Why can't qv equal delta H?

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KNguyen_1I
Posts: 101
Joined: Sat Aug 17, 2019 12:16 am

Why can't qv equal delta H?

Postby KNguyen_1I » Wed Jan 29, 2020 12:14 pm

I know Dr. Lavelle many times relates qp being delta H, what does that mean and why can't qv be delta H either?

Rachel Yu 1G
Posts: 113
Joined: Fri Aug 09, 2019 12:15 am

Re: Why can't qv equal delta H?

Postby Rachel Yu 1G » Wed Jan 29, 2020 12:45 pm

qv actually equals delta U. Since deltaU = q + work or deltaU = q - PdeltaV. Since qv means that there is a constant volume, then deltaV = 0 and work = 0, leaving deltaU = qv. On the other hand, qp=delta H because enthalpy (H) is defined as the amount of heat released/absorbed at a constant pressure. Therefore, qp, being heat at a constant pressure, equals delta H.

Victoria Zheng--2F
Posts: 103
Joined: Fri Aug 09, 2019 12:17 am

Re: Why can't qv equal delta H?

Postby Victoria Zheng--2F » Wed Jan 29, 2020 9:18 pm

qv can not equal delta H because qv is constant volume, but the pressure of the system changes as the temperature increases(more random motion by gas particles), and the definition of enthalpy states that enthalpy is the amount of heat absorbed or released at constant pressure, so since the pressure of the system changes, qv can not equal delta H.

ValerieChavarin 4F
Posts: 99
Joined: Wed Sep 18, 2019 12:18 am

Re: Why can't qv equal delta H?

Postby ValerieChavarin 4F » Thu Jan 30, 2020 4:13 pm

qv is at constant volume, with changing pressure, while delta H is heat transfer at constant pressure.


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