## Why does heat at constant pressure equal enthalpy?

Nisarg Shah 1C
Posts: 54
Joined: Sat Jul 22, 2017 3:00 am

### Why does heat at constant pressure equal enthalpy?

Why does the heat exchanged by a system and its environment at constant pressure equal $\Delta H$? What happens when pressure changes, and how would that mathematically change how you calculate enthalpy?

Jingyi Li 2C
Posts: 56
Joined: Fri Sep 29, 2017 7:06 am

### Re: Why does heat at constant pressure equal enthalpy?

By definition, H = U + PV, where U is internal energy, P is pressure of the system, V is volume.
Also, ΔU = w + q, where q is heat and w is work.
As a result, ΔH= ΔU + PΔV = w + q + PΔV, and w = -P(external) ΔV
when you combine them together, ΔH = q - PexΔV + PΔV
if pressure is constant, Pex = P, ΔH = q

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