According to second law of thermodynamics, all chemical reaction occurs spontaneously in one direction until the equilibrium state is obtained. When equilibrium state is obtained, the concentration of reactants and products remains constant.
This law also predicts that entropy of all the spontaneous processes increases with time and the free energy of the system in all the spontaneous reaction decreases.
Mathematically, the relation between enthalpy (H), entropy (S) and free energy (G) of the system can be shown by following equation.
ΔG = H – TS
ΔG = H – T S
The value of ΔG is negative because the free energy of the system in all spontaneous reaction decreases.
Here, G is the symbol of Gibbs’ free energy and is a state function and an extensive property of a substance.