qp and qv

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Diana_Diep2I
Posts: 130
Joined: Sat Aug 17, 2019 12:17 am

qp and qv

Postby Diana_Diep2I » Thu Jan 30, 2020 9:48 am

Why is qp=∆H and not qv?

Abby Soriano 1J
Posts: 103
Joined: Sat Aug 24, 2019 12:16 am

Re: qp and qv

Postby Abby Soriano 1J » Thu Jan 30, 2020 11:38 am

By definition, enthalpy is the amount of heat absorbed/released at a constant pressure. So that means that delta H only equals qp rather than qv.

Sjeffrey_1C
Posts: 108
Joined: Wed Feb 20, 2019 12:17 am

Re: qp and qv

Postby Sjeffrey_1C » Thu Jan 30, 2020 7:16 pm

Enthalpy in this case is referring to heat at a specific pressure, not a specific volume. Thus, the p subscript instead of v.

Hailey Kim 4G
Posts: 110
Joined: Sat Jul 20, 2019 12:16 am

Re: qp and qv

Postby Hailey Kim 4G » Thu Jan 30, 2020 7:57 pm

Enthalpy, which is represented by delta(h), is heat at a constant pressure. Heat is represented by "q", and because the pressure is constant, it will have the subscript "p" for pressure, rather than "v" for volume.

Daria Azizad 1K
Posts: 116
Joined: Thu Jul 25, 2019 12:15 am

Re: qp and qv

Postby Daria Azizad 1K » Thu Jan 30, 2020 10:12 pm

When volume is not constant, the gas is either expanding or compressing, so there is some work done. Therefore, that energy from work contributes to the delta H and must be factored in.

Sydney Myers 4I
Posts: 100
Joined: Fri Aug 09, 2019 12:17 am

Re: qp and qv

Postby Sydney Myers 4I » Fri Jan 31, 2020 11:43 am

It's simply that the definition of enthalpy is heat at a constant pressure. That's how it's defined as a function.

Junwei Sun 4I
Posts: 125
Joined: Wed Oct 02, 2019 12:16 am

Re: qp and qv

Postby Junwei Sun 4I » Fri Jan 31, 2020 1:18 pm

Because by definition H is the heat transferred at constant pressure.

Maya Pakulski 1D
Posts: 105
Joined: Thu Jul 11, 2019 12:17 am

Re: qp and qv

Postby Maya Pakulski 1D » Fri Jan 31, 2020 7:02 pm

This is because by definition delta H only equals q at constant pressure.

Orrin Zhong 4G
Posts: 51
Joined: Sat Jul 20, 2019 12:16 am

Re: qp and qv

Postby Orrin Zhong 4G » Fri Jan 31, 2020 8:03 pm

If the heat transfer is occurring while pressure is changing, q is not equal to change in enthalpy, since the change in enthalpy is defined at a constant pressure. Therefore, change in enthalpy only equals q when pressure is held constant: qp = change in enthalpy.


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