free energy

free energy
Gibbs free energy (G) the thermodynamic function G = HTS, where H is enthalpy, T absolute temperature, and S entropy. For chemical reactions occurring at a constant temperature and pressure, the free energy change ΔG = ΔHTΔS determines the direction in which a reaction proceeds; ΔG is negative for a spontaneous (exergonic) reaction; ΔG is positive for a nonspontaneous (endergonic) reaction. The free energy change can be determined from the equation ΔG = ΔG° + RT ln Q, where R is the gas constant, Q is the reaction quotient (q.v.), and ΔG° is the standard free energy change (the difference between the sum of the free energies of the products and the sum of the free energies of the reactants when all products and reactants are in their standard states; solids and liquids are pure substances; gases are at 1 atm pressure; the temperature is 25°C; and all solutions have a concentration of 1M). For a reaction at equilibrium, ΔG = 0; thus ΔG° = −RT ln K, where K is the equilibrium constant.

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