Nernst equation

The standard cell potential (E°) applies to cells where all substances are present in their standard state (1 mol L^–1 for solutes and 100 kPa for gases). The cell potential (E) for cells having substances in nonstandard concentrations or pressures can be calculated using the the Nernst equation (1).

(1) E = E° –	RT	ln Q
	zF

RT/F = 0.0257 V at 298 K
z = the number of electrons transferred from oxidant to reductant
Q is reaction quotient for the cell reaction into which the nonstandard concentrations and pressures are substituted.

When the system is at equilibrium there is no difference in potential between the electrodes
Thus E = 0 and Q = K .

(1) becomes (2) at equiliibrium

(2) E° =	RT	ln K
	zF

(2)

Hence knowledge of E for a cell under nonstandard conditions allows calculation of the equilibrium constant K for the cell reaction by

first using the Nernst equation to calculate E^o and
then using E^o to calculate K.