# Gibbs free energy

1. Oct 14, 2013

### iScience

the gibbs free energy is defined to be H-TS. In my thermo book, it says that if i were to create a system out from volume V=0 at constant pressure and temperature, the work that i would need to provide is G=H-TS. But for constant pressure i thought the work terms in enthalpy cancelled out and were zero and that all that was left was the heat (Q), i thought that's what made the enthalpy quantity useful. why then does the H-TS quantity have work in it? ie, i thought H-TS would just be Q-TS but apparently it's not.. why is this not true?

2. Oct 15, 2013

### Staff: Mentor

You have
$$G = H - TS = U + PV - TS$$
Considering that you are creating a system from nothing, enthalpy is the total energy you will have to furnish to create the system and puch away the atmosphere to make place for it. With the Gibbs free energy, you can consider that you do not have to furnish all the energy, as some can come from the environment in the form of heat (assuming constant temperature). So, when creating a system from nothing, $G$ is what you need to supply to create the system at $T=0$ and puch away the atmosphere to make room for it, considering that the enviroment will take care of bringing it to the right temperature.