Atoms and Molecules

Redox Reactions

Oxidant                                 molecule which accepts electrons
Reductant                            molecule which donates electrons
Redox pair/couple             an electron-donating molecule and its oxidised form
Half-cell                                 one electrode in an electrolytic cell and the solution in contact

In oxidation-reduction reactions, the ability to donate or accept electrons is given by the redox potential, E. This is related to the standard redox potential, E°' as given below

A positive E°' indicates ability to accept electrons (i.e. it is an oxidising agent)
A negative E°' indicates ability to donate electrons (i.e. it is a reducing agent)

Negative electrons are passed from materials of lower E°' to materials with a more positive E°'

DG°'= -nFE°'
where     F = Faraday (96,485 J V^-1 mol^-1)
      and       n = number of electrons transferred

the Nernst equation

consider

2H₂ + O₂ 2H₂O
DG°' =    -474.3 kJ mol^-1

The half reactions are: (the electrons (e^-) are from an electrode)

    easy reduction of oxygen               hard oxidation of water
O₂ + 4H⁺ + 4e^- 2H₂O
E°' = +0.815 V

hard reduction of hydrogen ions                easy oxidation of hydrogen
2H⁺ + 2e^- H₂
E°' = -0.414 V

A compound with the more positive potential will oxidise the reduced form of a substance of lower (more negative) potential.

Electrons flow from negative potential to positive potential. In the following example, the electrons flow from the hydrogen (E°' = -0.414 V) to the oxygen (E°' = +0.815 V).

O₂ + 4H⁺ + 4e^- 2H₂O         E°' = +0.815 V
           2H₂ 4H⁺ + 4e^-             E°' = -(-0.414 V)
                     2H₂ + O₂ 2H₂O            E°' = +0.815 + 0.414V

    DG°' = -nFE°'
    DG°' = -4 x 96.49 x 1.229 kJ mol^-1
    DG°' = -474.3 kJ mol^-1

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This page was last updated by Martin Chaplin
on 10 February, 2005