Generally defined in terms of changes in free energy: Δ‎F = Δ‎H – TΔ‎S, where H = enthalpy, T = absolute temperature, S = entropy. In a mixture of reactants, if Δ‎F is less than 0 a reaction may take place spontaneously, whereas if Δ‎F is positive energy must be supplied to the system for a reaction to occur at all. In geochemistry, the sign and magnitude of Δ‎F are important. The sign indicates whether a given reaction can occur spontaneously, and the magnitude indicates how far the reaction can go before equilibrium is attained.