Equilibrium Chemistry - Redox Equilibria

Redox Equilibria

A reduction-oxidation (redox) equilibrium can be handled in exactly the same way as any other chemical equilibrium. For example

However, in the case of redox reactions it is convenient to split the overall reaction into two half-reactions. In this example

The standard free energy change, which is related to the equilibrium constant by

can be split into two components,

The concentration of free electrons is effectively zero as the electrons are transferred directly from the reductant to the oxidant. The standard electrode potential, E0 for the each half-reaction is related to the standard free energy change by

where n is the number of electrons transferred and F is the Faraday constant. Now, the free energy for an actual reaction is given by

where R is the gas constant and Q a reaction quotient. Strictly speaking Q is a quotient of activities, but it is common practice to use concentrations instead of activities. Therefore

For any half-reaction, the redox potential of an actual mixture is given by the generalized expression

This is an example of the Nernst equation. The potential is known as a reduction potential. Standard electrode potentials are available in a table of values. Using these values, the actual electrode potential for a redox couple can be calculated as a function of the ratio of concentrations.

The equilibrium potential for a general redox half-reaction (See #Equilibrium constant above for an explanation of the symbols)

is given by

Use of this expression allows the effect of a species not involved in the redox reaction, such as the hydrogen ion in a half-reaction such as

MnO₄- + 8H+ +5e- Mn2+ + 4H₂O

to be taken into account.

The equilibrium constant for a full redox reaction can be obtained from the standard redox potentials of the constituent half-reactions. At equilibrium the potential for the two half-reactions must be equal to each other and, of course, the number of electrons exchanged must be the same in the two half reactions.

Redox equilibria play an important role in the electron transport chain. The various cytochromes in the chain have different standard redox potentials, each one adapted for a specific redox reaction. This allows, for example, atmospheric oxygen to be reduced in photosynthesis. A distinct family of cytochromes, the cytochrome P450 oxidases, are involved in steroidogenesis and detoxification.