Electron Activities and Free Energy Changes

In the same way that acid-base reactions involve the transfer of protons between proton donors and proton acceptors, redox reactions involve the transfer of electrons between electron donors, called reducing agents or reductants, and electron acceptors, called oxidizing agents or oxidants. Thus when a redox reaction takes place, a reductant loses electrons and is oxidized to its conjugate oxidaut  

The Biogeochemistry of Submerged Soils Gity Bvirk 2004 John Wiley Sons, Ltd ISBN 0-470-86301-3 

Which redox couple in a redox reaction has the oxidizing role and which the redncing role depends on the relative abilities of the two couples to accept or donate electrons. For example O2 has a greater affinity for electrons than other potential oxidants in natnral systems, and is therefore reduced preferentially. The means of quantifying the relative abilities of redox conples to accept or donate electrons and the corresponding free energy changes is as follows. 

Jnst as free protons do not exist in solution in acid-base reactions, there are no free electrons in redox reactions. However it is possible to define the activity of electrons relative to a specified standard state and thereby treat electrons as discrete species in equilibrinm calcnlations in the same way as ions and molecules. The standard state of electron activity for this pnrpose is by convention defined with respect to the redox conple made by hydrogen ions and hydrogen gas  

By convention the standard free energy change, AG°, for this reaction is set at zero, i.e. since AG° = —RT nK  

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