Question

Define Gibbs free energy and change in free energy. What are the units of Gibbs free energy? 

OR 

Derive the relation between ΔG and ΔS Total.

Answer 1

(i) Gibbs free energy, G is defined as, 

G = H – TS

Where H is the enthalpy, S is the entropy of the system at absolute temperature T. Since H, T and S are state functions, G is a state function and a thermodynamic function.

(ii) At constant temperature and pressure, change in free energy ΔG for the system is represented as, ΔG = ΔH – TΔS

This is called Gibbs free energy equation for ΔG. In this ΔS is total entropy change, i.e., ΔS_Total.

(iii) The SI units of ΔG are J or kJ (or Jmol^-1 or kJmol^-1).

The c.g.s. units of ΔG are cal or kcal (or cal mol^-1 or kcal mol^-1).