Difference between U and H?  [ENDORSED]


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Mike Matthews 1D
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Joined: Fri Sep 29, 2017 7:04 am

Difference between U and H?

Postby Mike Matthews 1D » Mon Jan 22, 2018 12:15 am

Could someone please explain, in detail, the difference between U (internal energy) and H (enthalpy). Why does the equation for Cv use U while the equation for Cp use H?

Priyanka Bhakta 1L
Posts: 50
Joined: Fri Sep 29, 2017 7:04 am

Re: Difference between U and H?

Postby Priyanka Bhakta 1L » Mon Jan 22, 2018 9:26 am

U is the internal energy, which is the amount of energy a system holds in both kinetic and potential energy. H is the enthalpy, which is the amount of heat released or absorbed by a system at constant pressure.

Thus, U is the total energy held by a system while H is specifically the energy that is transferred to/from a system in the form of heat.

Cv utilizes U as its q value because deltaU = q + P*deltaV; Cv is the heat capacity when the volume is constant and since C = q / deltaT and deltaU at a constant volume is merely q (since P*deltaV = P*0 = 0) --> Cv utilizes deltaU for its q value so that Cv = deltaU / deltaT.

On the other hand, Cp utilizes H as its q value because deltaH is equal to q at constant pressure so that Cp = deltaH / deltaT.

Hope that helps!

K Stefanescu 2I
Posts: 68
Joined: Fri Sep 29, 2017 7:04 am

Re: Difference between U and H?

Postby K Stefanescu 2I » Mon Jan 22, 2018 12:13 pm

Also, don't forget that U= q+ w (and any other substitutions for q and w)! So, U is the overall internal energy due to heat and work, whereas q only reflects energy due to heat.

Ryan Sydney Beyer 2B
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Joined: Fri Sep 29, 2017 7:07 am
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Re: Difference between U and H?  [ENDORSED]

Postby Ryan Sydney Beyer 2B » Mon Jan 22, 2018 12:47 pm

It could also be helpful to know the relationship between the two::

Enthalpy is obtained from internal energy when the variable entropy S is substituted by the variable temperature T.


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