## DeltaU and DeltaH

$\Delta U=q+w$

104969854
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### DeltaU and DeltaH

When does deltaU equal deltaH?
Does this happen in isochoric or isobaric conditions? What about at constant heat?

Angela 1K
Posts: 80
Joined: Fri Sep 29, 2017 7:05 am

### Re: DeltaU and DeltaH

$\Delta U=\Delta H$ 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.

Cristina Sarmiento 1E
Posts: 52
Joined: Wed Nov 16, 2016 3:02 am

### Re: DeltaU and DeltaH

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.

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### Re: DeltaU and DeltaH

What if there were a change in pressure?

104969854
Posts: 36
Joined: Fri Sep 29, 2017 7:05 am

### Re: DeltaU and DeltaH

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.