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Example from Lecture

Posted: Tue Feb 13, 2018 11:39 am
by Sarah_Stay_1D
In lecture on Monday Lavelle gave the following example problem:

Oxygen diflouride is a colorless gas the reacts rapidly with water vapor to produce O2, HF, and heat:
OF2 (g) + H2O (g) --> O2 (g) +2HF (g) ∆H = -318 kJ
What is the change in internal energy for the reaction of 1.00 moles of OF2 gas at 298k.

I am slightly confused, because I thought when we have a system at constant temperature ∆U = 0, however in this case we found that ∆U = -320 KJ. Does this have to do with the fact that we are not using ideal gases?

Re: Example from Lecture

Posted: Tue Feb 13, 2018 4:55 pm
by Chem_Mod
Hi, Sarah,
In this case, deltaU is not zero, as it is not an isothermal expansion (it is not an expansion at all). You generate gas, so you can use the formula:
w = -deltan*R*T (where delta n is the change in moles of gas) to find work and then plug it into delta U = q+w, where q = -318J.