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I think you have a misunderstood, the quantity entalphy is a definition, it comes from the second law (conserved energy, if there is constant pressure ($\delta W =pdV$) $dU=\delta Q-\delta W \Rightarrow dU=\delta Q-pdV$ from the chain rule $d(pV)=Vdp+pdV \Rightarrow pdV=d(pV)-Vdp$, so \$ dU=\delta Q -d(pV)+Vdp \Rightarrow d(U+pV)=\delta Q+Vdp ...