standard free energy ΔG
Summary
Standard free energy ΔG° is the Gibbs free energy change for a reaction when all species are in their standard states (1 M solutes, 1 atm gases, usually 298 K). It sets the equilibrium constant: ΔG° = −RT ln Keq, so a negative ΔG° means Keq > 1 and products are favored at equilibrium.
Detail
Meaning: ΔG° is the change in Gibbs free energy when reactants in their standard states (1 M solutes, 1 atm gases, usually 298 K) are converted to products in their standard states. It is a fixed value for a reaction at a given temperature and reflects where equilibrium lies, not how fast it gets there. Negative ΔG° means Keq > 1, positive means Keq < 1, and zero means Keq = 1.
Equation: ΔG° = −RT ln Keq = ΔH° − TΔS° = −nFE°. R = 8.314 J/(mol·K); T is temperature in K; Keq is unitless; ΔH° is in J/mol; ΔS° is in J/(mol·K); n is moles of electrons transferred (mol); F = 96,485 C/mol; E° is the standard cell potential in V; ΔG° is in J/mol.
On the MCAT: Expect to find the sign of ΔG° from a given Keq or E°, or to predict how raising T changes the sign from the signs of ΔH° and ΔS°. A larger Keq means a more negative ΔG°. Coupled reactions have additive ΔG° values.
Common confusion: ΔG° is not ΔG. The real direction of a reaction depends on ΔG = ΔG° + RT ln Q, which is zero at equilibrium. A reaction with a positive ΔG° can still go forward if Q is small enough.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.