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enter image description here

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?

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  • $\begingroup$ Youtube has Feinmann’s lectures on physics - audio you mught find them wirth listening to. $\endgroup$
    – Solar Mike
    Jun 7 at 10:41
  • $\begingroup$ $\delta$ H is only for constant pressure. There is never a change in pressure for $\delta$ H. $\endgroup$
    – S.M.T
    Jun 7 at 10:42
  • $\begingroup$ @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 . $\endgroup$
    – NMech
    Jun 7 at 10:51
  • $\begingroup$ @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 $\endgroup$
    – S.M.T
    Jun 7 at 10:52
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    $\begingroup$ 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.. $\endgroup$
    – NMech
    Jun 7 at 10:55
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I understood the answer to my Q.

ΔH=ΔU+Δ(pV)is the general statement for Δ q and ΔH

Whereas , ΔH=ΔU+pΔ(V) is a subcase when pressure is constant.

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