A question which sometimes arises is why, if the reduction potentials of copper(II) to copper(I) and copper(I) to copper metal are added, the result is not the same as the reduction potential of copper(II) to copper metal?

As is often the case, the explanation can only be given in terms of free energy changes – and since entropy is excluded from the London scheme this can only be covered properly if your teacher is prepared to move somewhat outside the syllabus. A simple coverage of entropy and its importance to chemical processes is given in the ‘Why do reactions go?’ page; here we take the results and apply them to electrochemical cells. Note that this material is not examinable under the London scheme.

The relationship between the free energy change DG and the potential of an electrochemical cell is

DG = -nFE^o

where F is the Faraday constant, 96485 C mol^-1 – the charge on a mole of electrons – and n is the number of electrons transferred in the reaction. Note this is the moles of electrons per mole of reaction, not the total number, which would be pretty big!

The electrode potentials do not, indeed, add up; however the free energy changes do. This is shown below; the free energy change for the conversion of Cu²⁺ to Cu is the same as that for the conversion of Cu²⁺ to Cu⁺ plus that of Cu⁺ to Cu.

Bearing in mind the experimental error, the two values of DG are the same.

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Dr Rod Beavon 17 Dean’s Yard London SW1P 3PB 

e-mail:  rod.beavon@westminster.org.uk