Gibbs Free Energy

Gibbs created this function to simplify determining spontaneity. The second law of thermodynamics requires measuring entropy changes for both the system and surroundings, which is difficult. Gibbs's function allows spontaneity to be determined using only the system's enthalpy and entropy, eliminating the need to measure surroundings' entropy.

Gibbs free energy (ΔG) is mathematically derived from the second law of thermodynamics direction spontaneous changes. When ΔG is negative, the reaction is spontaneous and the universe's entropy increases. When ΔG is positive, the reaction is nonspontaneous. This relationship makes ΔG a reliable indicator of spontaneity based solely on system properties.

When ΔG = 0, the system is at equilibrium. The reactants and products are present in amounts where no net change occurs. At this point, the free energy of the system is neither increasing nor decreasing, indicating the reaction has reached its equilibrium state.

Gibbs free energy is also called chemical potential because it functions like mechanical potential energy. Just as a ball rolls downhill to lower its potential energy, a chemical reaction proceeds spontaneously to lower its chemical potential. Systems naturally move toward states of lower free energy, driving reactions forward.

At constant temperature and pressure, Gibbs free energy is defined as ΔG = ΔH − TΔS, where ΔH is enthalpy change, T is absolute temperature, and ΔS is entropy change of the system. This equation shows that spontaneity depends on both the enthalpy and entropy contributions, weighted by temperature.

Gibbs free energy is a state function because it depends only on the current state of the system, not on the path taken to reach that state. Since both enthalpy and entropy are state functions, and ΔG is defined mathematically from these properties, ΔG is also a state function.

Temperature directly influences ΔG through the equation ΔG = ΔH − TΔS. The temperature term multiplies entropy change, so increasing temperature amplifies the entropy contribution to spontaneity. This means some reactions that are nonspontaneous at low temperatures may become spontaneous at higher temperatures.