ISOTHERMAL and isobaric (constant TEMPERATURE and pressure) thermodynamic system. This is where the TERM "free" comes from; it refers to the amount of energy in a system that is easily available for usage.Gibb’s free energy is given by\(G\; = \;H\;-\;TS\)The Gibbs energy is thus the enthalpy (essentially the TOTAL energy of the system) minus the entropy times the temperature (a measure of the amount of energy that is unavailable).