### Delta H and Delta U

Posted:

**Sat Jan 13, 2018 8:21 pm**Why can we equate delta H (heat transfer at constant pressure) and delta U (heat transfer at constant volume)?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=25440

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Posted: **Sat Jan 13, 2018 8:21 pm**

Why can we equate delta H (heat transfer at constant pressure) and delta U (heat transfer at constant volume)?

Posted: **Sat Jan 13, 2018 9:04 pm**

Mary Becerra 2D wrote:Why can we equate delta H (heat transfer at constant pressure) and delta U (heat transfer at constant volume)?

You can not always equate delta H (enthalpy) to delta U (internal energy). The two are only equal when you have constant pressure and constant volume. This is true because at constant pressure ΔH = ΔU + PΔV. So, if the volume is constant, the change in volume would be 0. Therefore, ΔH = ΔU + P(0) so, ΔH =ΔU.

Posted: **Sun Jan 14, 2018 1:59 pm**

Yes this is only true when the pressure and volume are constant

Posted: **Sun Jan 14, 2018 4:03 pm**

How are the equations for delta h and delta u different? Are there differenet variables in each equation?