# Why ΔH has two separate equation in my textbook?

ΔQ = ΔU + Δ(PV) (First law of thermodynamics) which has the same statement as the 1st equation.

$$Q_p$$ or ΔH= ΔU + PΔV which is the heat exchanged at constant pressure.

Then why in the image above in my textbook . ΔH has two equations?

• Youtube has Feinmann’s lectures on physics - audio you mught find them wirth listening to. Jun 7 at 10:41
• $\delta$ H is only for constant pressure. There is never a change in pressure for $\delta$ H. Jun 7 at 10:42
• @Srijan $\Delta (p\cdot V)$ is equal to the work ($dW$) done/extracted from a system. Because the pressure is constant the work is equal to the pressure times the change in volume . Jun 7 at 10:51
• @NMech Right that is. I meant to say there is only one equation for $\delta$ H where pressure is const. Not changing unlike the 1st equation of image. Since that equation is the 1st law of thermodynamics Jun 7 at 10:52
• Maybe I did not understand your question. $\Delta H = \Delta U + \Delta (pV)$ is the more generic, while $\Delta H = \Delta U + p\Delta (V)$ is a subcase when pressure is constant.. Jun 7 at 10:55