Consider a redox reaction between molecules A and B. Molecule A has a redox potential of –100 mV and molecule B has a redox potential of +100 mV
For the transfer of electrons from A to B, is the ?G° positive or negative or zero? Under what conditions will the reverse reaction, transfer of electrons from B to A, occur?
The ?G° is negative. The sign of ?G° is the opposite of that of ?E0? = E0? (acceptor) –E0? (donor). The acceptance of electrons by B from A has a ?E0? = 100 – (–100) = 200. The reverse reaction, the donation of electrons from B to A, has a positive ?G° and is therefore unfavorable under standard conditions. Remember that, by definition, the concentrations of A and its redox pair A? are equal under standard conditions; similarly, the concentration of B is equal to the concentration of its redox pair B?. B will be able to donate electrons to A only when [B] > [B?] and/or [A] < [A?] to such an extent that the ?G for electron transfer becomes negative.
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