Question

Hess’s Law. Nitric oxide (NO) is a pollutant formed by the combustion of fuels in the presence of air, which is composed of approximate 80 % nitrogen gas (N2). In the atmosphere, NO and NO2 can inter convert by reacting with oxygen in the atmosphere.
Thermodynamic data

2 O3(g) --> 3 O2(g)   ΔH° = –427 kJ  

O2(g) --> 2 O(g)    ΔH° = 495 kJ  

NO(g) + O3(g) --> NO2(g) + O2(g) ΔH° = –199 kJ

Using the above data, calculate ΔH° for the following reaction:

NO(g) + O(g) --> NO2(g)

I know the answer (-233 kJ/mol) but I'm not sure how they got the answer please show steps so I can understand how you got your answer. Thank you!

Answer 1

In this kind of exercices you must arrange the reaction in order to get the one asked.

NO + O3 ----> NO2 + O2 DH0= -199 kJ

(3O2 ----> 2O3)x1/2 DH0= 427/2 kJ (this is the opposite of the reaction given, so DH0 is also the opposite, it will be positive. Also we multiply it by 1/2 so the value of DH0 will be 1/2 of the original value).

(2O ---> O2)x1/2 DH0= -495/2 kJ (opposite sign and x1/2 for the same reasons that the above one)

-----------------------------

NO + O ---> NO2

DH0 final= -495/2 + 427/2 -199= -233 kJ