Enthalpy

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Emma Li 2C
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Joined: Fri Sep 29, 2017 7:07 am
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Enthalpy

Postby Emma Li 2C » Sat Jan 20, 2018 8:28 pm

For the enthalpy equation: H = U + PV, if change in enthalpy measures the change in heat without work, why is PV added to the internal energy U?

Thank you!

Cynthia Bui 2H
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am

Re: Enthalpy

Postby Cynthia Bui 2H » Sat Jan 20, 2018 9:31 pm

The change in enthalpy isn't the change in heat without work, it refers to the transfer of heat at a constant pressure, which implies that work is being done. Enthalpy is the total energy content of a system so the change in enthalpy is equal to the change in internal energy due to transfer of heat plus work (PV) because work is being done on the system so that it stays at the constant pressure. Remember that U = H - PV and PV is negative because work is being done by the system. That equation is also just another form of your equation.

Phi Phi Do 2E
Posts: 27
Joined: Fri Sep 29, 2017 7:05 am

Re: Enthalpy

Postby Phi Phi Do 2E » Sat Jan 20, 2018 9:56 pm

PV is also negative because as work is being done by the system, it is losing energy.


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