The free energy change for a process may be viewed as a measure of its driving force. A negative value for Δ*G* represents a driving force for the process in the forward direction, while a positive value represents a driving force for the process in the reverse direction. When Δ*G* is zero, the forward and reverse driving forces are equal, and the process occurs in both directions at the same rate (the system is at equilibrium).

The reaction quotient, *Q*, is a convenient measure of the status of an equilibrium system. *Q* is the numerical value of the mass action expression for the system, and it can be used to identify the direction in which a reaction will proceed in order to achieve equilibrium. When *Q* is lesser than the equilibrium constant, *K*, the reaction will proceed in the forward direction until equilibrium is reached and *Q* = *K*. Conversely, if *Q* > *K*, the process will proceed in the reverse direction until equilibrium is achieved.

The free energy change for a process taking place with reactants and products present under nonstandard conditions (pressures other than 1 bar; concentrations other than 1 M) is related to the standard free energy change according to this equation:

Δ*G *= Δ*G*° + *RT *ln*Q*

*R* is the gas constant (8.314 J/K mol), *T* is the kelvin or absolute temperature, and *Q* is the reaction quotient. For gas-phase equilibria, the pressure-based reaction quotient, *QP*, is used. The concentration-based reaction quotient, *QC*, is used for condensed phase equilibria.

For a system at equilibrium, *Q* = *K* and Δ*G* = 0, and the previous equation may be written as

0 = Δ*G*° + *RT *ln*K *(at equilibrium)

Δ*G*° = −*RT *ln*K*

This form of the equation provides a useful link between these two essential thermodynamic properties, and it can be used to derive equilibrium constants from standard free energy changes and vice versa.

*This text is adapted from Openstax, Chemistry 2e, **Section 16.4: **Free Energy*.

ABOUT JoVE

Copyright © 2024 MyJoVE Corporation. All rights reserved