ABSTRACT
The total entropy change which accompanies a process is the sum of the entropy change in the system and its surroundings:
∆Stotal=∆Ssystem+∆Ssurroundings
∆Ssurroundings is related to the enthalpy change in the system at constant pressure through the relationship: ∆Ssurroundings=−∆Hsystem/T. Substitution of this expression into the previous one, and subsequent multiplication by −T yields the relationship:
−T∆Stotal=∆Hsystem−T∆Ssystem
The Gibbs free energy, G, (occasionally referred to as the Gibbs energy or Gibbs function) is defined by G=H−TS. At constant pressure and temperature, finite changes may be expressed as:
∆G−∆Hsystem−T∆Ssystem
∆G is therefore equal to −T∆Stotal, and the free energy may be regarded as a measure of the total entropy change (both in the system and the surroundings) for a process. Whilst a spontaneous process gives rise to a positive value of ∆S, ∆G must be negative because of the minus sign in ∆G=−T∆Stotal.
