## Conceptualizing enthalpy

andrewr2H
Posts: 29
Joined: Fri Sep 29, 2017 7:05 am

### Conceptualizing enthalpy

I was a little confused about the equation q/p = enthalpy. During Monday's lecture Dr. Lavelle defined enthalpy as the heat released or absorbed during a chemical reaction or phase change (change in heat), but I'm confused over why the heat released or absorbed is equal to heat at constant pressure.

Specifically, the questions I have are as follows:

Firstly, what specifically is q/p the heat of?
Secondly, does this equation not say that the change in some heat, is equal to that "same" heat itself, but just at constant pressure??
And thirdly, if the second is true, how?

Britney Alvey 1B
Posts: 31
Joined: Fri Jun 23, 2017 11:39 am

### Re: Conceptualizing enthalpy

1. $q_{p}$ is the heat of the system at constant pressure.
2. It might help if you look at it this way: $\Delta H = \Delta U +PV$ . If you assume that the system can only do expansion work then you can substitute in that $\Delta U=q+w$ and $w=-P_{ex}\Delta V$ . Also, because the system is at a constant pressure, then the external pressure equals the pressure of the system, $P_{ex}=P$.This leaves you with $\Delta H=q_{p}$ .

Anna Okabe
Posts: 30
Joined: Fri Sep 29, 2017 7:06 am

### Re: Conceptualizing enthalpy

Q/p is the heat of the system, or the heat required/generated by the formation or breaking of the bonds. The pressure must be constant because according to the ideal gas law, if one part of the system, such as pressure, changes, temperature can also change.

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