## 4D.23

dtolentino1E
Posts: 101
Joined: Thu Jul 11, 2019 12:17 am

### 4D.23

Calculate the standard enthalpy of formation of dinitrogen pentoxide from the following data: 2 NO(g) + O2(g) --> 2 NO2(g); delta H° = -114.1 kJ
4 NO2(g) + O2(g) --> 2 N2O5(g); delta H° = -110.2 kJ
and from the standard enthalpy of formation of nitric oxide, NO (see Appendix 2A).

how do i calculate the standard enthalpy of formation of dinitrogen pentoxide when the equations listed don't have the elements in their simplest forms?

DarrenKim_1H
Posts: 123
Joined: Fri Sep 20, 2019 12:17 am
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### Re: 4D.23

With Hess's law, you can add or subtract different parts of the equation in order to get delta H

Rebecca Remple 1C
Posts: 137
Joined: Wed Sep 18, 2019 12:16 am
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### Re: 4D.23

dtolentino1E wrote:Calculate the standard enthalpy of formation of dinitrogen pentoxide from the following data: 2 NO(g) + O2(g) --> 2 NO2(g); delta H° = -114.1 kJ
4 NO2(g) + O2(g) --> 2 N2O5(g); delta H° = -110.2 kJ
and from the standard enthalpy of formation of nitric oxide, NO (see Appendix 2A).

how do i calculate the standard enthalpy of formation of dinitrogen pentoxide when the equations listed don't have the elements in their simplest forms?

Hi there,

In the solutions manual the equations are manipulated and simplified. The two equations cancel out until you get 2 NO + 3/2 O2 -> N2O5. This has the same enthalpy as the formation as N2 + 5/2 O2 -> N2O5, minus the standard enthalpy of formation of NO. After subtracting the enthalpy of formation of NO, you can get the enthalpy of formation of N2O5. The solutions manual has a nice walk-through of this process if you need help visualizing it. I hope this helps!

-Rebecca Remple

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