Hess’s Law. Nitric oxide (NO) is a pollutant formed by the combustion of fuels i
ID: 958190 • Letter: H
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) H° = –427 kJ
O2(g) --> 2 O(g) H° = 495 kJ
NO(g) + O3(g) --> NO2(g) + O2(g) H° = –199 kJ
Using the above data, calculate H° 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!
Explanation / Answer
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)
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NO + O ---> NO2
DH0 final= -495/2 + 427/2 -199= -233 kJ