ΔG = ΔH - TΔS
Where ΔH is the enthalpy change, T is the absolute temperature in Kelvin, and ΔS is the entropy change.
Gibbs free energy (G), named after American scientist Josiah Willard Gibbs, is a thermodynamic potential that measures the maximum amount of reversible work that can be performed by a thermodynamic system at constant temperature and pressure. The change in Gibbs free energy (ΔG) is one of the most important concepts in chemistry because it tells us whether a reaction will occur spontaneously under the given conditions.
The Gibbs free energy combines two fundamental thermodynamic quantities: enthalpy (H), which represents the heat content of the system, and entropy (S), which represents the degree of disorder or randomness. By accounting for both energy and entropy changes, Gibbs free energy provides a complete picture of whether a process will be thermodynamically favorable.
When ΔG is negative, the reaction releases free energy and is thermodynamically favorable, meaning it can occur spontaneously without external energy input. These reactions are called exergonic reactions. Examples include the combustion of fuels and the hydrolysis of ATP in biological systems.
When ΔG is positive, the reaction requires energy input and is non-spontaneous. These endergonic reactions will not proceed on their own but can be driven by coupling them with exergonic reactions or by supplying external energy. When ΔG equals zero, the system is at equilibrium and there is no net change in the forward or reverse direction.
Always Spontaneous (ΔG < 0)
When ΔH < 0 (exothermic) and ΔS > 0 (entropy increases), the reaction is spontaneous at all temperatures. Both thermodynamic factors favor the forward reaction.
Never Spontaneous (ΔG > 0)
When ΔH > 0 (endothermic) and ΔS < 0 (entropy decreases), the reaction is non-spontaneous at all temperatures. Both factors oppose the forward reaction.
Temperature Dependent
When ΔH and ΔS have the same sign, spontaneity depends on temperature. High temperatures favor entropy-driven reactions, while low temperatures favor enthalpy-driven reactions.
Calculations assume constant temperature and pressure conditions. Real reactions may vary due to non-ideal behavior, kinetic limitations, or changes in conditions. A negative ΔG indicates thermodynamic favorability but does not guarantee a fast reaction rate. Some spontaneous reactions may be extremely slow without a catalyst. Additionally, standard Gibbs free energy values (ΔG°) assume standard conditions (1 atm, 1 M concentrations) which may differ from actual experimental conditions.