Chapter 11, Problem 1CO

Interpretation Introduction

Interpretation:

To understand the role of chemical kinetics in the formation of ozone and other pollutants in urban air (near earth surface).

Concept Introduction:

Different mechanisms and different rate of reactions are responsible for contrasting level of ozone concentrations in troposphere and stratosphere.

Answer to Problem 1CO

Ozone and other pollutants form in the troposphere due to different rate of reactions involved.

Explanation of Solution

Ozone also formed at ground level but the mechanism and kinetics are very different from that in upper atmosphere. The formation of ozone in smog comprises several steps. The oxide of nitrogen ${\text{NO}}_{\text{2}}$ forms in the high temperature of automobile engines.

When NO₂ absorbs sunlight, it dissociates to:

$N{O}_2\text{ + h}\nu \to \text{ NO + O}$

The oxygen atom generated by this reaction can now form ozone:

$O_2\text{ + O }\to {\text{ O}}_{\text{3}}$

Thus, formed ozone will quickly react with NO:

$O_3\text{ + NO }\to {\text{ NO}}_{\text{2}}{\text{ + O}}_{\text{2}}$

This reaction is much faster so ozone is unlikely to form if no other compound reacts with NO to consume it and make it unavailable for dissociation of ozone. The chemical which can consume NO are volatile organic compounds (VOC). Most of the VOC’s are industrial solvents.

The reactions to remove NO are much faster than the reaction of NO with O₃. Thus, in the presence of sunlight smog can form along with ozone.

Conclusion

Ozone is also a pollutant at concentrations higher than 1 ppm along with other oxides of nitrogen and sulphur.