# Standard Potential

The overall energy of the outer electrons for a material depends on the temperature of the systems and also, for a material that is not a pure solid, also depends on the concentration of the reacting species. Thus, in measuring the electrochemical potentials, both the temperature and the concentration must be specified. The standard potential, given the symbol E0, is the electrochemical potential at 25 °C, with gasses having a pressure of 1 atm and solutions a concentration of 1 Molar (1 M or 1 mol/liter).

When measuring standard potentials, as with all voltages, only a difference in potential or voltage can be measured and hence only a potential between an oxidation and reduction reaction can be measured. To determine the potential of a single oxidation or reduction reaction, the potential of one reaction is measured compared to the potential of a specified reactions, which is oxidation of H. The standard potential of this reaction is by definition set to zero.

## Hydrogen Reduction

$2{H}^{+}\left(aq\right)+2{e}^{-}\to {H}_{2}\left(g\right)$

The standard potential of the reduction /oxidation reaction pair is the sum of the standard potentials for the half reactions (to avoid continually having to say the oxidation and reduction reaction, they are more generally termed a half reactions, since they make up half of the overall redox reaction). The values of common half reactions are listed in Tables of Standard Potentials. The standard potential of a redox reaction is used to determine if a redox reaction will occur spontaneously (ie if it will generate a voltage between the reduction and oxidation reaction). If the difference between the standard potentials is positive, then the reaction will proceed spontaneously. If the standard potential is negative, a voltage needs to be applied in order for the reaction to proceed.

From the table of standard potentials for reduction reactions for copper and zinc are:

## Copper Reduction with Standard Potential

The zinc undergoes an oxidation reaction, so the direction of the reaction and the sign of the standard potential must be reversed to give the oxidation/reduction reaction pair is:

## Copper Reduction with Standard Potential

and the standard potential for the reaction is 1.1V.