Nernst equation In electrochemistry the Nernst equation gives the reduction potential of a half‐cell in equilibrium. In further cases, it can also be used to determine the emf (electromotive force) for a full electrochemical cell.
(half‐cell reduction potential)
(total cell potential) where
Ered is the half‐cell reduction potential at a certain T o E red is the standard half‐cell reduction potential
Ecell is the cell potential (electromotive force) o E cell is the standard cell potential at a certain T R is the universal gas constant: R = 8.314 472(15) J K−1 mol−1 T is the absolute temperature in Kelvin
a is the chemical activity for the relevant species, where aRed is the reductant and aOx is the oxidant F is the Faraday constant; F = 9.648 533 99(24)×104 C mol−1 z is the number of electrons transferred in the cell reaction or half‐reaction Q is the reaction quotient (e.g. molar concentrations, partial pressures …)
As the system is considered not to have reached equilibrium, the reaction quotient Q is used instead of the equilibrium constant k.
The electrochemical series is used to determine the electrochemical potential or the electrode potential of an electrochemical cell. These electrode potentials are measured relatively to the standard hydrogen electrode. A reduced member of a couple has a thermodynamic tendency to reduce the oxidized member of any couple that lies above it in the series.
The standard hydrogen electrode is a redox electrode which forms the basis of the thermodynamic scale of these oxidation‐ reduction potentials. For a comparison with all other electrode reactions, standard electrode potential E0 of hydrogen is defined to be zero at all temperatures.
+ ‐ 0 + + 2H (aq) + 2e → H2 (g) E (H / H2) = 0 for p(H2) = 1 bar and c(H ) = 1mol/L
References:
Shriver & Atkins, Inorganic Chemistry, Oxford University Press, Fourth edition, 2006, p. 186
Questions:
1. Calculate the emf of the following cell at 25°C: Sn(s)|Sn2+(0.025M)||Ag+(2M)|Ag(s) 2. Why is platinized platinum used in the standard hydrogen electrode instead of normal solid platinum?