8.65, where did that equation come from?

[sofiakavanaugh]

Hi,

For 8.65, you are given the rxns 2NO(g)+ O2(g) --> 2NO2(g) with deltaH=-114.1 KJ and 4NO2(g) +O2(g) ---> 2N2O5(g) with delta H=-110.2 KJ. you are asked to use this and deltaH formation of NO (90.25 KJ/mol) to find the delta H formation of N2O5. Then in the solutions manual, it says that the reaction you want is N2 + 5/2 O2 ---> N2O5, and I was wondering where that rxn came from? How would you know that? Especially the N2 part since that isn't part of either of the equations we were given?

Thanks!

[Rachel Brown 3A]

I was wondering the same thing... Is there possibly a chart or something that we should be using to get that equation or would it be given?

[Farah Ahmad 2A]

In the textbook standard enthalpy of formation is defined as:
"The standard enthalpy of formation of a substance is the standard reaction enthalpy per mole of formula units for the formation of a substance from its elements in their most stable form" (294).
The most important sentence from that definition being "from its elements in their most stable form". So, if we take N2O5, it is made up of the elements Nitrogen and Oxygen. The most stable forms of Nitrogen and Oxygen are N2 and O2 (since they are diatomic molecules). The reason for the 5/2 in front of O2 in the formation reaction is for balancing purposes.

[Tiffany Dao 1A]

It's because the question states to find the enthalpy of formation for dinitrogen pentoxide (N2O5) from the two given equations as wells as the enthalpy of formation of nitric oxide (NO), which would be
N2 +O2 ---> 2NO.
That means that the equation of enthalpy of formation of N2O5 should be N2 + 5/2 O2 --->N2O5.
Which would be found by combining all of the given ones.