## Question 8.65

$\Delta U=q+w$

LannaP
Posts: 11
Joined: Fri Sep 25, 2015 3:00 am

### Question 8.65

Calculate the standard enthalpy of formation of dinitrogen pentoxide from the following data,
2 NO(g) + O2(g) --> 2 NO2 (g) ΔH = -114.1 kJ
4 NO2(g) + O2(g) --> 2 N2O5 (g) ΔH = -110.2 KJ

and from the standard enthalpy of formation of nitric oxide, NO, which is +90.25 kJ

How do we know that the reaction that we want is
N2(g) + 5/2 O2(g) --> N2O5 (g) ?

Sandeep Gurram 2E
Posts: 42
Joined: Fri Sep 25, 2015 3:00 am

### Re: Question 8.65

The desired reaction for a standard enthalpy of formation of any compound is just a balanced chemical reaction between the constituent elements in their standard states as reactants forming the desired compound as the product.

For example, in this problem the desired standard enthalpy of formation is N2O5. The constituent elements are nitrogen and oxygen. So just balance both of these elements in their standard states to form dinitrogen pentoxide.

Hope that helped!