Constant Volume vs. Constant Pressure in a Calorimeter

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Matthew Gutierrez 2D
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Joined: Fri Sep 25, 2015 3:00 am

Constant Volume vs. Constant Pressure in a Calorimeter

Postby Matthew Gutierrez 2D » Sat Jan 09, 2016 1:37 pm

I am still confused why at constant volume, the heat transfer is called "deltaU", while in a constant pressure environment, it is called "deltaH". How do the difference in conditions change what value is being measured and why?

Karla Santos 3J
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: Constant Volume vs. Constant Pressure in a Calorimeter

Postby Karla Santos 3J » Sat Jan 09, 2016 3:21 pm

Delta U is internal energy, while Delta H is external energy, if I remember correctly.

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Re: Constant Volume vs. Constant Pressure in a Calorimeter

Postby Chem_Mod » Sat Jan 09, 2016 3:28 pm

Recall the U= q + w => dU=dq + dw; dw=-PdV. With constant volume, dw=0 and therefore dU=dq or heat transfer equals dU. For constant pressure, there is some work done so apparently dU no longer equals to dq and people don't want to leave dq in the equation, so they call dq at constant pressure dH which is enthalpy as we know.


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