The Gibbs free energy can be written as a function of enthalpy and entropy as follows,
(B) G=H−TS
At constant T
ΔG=ΔH−TΔS
(A) According to first law of thermodynamics, ΔU=Q+W
If we apply constant P and reversible work.
W=−PΔV
ΔU=Q−PΔV
(C) By definition of entropy change dS=Tdqrev
At constant T
ΔS=Tqrev
Enthalpy can be written as a function of internal energy, pressure and volume as follows,
(D) H=U+PV
For ideal gas equation, PV=nRT
H=U+nRT
At constant T
ΔH=ΔU+ΔnRT