⁺(aq) + 2e⁻ ⇌ H₂(g), with E° = 0.00 V.
Why is platinum used in the SHE?: It is inert and conducts electricity; platinum black has a large surface area that adsorbs H₂.
Why is a reference electrode needed?: Only a potential difference between two half-cells can be measured, not the potential of a single half-cell.
What is the role of the salt bridge?: It completes the circuit by letting ions move between the half-cells without the solutions mixing.
Why must the voltmeter have a high resistance?: So negligible current flows and the concentrations stay at their standard values.
How is E° measured for Cl₂|Cl⁻?: A platinum electrode in 1.00 mol dm⁻³ Cl⁻(aq) with Cl₂ gas at 100 kPa bubbled over it, connected to the SHE.
How is E° measured for Fe³⁺|Fe²⁺?: A platinum electrode in a solution of 1.00 mol dm⁻³ Fe³⁺ and 1.00 mol dm⁻³ Fe²⁺, connected to the SHE.
What does a very positive E° tell you?: The species on the left of the half-equation is a strong oxidising agent, as it gains electrons readily.

Exam questions on Standard electrode potentials and the hydrogen electrode

Acidified potassium dichromate(VI) is used to oxidise iron(II) ions to iron(III) ions in a laboratory analysis. The ionic equation for the reaction is: Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
Explain, in terms of electrons and oxidation number, why the dichromate(VI) ion acts as an oxidising agent in this reaction.2 marks
A student sets up a standard hydrogen electrode (SHE) to use as the reference when measuring electrode potentials.
Explain why platinum is used as the electrode in the standard hydrogen electrode.2 marks
A student measures the standard electrode potential of the Fe³⁺(aq)|Fe²⁺(aq) half-cell and, separately, of the Zn²⁺(aq)|Zn(s) half-cell, in each case by connecting the half-cell to a standard hydrogen electrode.
Describe how the student would set up the Fe³⁺(aq)|Fe²⁺(aq) half-cell and complete the circuit so that its standard electrode potential can be measured.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).