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Electrochemistry

14 min read

Electrode potentials

A half-cell is a metal in a solution of its ions. A metal tends to lose electrons (be oxidised) to a different extent in different half-cells. The standard electrode potential E° of a half-cell is its voltage measured against the standard hydrogen electrode (platinum in 1 mol/dm³ H⁺ with hydrogen gas at 1 atmosphere, 298 K), which is given E° = 0.00 V.

Half-reaction (reduction)E° / V
Li⁺ + e⁻ → Li−3.04
Zn²⁺ + 2e⁻ → Zn−0.76
Fe²⁺ + 2e⁻ → Fe−0.44
2H⁺ + 2e⁻ → H₂0.00
Cu²⁺ + 2e⁻ → Cu+0.34
Ag⁺ + e⁻ → Ag+0.80
Cl₂ + 2e⁻ → 2Cl⁻+1.36
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O+1.51

Electrochemical cells

The Daniell cellTwo beakers. One has a zinc rod in zinc sulphate solution and the other a copper rod in copper sulphate solution. A salt bridge joins the solutions and a wire with a voltmeter joins the rods. Zinc is oxidised to zinc ions at the negative electrode, and electrons flow through the wire to the copper, where copper ions are reduced to copper.Vsalt bridgee⁻Zn²⁺Cu²⁺Zn → Zn²⁺ + 2e⁻Cu²⁺ + 2e⁻ → Cuanode (−) oxidationcathode (+) reductionE° = +1.10 V
The cathode is the half-cell with the more positive E° (reduction). A positive E°_cell means the reaction is spontaneous. n = electrons transferred, F = 96 500 C/mol.
Worked exampleThe Daniell cell

Find the standard cell potential of a zinc–copper cell. (E° Zn²⁺/Zn = −0.76 V; E° Cu²⁺/Cu = +0.34 V)

  1. Copper has the more positive E°, so it is the cathode.

Answer: E°_cell = +1.10 V; zinc dissolves and copper is deposited.

Try itCell voltage and free energy

Choose the two electrode potentials and the number of electrons transferred. See the cell voltage and the free energy change.

Effect of concentration: the Nernst equation

At 298 K, Q is the reaction quotient. Changing the concentration of an ion changes the electrode potential.

Electrolysis, batteries and fuel cells

CellReactionsUse
Dry cell (Leclanché)Zinc case is the negative electrode; manganese(IV) oxide and carbon rod are positiveTorches, radios
Lead–acid accumulatorPb and PbO₂ in sulphuric acid, rechargeable, about 2 V per cellVehicle batteries
Lithium-ionLithium ions move between electrodesPhones, laptops, electric vehicles
Hydrogen–oxygen fuel cellH₂ is oxidised at the anode and O₂ is reduced at the cathode; the product is waterSpacecraft, clean vehicles
Hydrogen collects at the cathode and oxygen at the anode, in the ratio 2 : 1 by volume.
Check yourself In a galvanic cell, oxidation happens at the…
Check yourself E° Mg²⁺/Mg = −2.37 V and E° Cu²⁺/Cu = +0.34 V. What is E°_cell for the Mg/Cu cell?
Check yourself What is the standard hydrogen electrode potential?

Key terms

Standard electrode potential
The voltage of a half-cell measured against the standard hydrogen electrode under standard conditions.
Salt bridge
A link that lets ions move between half-cells and completes the circuit.
Galvanic cell
A cell that produces electricity from a spontaneous reaction.
Fuel cell
A cell that makes electricity from a fuel and oxygen supplied continuously.

Summary

  • E° values are measured against the standard hydrogen electrode.
  • E°_cell = E°_cathode − E°_anode; positive means spontaneous; ΔG° = −nFE°.
  • The Nernst equation links E to concentration.
  • Batteries and fuel cells turn chemical energy directly into electricity.
Chemical Kinetics Acids, Bases and Salts