You have learned that reactions like to release heat, thus having negative 
H, and more entropy, thus having positive S. In the 19th century, an American named Josiah Gibbs combined enthalpy and entropy values to determine the spontaneity of a reaction. The equation used to determine the free energy in a chemical reaction is as follows:
G = H - T S G is the free energy of the reaction, H is the enthalpy of the reaction, S is the entropy, and T is the temperature, in Kelvin. This equation is very useful, because it calculates free energy, a value that can be used to determine the spontaneity of a reaction by simply looking at its sign.
G is negative, the reaction is spontaneous. G is zero, the reaction is at equilibrium. G is positive, the reaction will not proceed spontaneously.
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