Elicit the two thermodynamic quantities which affect spontaneity of a reaction, entropy and enthalpy. Introduce Gibb's free energy expression, which ties together both entropy and enthalpy. Define free energy. Explain the how a negative free energy and a positive free energy affect a reaction. Negative describes useful work which can be obtained from a reaction and positive describes work which must be supplied. introduce delta G=delta H - T delta S. . Explain the direction of a chemical reaction when delta G is negative, positive, and zero.