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I have 3 questions:

Enthalpy is defined as:

H = U + PV

  1. What exactly does PV signify?

Also from the First Law of Thermodynamics

∆U = q + w

w here means work (done by, say, the system).
So,
∆H = q + w + ∆(PV)
2. In the above equation are we adding work twice?
I asked chatGPT about it, it said we are not. I need a more deeper explanation of what the distinction between PV and w is. There seems to be a distinction between the PV work in

w = -P∆V (if PV work is being done ex: gas expansion or compression) and

the PV term in

H = U + PV

  1. What is this distinction? Could someone please explain with examples.
apm
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  • U is internal energy of an object. H includes squeezing by pressure - atmospheric in particular. BTW this was likely asked here like half a dozen times :/ – Mithoron Feb 20 '24 at 16:18
  • Not twice, it cancels out if the pressure is the same before and after (and you keep the sign conventions straight). – Karsten Feb 20 '24 at 16:57

1 Answers1

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Your equations should read $$\Delta U=q-\int{P_{ext}dV}$$ and $$\Delta H==q-\int{P_{ext}dV}+\Delta PV$$If $P_{ext}=P=const, $, these equations reduces to $$\Delta U=q-P\Delta V$$and $$\Delta H=q$$

Chet Miller
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