ΔG is net energy available to do useful work and is thus a measure of “free energy”. Show mathematically that ΔG is a measure of free energy. Find the unit of ΔG. If a reaction has positive enthalpy change and positive entropy change, under what condition will the reaction be spontaneous?
Gibbs energy
G = H - TS
H = Enthalpy of the system
T = Temperature
S = Entropy of the system
We know
H = U + PV
Therefore G = U + PV – TS
G is a state function and an extensive property. The change in Gibbs energy for the system can be
∆G = ∆U + ∆(PV) - ∆(TS)
If the process is done at constant temperature and pressure then
∆Gsys = ∆Usys + P∆V - T∆Ssys
∆Gsys = ∆H - T∆Ssys
∆G has the unit of energy because both ∆H and T∆Ssys are terms of energy
Unit of ∆G is joules.
∆G is the energy available to do useful work and also known Gibbs free energy
∆G gives the criteria for spontaneity at constant pressure and temperature
If ∆G is positive then non-spontaneous
If ∆G is negative then spontaneous.