The reaction between nitrogen dioxide and ozone, 2NO 2 + O 3 -> N 2 O 5 + O 2 ha

Question

The reaction between nitrogen dioxide and ozone,

       2NO2 + O3 -> N2O5 + O2 has been studied at 231 K. The experimental rate equation is

       rate = k[NO2]O3].

a) What is the order of the reaction?

b) Is either of the following proposed mechanism s consistent with the given kinetic data?

(1)         NO2 + NO2 <-->N2O4                     (fast, equilibrium)

N2O4 +O3--> N2O5 + O2                  (slow)

(2)          NO2 + O3 --> NO3 + O2                     (slow)

NO3 +NO--> N2O5                           (fast)

Explanation / Answer

order of reaction is 2

as rate equation is  rate = k[NO2]O3].

and order of reaction of a general reaction of type say A+B ---> C + D

rate = k [A]a[B]b

order here equal to a+b

so comparing with this equation a=1 and b=1

so overall order of reaction = 1+1 =2

and for second part

you need to know that rate law is being decided only by the slow step of the reaction mechanism , if theany of the slow step in the two mechanism matches with the rate law proposed then that mechanism is consistent with the given rate law

for example here in second mechanism the slow step is NO2 + O3 --> NO3 + O2

and any single reaction in the given mechanism is an elementary reaction means their rate law can be directly written by seeing the stoichiometric coeffiecnt of the elementary reaction which here is 1 and 1 wrt NO2 and O3

so the final rate equation by looking at the second reaction is rate = k[NO2]O3].

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