Free Energy and Equilibrium

A negative ΔG (free energy change) indicates the reaction has a driving force in the forward direction and will proceed spontaneously. As reactant concentrations decrease and product concentrations increase, ΔG becomes less negative. This relationship helps predict whether endergonic and exergonic reactions in the cell will occur under given conditions.

The reaction quotient Q measures the status of an equilibrium system using the mass action expression. When Q is less than the equilibrium constant K, the reaction proceeds forward until equilibrium is reached. Conversely, if Q exceeds K, the reaction reverses until Q equals K and the system reaches equilibrium.

At equilibrium, ΔG equals zero, and the equation rearranges to ΔG° = −RT lnK. This fundamental relationship links standard free energy change to the equilibrium constant, allowing scientists to derive one property from the other. R is the gas constant, T is absolute temperature, and K is the equilibrium constant.

At equilibrium, forward and reverse reaction rates are equal, creating balanced driving forces in both directions. The ratio of product to reactant concentrations equals the equilibrium constant K. When ΔG reaches zero, the system is at equilibrium and no net change occurs, though reactions continue in both directions simultaneously.

Under nonstandard conditions (concentrations other than 1 M or pressures other than 1 bar), free energy is calculated using ΔG = ΔG° + RT lnQ. The reaction quotient Q accounts for actual concentrations or pressures present. For gases, pressure-based quotient QP is used; for condensed phases, concentration-based quotient QC applies.

A positive ΔG represents a driving force in the reverse direction, meaning the reaction will not proceed forward spontaneously under current conditions. The system will shift backward until equilibrium is achieved. Positive ΔG indicates the reaction is thermodynamically unfavorable in the forward direction.

The equation ΔG = ΔG° + RT lnQ bridges standard free energy to actual cellular environments. Standard free energy (ΔG°) represents conditions at 1 M concentration and 1 bar pressure, while the RT lnQ term adjusts for real concentrations and temperatures. This allows prediction of reaction spontaneity in living cells where conditions differ from standard.