## 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} = pΔVq = 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.