Gibbs energy is defined as the energy 'available' to do work at constant temperature & pressure.The energy required to make the system from none under constant pressure is the enthalpy, that must be given to make the system. However, owing to the energy dispersal tendency to bring disorder to the system owing to its entropy is supplied by the environment. So, the energy given to form the system is $H - TS$ which is the Gibbs Energy.

This is the line noteworthy to be quoted from my book:

It is the energy available for doing work as the energy owing to the entropy cannot be used to do work.

So, can anyone tell me why the energy gained due to entropy can't be used to do work?

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    $\begingroup$ I'm sorry user, your question isn't clear. Could you rephrase it? Meanwhile can I suggest that we set enthalpy aside and recall that energy is always conserved whilst work involves the transfer of energy. It's transferred from a place where the energy-density is higher to a place where it's lower, whereafter the energy-density is more uniform and entropy has increased. I think it's useful to think of entropy is something like "uniformity". When the energy-density is totally uniform, you can't do any work. There is no available energy. $\endgroup$ – John Duffield Jul 11 '15 at 15:12
  • $\begingroup$ @John Duffield: Thanks sir for providing the link; I have googled it but couldn't get the proper result. $\endgroup$ – user36790 Jul 11 '15 at 15:23

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