Page 1 of 1

DeltaU and DeltaH

Posted: Thu Jan 25, 2018 8:38 pm
When does deltaU equal deltaH?
Does this happen in isochoric or isobaric conditions? What about at constant heat?

Re: DeltaU and DeltaH

Posted: Thu Jan 25, 2018 9:03 pm
$\Delta U=\Delta H$ occurs when the volume is constant. If you want to derive this, the equation for $\Delta U$ is $\Delta U=\Delta H-P\Delta V$. Thus, when there is no change in volume, $\Delta V=0$ and so $P\Delta V=0$. Thus, $\Delta U=\Delta H$. Not 100% sure, but I hope this helped.

Re: DeltaU and DeltaH

Posted: Thu Jan 25, 2018 9:47 pm
Delta U = Delta H when there is constant pressure and constant volume. Remember that Delta U = q +w. Delta H = q when there is constant pressure. w= -P Delta V. So when volume is constant, Delta V = 0. Therefore when there is constant pressure and volume, you can substitute q & w and get delta U = delta H.

Re: DeltaU and DeltaH

Posted: Wed Jan 31, 2018 6:55 pm
What if there were a change in pressure?

Re: DeltaU and DeltaH

Posted: Wed Feb 14, 2018 12:20 am
Angela 1K wrote:$\Delta U=\Delta H$ occurs when the volume is constant. If you want to derive this, the equation for $\Delta U$ is $\Delta U=\Delta H-P\Delta V$. Thus, when there is no change in volume, $\Delta V=0$ and so $P\Delta V=0$. Thus, $\Delta U=\Delta H$. Not 100% sure, but I hope this helped.

Hi, I'm looking back at this and I'm just confused because your explanation totally makes sense with the equation you gave, and because I know that with constant volume work equals 0 so q=delta U. with your explanation, would this make delta H=delta U=q under constant volume, and people are saying that q is only equal to delta H under constant pressure....I'm just lost with the inconsistency and was wondering if you could elaborate a bit.