Chemical Thermodynamics and the Equilibrium State
|The reduction potential measures the intrinsic tendency for a species to undergo reduction. How strongly does it pull electrons at its electrode? Elements can be arranged by order of proclivity to be reduced. The graph of reduction potentials has general agreement with the graph of electronegativities with the divergence deriving from the fact that the reduction potential also includes the contribution of the entropy term to the standard free energy change. Because in a redox process, one species is oxidized while the other is reduced, only the total cell potential can be measured. The individual reduction potentials are actually the cell potential with the substance up against a hydrogen electrode as a standard. This means that when you subtract the standard reduction potential of the anode reaction from the standard reduction potential of the cathode reaction to derive the standard cell potential, you are, in fact, applying Hess's Law of Heat Summation and imagining a pathway with an imaginary hydrogen electrode intermediating. This is fine, because the electrode potential is a variation of the free energy change, a thermodynamic function of differences in state, not the path between states.|
Special note on problem solving: To balance electron transfer, half-reactions must often be multiplied by factors before they are combined, but because voltage is measured in joules/coulomb, the reduction potential is an intensive quantity that reflects the energy involved per unit of charge. So the reduction potentials are not multiplied in balancing the reactions.