Significance of Gibbs Free Energy Change:
ΔG is a measure of the spontaneity of a chemical reaction. Gibbs energy change is also related to useful work other than pressure-volume work that can be obtained from the system. It can be shown that Gibbs energy of a process is equal to maximum possible useful work that can be derived from the process as given below-
From the first law of thermodynamics, we know-
ΔU = q – w or q = ΔU + w ……………….(I) |
Where q is the heat absorbed by the system, ΔU is the change in internal energy and w is the work done by the system. The work may be expansion work as well as non-expansion work.
or q = ΔU + wexpansion + wnon-expansion But wexpansion = pΔV q = ΔU + pΔV + wnon-expansion But, ΔU + pΔV = ΔH Thus, q = ΔH + wnon-expansion ………………..(II) |
For a process carried out reversibly at a constant temperature,
ΔS = qrev/T or qrev = TΔS |
Substitute the value of q in equation (II) we get,
TΔS = ΔH + wnon-expansion ……………….(III) or ΔH – TΔS = – wnon-expansion But ΔH – TΔS = ΔG so that ΔG = – wnon-expansion – ΔG = wnon-expansion – ΔG = wuseful |
Thus, the decrease in the free energy change of a system is the measure of useful work done by the system. This is the significance of the Gibbs free energy change.
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