standard free energy

Meaning: ΔG° is the free energy change for converting reactants to products with every species at standard state: 1 M for solutes, 1 atm for gases, usually 298 K. For a given reaction and temperature it is a fixed number that measures where equilibrium lies. If ΔG° < 0, Keq > 1 and products are favored at equilibrium. If ΔG° > 0, Keq < 1. If ΔG° = 0, Keq = 1. (Biochemistry often uses ΔG°′, the same idea at pH 7.)

Equation: ΔG° = −RT ln Keq = ΔH° − TΔS° = −nFE°, and ΔG = ΔG° + RT ln Q. Here ΔG° is in J/mol, R = 8.314 J/(mol·K), T is in K, Keq and Q are unitless, ΔH° is in J/mol, ΔS° is in J/(mol·K), n is moles of electrons transferred, F = 96,485 C/mol, and E° is the standard cell potential in V.

On the MCAT: You may be asked to find the sign or size of Keq from ΔG°, or the reverse. You may be asked to predict whether ΔG is negative when Q < Keq, or to add the ΔG° values of coupled steps. Electrochemistry passages link E° to ΔG°. A positive E° gives a negative ΔG°.

Common confusion: A negative ΔG° does not mean the reaction proceeds forward under every set of conditions. The sign of ΔG, not ΔG°, sets the direction. If Q > Keq, ΔG is positive even when ΔG° is negative. At equilibrium ΔG = 0, while ΔG° stays at its fixed value.