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### ΔU vs. ΔH

Posted: Sun Jan 21, 2018 10:57 pm
Can someone explain more in-depth the difference between ΔU & ΔH? I have a general understanding, but I’d like to have a more solid knowledge for the test. (for ex: in which circumstances do we use which one?)

Thanks!

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:10 pm
When heat is released or absorbed during a reaction that takes place at constant volume, you use deltaU. When heat is released or absorbed during a reaction that takes place under constant pressure, you use deltaH.
I found the following website to be very helpful in explaining the difference if you read "Enthalpy Versus Internal Energy." Hope this helps!

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:10 pm
I also was wondering what the relationship between the two was. Like are they completely different variables or do they work together?

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:11 pm

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:15 pm
ΔH is the thermodynamic potential of the system, and thus includes work done and ΔU, ΔH=ΔU + PΔV. ΔU is the summation of energies in a system. When work done=0, ΔH=ΔU.

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:29 pm
Delta U is a summation of the work done in the system and the heat in the system. In other words, it is a measure of the internal energy of the system. When pressure is constant, there is no work being done. Therefore delta U is equal to delta H under these circumstances.

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:35 pm

This is a really helpful video that explains ΔU and ΔH.

### Re: ΔU vs. ΔH

Posted: Sun Jan 21, 2018 11:46 pm
deltaU= change in internal energy. Internal energy is the energy in every molecule of in the substance.

deltaU = Q+W

Q= heat added to system and W= work done on the system

deltaU+PdV= W

change in enthalpy (deltaH) is change in total energy of the system and enthalpy H can be written as

H= U+PV

delH= delU+PdV+VdP

### Re: ΔU vs. ΔH

Posted: Mon Jan 22, 2018 12:21 am
Delta U is internal energy, so delta U = q + w. Delta H is enthalpy (heat under constant pressure), so it doesn't account for work.

### Re: ΔU vs. ΔH

Posted: Mon Jan 22, 2018 12:40 am
Delta H deals with internal energy at a constant pressure versus delta U is just change in internal energy.

### Re: ΔU vs. ΔH

Posted: Mon Jan 22, 2018 12:53 pm
It might also be helpful to know the relationship between the two::

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