What is the relationship between ∆G and ∆G° in electrochemical cells?

The correct relationship between the standard Gibbs free energy change (∆G°) and the Gibbs free energy change (∆G) for a reaction occurring in an electrochemical cell is articulated by the equation ∆G = ∆G° + RT ln(Q).

In this equation, ∆G represents the Gibbs free energy change under non-standard conditions, while ∆G° reflects the free energy change when reactants and products are in their standard states (1 M concentration, 1 atm pressure, and at a specified temperature). The term RT ln(Q) accounts for the effect of the reaction quotient (Q), which reflects the current concentration of reactants and products at any given moment.

As the reaction proceeds, the concentrations of reactants and products change, thus altering Q. This relationship shows how the favorability of a reaction (as indicated by ∆G) can vary based on the conditions of that reaction, with respect to its standard state. If Q increases, the ln(Q) term increases, and thus ∆G will be influenced by this term, indicating the shift towards the products or reactants based on current conditions.

This formulation helps to determine how the spontaneity of the reaction changes when it moves away from standard conditions,