Free Energy and Equilibrium

A big negative delta G degree tells you a reaction favors products at standard conditions, but it never means the reaction runs to completion. As reactants turn into products, concentrations change, and the actual delta G changes with them, through delta G = delta G degree + RT ln Q. The reaction keeps moving forward only as long as delta G stays negative. Once Q climbs high enough that delta G hits exactly zero, the forward and reverse rates balance and net change stops. That stopping point is equilibrium, and it happens before all the reactant is used up because delta G, not delta G degree, is what governs whether the reaction still wants to move.

The equation delta G degree = -RT ln K makes this connection exact. A very negative delta G degree forces ln K to be large and positive, so K is huge and the equilibrium mixture sits mostly on the product side. A positive delta G degree gives a small K, favoring reactants. Delta G degree of zero gives K equal to 1, reactants and products equally present at equilibrium. Plotted against extent of reaction, total free energy traces a curve that dips to a minimum; that minimum is the equilibrium composition, and its position depends on the sign and size of delta G degree.

This ties the whole unit together: entropy and enthalpy set delta G degree, delta G degree sets K, and K describes the same chemistry that electrochemistry will soon describe in volts, since delta G degree also equals -nFE degree for a cell.