Q. Describe Gibbs Free Energy and Spontaneity?

Ans.

This is the end of the quest to find spontaneity.  Free Energy, or Gibbs Free Energy is a quantity which balances the relative contributions of enthalpy, entropy and temperature.   

To calculate the change in Gibbs Free Energy, just subtract:

ΔG = ΔG_final - G_intial

or in terms of the reaction:

ΔG_products - G_reactants

A long as the pressure is constant and the temperature is constant throughout the reaction, this is the relationship between Gibbs Free Energy, enthalpy, temperature and entropy:

ΔG = ΔH - TΔS

Simple.  Just remember T is on the Kelvin scale.

If the change in free energy is negative, then the reaction will be spontaneous.

If the change in free energy is positive then the reaction will not happen spontaneously.

Let's investigate how the values of H and S affect the spontaneity of a reaction: