For chemical systems, the value of ΔS was not always possible to determine. As a result, a new thermodynamic function was needed to predict the direction of spontaneity. J. Gibbs introduced a new function known as Gibbs' free energy. Denoted by the letter G, it is mathematically defined by
G = H - TS
H= Enthalpy of the system
T= Absolute temperature (Kelvin scale)
S= Entropy of a system
Let us consider an isothermal change at temperature T from initial state 1 to final state 2.
Then for the initial state,
G1= H1 - TS1
And, for final state,
The change of free energy ΔG is given by
This equation is also known as Gibbs Helmholtz Equation.