How is the EMF of a cell calculated from electrode potentials?

EMF = E(positive electrode) − E(negative electrode).

Which half-cell is the negative electrode when a cell supplies current?

The one with the more negative electrode potential, where oxidation occurs.

Why can a rechargeable cell be recharged?

Its electrode reactions are reversible and the products stay on the electrodes, so an external voltage can drive the reactions backwards.

What voltage must be applied to recharge a cell?

One greater than the EMF of the cell, connected so that it reverses the electrode reactions.

Write the positive electrode reaction of a lithium cell.

Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻

What happens to cobalt at the positive electrode of a lithium cell?

It is reduced from oxidation state +4 to +3.

Why does a fuel cell not need electrical recharging?

The fuel and oxygen are supplied continuously, so it gives current as long as they are fed in.

Write the negative electrode reaction in an alkaline hydrogen–oxygen fuel cell.

H₂ + 2OH⁻ → 2H₂O + 2e⁻

Write the positive electrode reaction in an alkaline hydrogen–oxygen fuel cell.

O₂ + 2H₂O + 4e⁻ → 4OH⁻

Give one risk of using hydrogen fuel cells.

Hydrogen is flammable and hard to store, and it is often made using fossil fuels.

Commercial electrochemical cellsAQA A-Level Chemistry: Flashcards

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What are the three types of commercial cell?

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What are the three types of commercial cell?: Non-rechargeable (irreversible), rechargeable and fuel cells.
How is the EMF of a cell calculated from electrode potentials?: EMF = E(positive electrode) − E(negative electrode).
Which half-cell is the negative electrode when a cell supplies current?: The one with the more negative electrode potential, where oxidation occurs.
Why can a rechargeable cell be recharged?: Its electrode reactions are reversible and the products stay on the electrodes, so an external voltage can drive the reactions backwards.
What voltage must be applied to recharge a cell?: One greater than the EMF of the cell, connected so that it reverses the electrode reactions.
Write the negative electrode reaction of a lithium cell.: Li → Li⁺ + e⁻
Write the positive electrode reaction of a lithium cell.: Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻
What happens to cobalt at the positive electrode of a lithium cell?: It is reduced from oxidation state +4 to +3.
Why does a fuel cell not need electrical recharging?: The fuel and oxygen are supplied continuously, so it gives current as long as they are fed in.
Write the negative electrode reaction in an alkaline hydrogen–oxygen fuel cell.: H₂ + 2OH⁻ → 2H₂O + 2e⁻
Write the positive electrode reaction in an alkaline hydrogen–oxygen fuel cell.: O₂ + 2H₂O + 4e⁻ → 4OH⁻
What is the EMF of the alkaline H₂/O₂ fuel cell?: +0.40 − (−0.83) = +1.23 V.
Give one risk of using hydrogen fuel cells.: Hydrogen is flammable and hard to store, and it is often made using fossil fuels.

Exam questions on Commercial electrochemical cells

A manufacturer makes rechargeable nickel–cadmium cells for cordless power tools. The cell is based on two half-equations with standard electrode potentials: NiO(OH) + H₂O + e⁻ ⇌ Ni(OH)₂ + OH⁻, E = +0.52 V; and Cd(OH)₂ + 2e⁻ ⇌ Cd + 2OH⁻, E = −0.88 V. The electrolyte is aqueous alkali.
Explain why this cell can be recharged.2 marks
A lithium cell in a mobile phone uses a lithium-containing negative electrode and a cobalt(IV) oxide positive electrode. When the cell supplies current, the simplified electrode reactions are: negative electrode, Li → Li⁺ + e⁻; positive electrode, Li⁺ + CoO₂ + e⁻ → Li⁺[CoO₂]⁻.
Explain how the electrode reactions in this cell generate an electric current.2 marks
A bus company is trialling hydrogen fuel-cell buses that use an alkaline hydrogen–oxygen fuel cell. Hydrogen and oxygen are fed continuously to separate porous electrodes in contact with aqueous potassium hydroxide. The relevant electrode potentials are: O₂ + 2H₂O + 4e⁻ ⇌ 4OH⁻, E = +0.40 V; and 2H₂O + 2e⁻ ⇌ H₂ + 2OH⁻, E = −0.83 V.
Deduce the equation for the reaction at each electrode when the cell is supplying current, and calculate the EMF of the cell.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).