Gibbs Free Energy

This is another thermodynamic quantity that helps in predicting the spontaneity of a process, is called Gibbs energy (G).  

It is defined mathematically by the equation.  

G = H - TS  

Where H = heat content, S = entropy of the system, T = absolute temperature  

Solved example: Which of the following will fit into the blank?  

When two phases of the same single substance remain in equilibrium with one another at a constant P and T, their molar _________ must be equal.  

             (A) Internal energy                              (B) Enthalpy  

             (C) Entropy                                         (D) Free energy  

Solution: (D)    

Free energy change  

For isothermal process.  

      ΔG = G₂ – G₁ = ΔH – TΔS  

ΔG = change in Gibbs free energy of the system.  

It is that thermodynamic quantity of a system the decrease in whose value during a process is equal to the maximum possible useful work that can be obtained from the system.  

Solved example: Calculate free energy change when one mole of NaCl is dissolved in water at 25°C. Lattice energy = 700 kJ/mol.ΔS at 25°C = 26.5 Jmol^–1, Hydration energy of NaCl = -696 kJ/mol.  

                        (A) -3.9 kJ                                         (B) -8kJ  

                        (C) -12kJ                                           (D) -16kJ    

Solution: (A)