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### qp and qv

Posted: Thu Jan 30, 2020 9:48 am
Why is qp=∆H and not qv?

### Re: qp and qv

Posted: 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.

### Re: qp and qv

Posted: 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.

### Re: qp and qv

Posted: 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.

### Re: qp and qv

Posted: 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.

### Re: qp and qv

Posted: 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.

### Re: qp and qv

Posted: Fri Jan 31, 2020 1:18 pm
Because by definition H is the heat transferred at constant pressure.

### Re: qp and qv

Posted: Fri Jan 31, 2020 7:02 pm
This is because by definition delta H only equals q at constant pressure.

### Re: qp and qv

Posted: 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.