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Ashley Macabasco 2K
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Postby Ashley Macabasco 2K » Thu Jan 11, 2018 5:07 pm

For Hess's law, do you only add the enthalpy changes of the same phase to find the standard reaction enthalpy? For example in question 8.59 to calculate the standard reaction enthalpy of 4HNO3(l) + 5N2H4(l) ---> 7N2(g) + 12H2O(l), nitrogen is not included in finding the standard reaction enthalpy and I was wondering if it was due to it being in the gas phase and not the liquid phase as the other components of the equation.

Ramya Lakkaraju 1B
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Joined: Fri Sep 29, 2017 7:03 am

Re: 8.59

Postby Ramya Lakkaraju 1B » Thu Jan 11, 2018 5:11 pm

From what I understand, the only reason that nitrogen is not included is because its enthalpy is 0 if you look at the table. I don't think it has to do with the fact that it is in its gas phase.

Gevork 2E
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Re: 8.59

Postby Gevork 2E » Thu Jan 11, 2018 5:39 pm

From my understanding, Nitrogen has its value as 0 because in standard conditions it will be diatomic. It would not require any energy loss or gain to be in that state, so therefore its value is 0.

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