## Constant Volume vs. Constant Pressure in a Calorimeter

Matthew Gutierrez 2D
Posts: 19
Joined: Fri Sep 25, 2015 3:00 am

### Constant Volume vs. Constant Pressure in a Calorimeter

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

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

Chem_Mod
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### Re: Constant Volume vs. Constant Pressure in a Calorimeter

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.