Dear student,
The Gibbs Free Energy is a thermodynamic quantity which can be used to  determine if a reaction is spontaneous or  not. The definition of the Gibbs free energy is
G = H- T*S
where G is the free energy, H is the enthalpy and S is the entropy. If  we consider the change in G for a reaction where the temperature does not  change, we have
DG = DH - TDS
DH and DS are computed from the usual procedures. Temperatures  for this equation must be in Kelvin. The above equation is often known as the  "Gibbs-Helmholtz equation". Since the values of DH and DS are usually gotten from tables of standard entropy  and enthalpy, the Gibbs free energy change usually computed is the standard  Gibbs free energy change, DG0 at  250C- for different temps the procedure is similar
The sign of DG determines if a reaction is  spontaneous or not.

DG < 0: the reaction is spontaneous 
DG > 0: the reaction is not spontaneous 
DG = 0: the reaction is at equilivrium

 
































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