R10 Non-standard reduction potentials

Illustration R10 shows the and actual reduction potential values, E, for the two half-reaction equations:

at various pH's

For each half-reaction equation there are four different representations depending on the pH of the medium in which they are to be found: two for bases (with OH^-) and two with acids (with H⁺). However only three E values are shown for each half-reaction equation.
For the top set of E values on illustration R10 i.e. the half-reaction 2H⁺/H₂ :

The middle values are not standard reduction potentials because the concentration of H⁺ is not 1 mol/L and therefore the concentration is quoted in the table. Lack of space prevents either the units or the physical state of the reactants being quoted. Strictly speaking these should be quoted as (g) (l) (s) (aq) depending on whether the reagent is a gas, a liquid, a solid or an aqueous solution. The values given at the top and bottom of the table are the standard reduction potentials for each of the half-reactions. Fundamentally the process is the same, the reduction of hydrogen, H, from oxidation number +I to 0. The values can be found in standard tables, but the non-standard reduction potentials (middle values) are not. These can be calculated at 25°C for the half-reaction 2H⁺/H₂ using the Nernst-equation:

Thus :

For the bottom set of E values on illustration R10 i.e. the half-reaction O₂/2H₂O :

The process represents the reduction of oxygen from oxidation number 0 to -II. Again the non-standard reduction potential can be calculated using the Nernst-equation.

Note :

Most reactions in neutral aqueous solution are carried out at non-standard reduction potentials. For the half-reaction H⁺/H₂ the reduction potential used is -0.41 V and for the half-reaction O₂/H₂O this is +0.81 V.
Illustration RR10 shows that the value does not change whether the half-reaction is written with OH^- or H⁺.
It can be concluded that hydrogen is a stronger reducing agent and oxygen a stronger oxidising agent in water at pH 7 than under standard conditions.
In a neutral solution hydrogen ions, which are present in very small concentration, are less likely to oxidise zinc metal than in an acid solution in which their concentration is high.

This model can also be extended to consider oxygen gas in acid solution in which it is more likely to oxidise iron than in a neutral solution because the concentration of hydrogen ions is greater.