What is the standard hydrogen electrode?

A platinum electrode in 1.00 mol dm⁻³ H⁺ with H₂ gas at 100 kPa and 298 K, defined as 0.00 V.

What are standard conditions for electrode potentials?

298 K, 100 kPa and 1.00 mol dm⁻³ solutions of ions.

What is the purpose of the salt bridge?

To complete the circuit by allowing ions to flow between the half-cells without the solutions mixing.

Why is a high-resistance voltmeter used?

So that almost no current flows and the EMF measured is the maximum value.

Which electrode is on the left of a cell diagram?

The more negative electrode, where oxidation occurs.

How is the EMF of a cell calculated?

E°(positive electrode) − E°(negative electrode).

What does a more positive E° tell you about a species?

The species on the left is a stronger oxidising agent.

What does a positive E°cell show?

The reaction is feasible under standard conditions.

Why is a platinum electrode used in an Fe³⁺/Fe²⁺ half-cell?

There is no solid metal, so an inert conducting electrode is needed.

Why must conditions be stated when measuring electrode potential?

Changing concentration, temperature or pressure changes the equilibrium position and so E.

Do you multiply E° by the number of electrons when balancing?

No. E° is unchanged when a half-equation is multiplied.

Electrode potentials and cellsAQA A-Level Chemistry: Flashcards

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In which form are half-equations written by IUPAC convention?

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In which form are half-equations written by IUPAC convention?: As reductions: oxidised form + ne⁻ ⇌ reduced form.
What is the standard hydrogen electrode?: A platinum electrode in 1.00 mol dm⁻³ H⁺ with H₂ gas at 100 kPa and 298 K, defined as 0.00 V.
What are standard conditions for electrode potentials?: 298 K, 100 kPa and 1.00 mol dm⁻³ solutions of ions.
What is E° for the standard hydrogen electrode?: 0.00 V (by definition).
What is the purpose of the salt bridge?: To complete the circuit by allowing ions to flow between the half-cells without the solutions mixing.
Why is a high-resistance voltmeter used?: So that almost no current flows and the EMF measured is the maximum value.
What does a double vertical line mean in a cell diagram?: The salt bridge.
Which electrode is on the left of a cell diagram?: The more negative electrode, where oxidation occurs.
How is the EMF of a cell calculated?: E°(positive electrode) − E°(negative electrode).
What does a more positive E° tell you about a species?: The species on the left is a stronger oxidising agent.
What does a positive E°cell show?: The reaction is feasible under standard conditions.
Why is a platinum electrode used in an Fe³⁺/Fe²⁺ half-cell?: There is no solid metal, so an inert conducting electrode is needed.
Why must conditions be stated when measuring electrode potential?: Changing concentration, temperature or pressure changes the equilibrium position and so E.
Do you multiply E° by the number of electrons when balancing?: No. E° is unchanged when a half-equation is multiplied.

Exam questions on Electrode potentials and cells

A cell is set up with a zinc half-cell and a copper half-cell joined by a salt bridge. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V.
Write the equation for the overall cell reaction and state the direction of electron flow in the external circuit.2 marks
A student measures the standard electrode potential of the Ag⁺(aq) | Ag(s) half-cell by connecting it to a standard hydrogen electrode using a salt bridge and a voltmeter.
Explain why a high-resistance voltmeter is used and what the purpose of the salt bridge is.2 marks
A chemist investigates the reactions of aqueous iron(II) and iron(III) ions with halogens and halide ions at standard conditions. Standard electrode potentials: Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V; I₂(aq) + 2e⁻ ⇌ 2I⁻(aq), E° = +0.54 V; Br₂(aq) + 2e⁻ ⇌ 2Br⁻(aq), E° = +1.07 V.
Use the electrode potentials to deduce whether iron(III) ions will oxidise iodide ions. Calculate the EMF and write an equation for any reaction.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).