Significance of Gibbs Free Energy Change

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 + w_expansion + w_{non-expansion}
But w_expansion = pΔV
q = ΔU + pΔV + w_{non-expansion}
But, ΔU + pΔV = ΔH
Thus, q = ΔH + w_{non-expansion} ………………..(II)

For a process carried out reversibly at a constant temperature,

ΔS = q_rev/T or q_rev = TΔS

Substitute the value of q in equation (II) we get,

TΔS = ΔH + w_{non-expansion} ……………….(III)
or ΔH – TΔS = – w_{non-expansion}
But ΔH – TΔS = ΔG so that
ΔG = – w_{non-expansion}
– ΔG = w_{non-expansion}
– ΔG = w_useful

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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