From the second law spontaneity is defined so that

The DS_surr can be replaced by the expression in which the enthalpy of the system effects the temperature of the surroundings.

By substituting into the first equation, we obtain

Rearranging the equation leads to a new expression in which the criteria for a spontaneous reaction is expressed in terms of system properties.

Gibbs Free Energy is the energy available to do work and is the ultimate test for spontaneity;