What is Gibbs Free Energy?

2 Answers

Gibbs free energy, G, is just the enthalpy change of a reaction, ΔH, minus the entropy change of the reaction system, ΔSsys, multiplied by the temperature of the reaction, T.

ΔG=ΔHTΔSsys

For a reaction to occur, it needs to cause the total entropy of the reaction system, Ssys, and its surroundings, Ssur, to increase.

ΔSoverall=ΔSsys+ΔSsur>0

But, because "the surroundings" is effectively the whole rest of the universe, it's quite difficult to accurately measure ΔSsur.

Therefore, the Gibbs free energy equation was invented, because if ΔG<0, then that means that the total entropy change of a reaction is greater than zero (ΔSoverall>0), and so the reaction will happen.

Derivation of the Gibbs free energy equation:

ΔSoverall=ΔSsur+ΔSsys>0

ΔSsur=ΔHT

ΔSoverall=ΔHT+ΔSsys>0

Multiplying through by T gives
ΔG=TΔSoverall=ΔHTΔSsys

NOTE: when doing actual calculations enthalpy ΔH values use kJ but entropy ΔS values are often expressed in J. Watch for this and convert ΔS to kJ (divide by 1000) in these types of calculations.

Sep 24, 2015

The Gibbs free energy is classically the energy associated with a chemical reaction that can be used to do work. It includes an enthalpy term, ΔH, and an entropy term, ΔS. It remains the unequivocal criterion for the spontaneity of chemical change.

Explanation:

By definition, the Gibbs free energy is defined by the relationship:

ΔG=ΔHTΔS

We can introduce specified standard state conditions, but if ΔG is negative, then the reaction as written is spontaneous. If ΔG is positive, then the reaction as written is non-spontaneous. It may be related to the equilibrium constant, Keq, by the relationship:

ΔG=RTlnKeq.

Given this relationship, only negative ΔG values will give rise to an equilibrium constant that is >1, and hence spontaneous.

Often you see values of ΔG tabulated for standard state conditions, 298 K, and near atmospheric pressure, in which case ΔG is specified.

I recommend this video that explains in details Gibbs free energy.