Question

Nitric oxide (NO) can catalyze ozone destruction through the catalytic process described below:

NO + O3 > NO2 + O2 (Reaction 1)

NO2 + O > NO + O2 (Reaction 2)

The reaction rate (how fast the reaction proceeds) is calculated as the product of the reaction rate constant k, and the respective concentrations of the reactants e.g. in the reaction A + B products, the reaction rate is given by Rate = d[A]/dt = d[B]/dt = k[A][B].

The reaction rate constants of reactions (1) and (2) are k1=3.5x10-15 cm3/(molecule sec) and k2=9.3x10-12 cm3/(molecule sec). Concentrations of reactants at an altitude of 20 km, are as follows:

[NO] = 2.0 x 109 molecules/cm3

[O3] = 3.0 x 1012 molecules/ cm3

[NO2] = 8.0 x 109 molecules/cm3

[O] = 2.0 x 107 molecules/ cm3

a. Use the rate low to write the reaction rate k for each of the two reactions. b. Calculate the reaction rates for the two reactions in units of molecules/(cm3 s). Pay attention to units. c. Which reaction proceeds at a faster rate? By what factor? What is the dominant parameter that influences the relative rates of these two reactions under these conditions, rate constant or concentration? d. Which of the two reactions is the limiting step for the overall destruction of O3 by the above mechanism?

Answer 1

a.For writing the rate law

assuming each step of the reaction to be elementary

So,Rate law for 1st reaction is given as

for 2nd rate llaw would be

b.Reaction rates for 1st reaction is

Reaction rate for 2nd reaction would be

c.We can see clearly reation 1 proceeds at a faster rate by factor

2.1/0.1488=14.11

d.Dominant factor that influences reaction rates are concentration because it contributes more as compared to rate constant.

e.Always slow reaction rate step is the rate limitng step so here reaction 2 is Rate dermining step.