Internal Energy in Thermodynamics


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Michelle_Tan_1G
Posts: 35
Joined: Fri Jun 17, 2016 11:28 am

Internal Energy in Thermodynamics

Postby Michelle_Tan_1G » Mon Jan 30, 2017 10:11 pm

As I understand it, internal energy is equal to work + heat in the equation ∆U = q + w. However, there's a second equation that reads ∆U = ∆H - P∆V. Change in heat represents q, and P∆V = work of expansion. What is the reasoning behind the positive sign in ∆U = q + w changing to a negative in ∆U = ∆H - P∆V?

Evan Lee 2D
Posts: 18
Joined: Wed Sep 21, 2016 2:58 pm

Re: Internal Energy in Thermodynamics

Postby Evan Lee 2D » Mon Jan 30, 2017 10:28 pm

From my understanding, w = -P∆V.

So ∆U = q + w becomes ∆U = q + (-P∆V), which then becomes ∆U = ∆H - P∆V when you replace the terms.

Work of expansion is negative work.

Michelle_Tan_1G
Posts: 35
Joined: Fri Jun 17, 2016 11:28 am

Re: Internal Energy in Thermodynamics

Postby Michelle_Tan_1G » Mon Jan 30, 2017 11:05 pm

Thank you, that makes sense! So w=-PV represents negative work done by expansion. When work is lost by expansion it is replaced by heat added into the system (in reversible reactions).


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