Example from Lecture

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Example from Lecture

Postby Sarah_Stay_1D » Tue Feb 13, 2018 11:39 am

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?

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Re: Example from Lecture

Postby Chem_Mod » Tue Feb 13, 2018 4:55 pm

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.

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