Question

A mechanism for a naturally occurring reaction that destroys ozone is Step I: \(\mathrm{O}_{3}(\mathrm{~g})+\mathrm{HO}(\mathrm{g}) \rightarrow \mathrm{HO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g})\) Step II: \(\mathrm{HO}_{2}(\mathrm{~g})+\mathrm{O}(\mathrm{g}) \rightarrow \mathrm{HO}(\mathrm{g})+\mathrm{O}_{2}(\mathrm{~g})\) What is the molecularity of the overall reaction? a. Bimolecular b. Unimolecular c. None of these because molecularity only refers to elementary steps d. None of these because molecularity is the difference of the exponents in the rate law

Short answer

The molecularity of the overall reaction is not defined, making option c correct.

Step-by-step solution

Understand Definition and Given Reactions

Molecularity refers to the number of molecules involved in an elementary reaction step. We are given two steps and asked about the overall molecularity.

Analyze Each Step

For Step I, two reactant molecules, \( \mathrm{O}_{3}(\mathrm{~g}) \) and \( \mathrm{HO}(\mathrm{g}) \), are involved. This step is bimolecular. For Step II, two molecules, \( \mathrm{HO}_{2}(\mathrm{~g}) \) and \( \mathrm{O}(\mathrm{g}) \), are involved. This step is also bimolecular.

Determine the Overall Molecularity

The overall reaction is the sum of these two steps. However, molecularity applies to individual elementary reactions and not the overall reaction. According to the definition, molecularity should be used only for individual steps.

Select the Correct Answer

Molecularity is not defined for the overall reaction, only for elementary steps, making option c, "None of these because molecularity only refers to elementary steps," correct.

Key concepts

Elementary Reactions

In chemistry, an elementary reaction is a single event where reactants directly form products without any intermediate steps. This is the simplest form of reaction and occurs in one step. The molecularity of an elementary reaction refers to how many molecules or atoms are involved at the start of this process. For example, Step I in our exercise involves the molecules

• a molecule of \(\mathrm{O}_{3}(\mathrm{~g})\), and
• a hydroxyl radical \(\mathrm{HO}(\mathrm{g})\)\(,\) making it a bimolecular reaction since there are two reactant molecules involved.Similarly, for Step II with \(\mathrm{HO}_{2}(\mathrm{~g})\) and \(\mathrm{O}(\mathrm{g})\), this is another bimolecular step. Understanding elementary reactions is crucial since they are the basic building blocks of more complex reaction mechanisms.Each elementary reaction step follows its own rate law based solely on its molecularity. This is different from overall reactions because the latter can involve multiple elementary steps and thus have a more complex rate law.
  • To summarize, individual steps, or elementary reactions, have a well-defined molecularity, highlighting the number of reactants involved directly leading to product formation.

Reaction Mechanisms

A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It gives insight into the pathway followed by the reactants to transform into the products. Each mechanism is composed of several elementary reactions.
The mechanism provided in this exercise describes the pathway of ozone destruction. Step I and Step II are elementary steps in this mechanism:

• In Step I, ozone reacts with the hydroxyl radical to form \(\mathrm{HO}_{2}\) and oxygen.
• In Step II, \(\mathrm{HO}_{2}\) reacts with atomic oxygen to regenerate the \(\mathrm{HO}\) radical and produce more \(\mathrm{O}_{2}\).

These steps combined provide a complete picture of how ozone is broken down through a series of intermediate stages within the reaction mechanism. Reaction mechanisms are vital because understanding how each intermediate forms and reacts helps chemists control and predict the outcomes of chemical processes.
Overall, mechanisms provide the detailed route of a reaction, while the kinetic study of these steps defines their rates and how they interact over the course of the reaction.

Ozone Destruction

Ozone destruction is a critical atmospheric reaction involving the breakdown of ozone (\(\mathrm{O}_3\)) in the Earth's stratosphere. This is a major environmental concern as ozone plays a crucial role in absorbing harmful ultraviolet radiation from the sun.The exercise illustrates a mechanism where ozone reacts with hydroxyl radicals and atomic oxygen:

• Ozone and hydroxyl radical form hydrogen peroxide (\(\mathrm{HO}_{2}\)) and molecular oxygen (\(\mathrm{O}_2\)) in the first step.
• Hydrogen peroxide then reacts with atomic oxygen to regenerate the hydroxyl radical and further molecular oxygen in the second step.

Ultimately, this set of reactions is catalytic in nature, with the hydroxyl radical facilitating both the destruction and reformation process, highlighting the complex balance in atmospheric chemistry. Understanding these pathways is paramount in our efforts to mitigate the deleterious effects of substances that lead to ozone depletion.

Chemical Kinetics

Chemical kinetics is the branch of chemistry that studies the speed of chemical reactions, revealing how changes in conditions affect the reaction rate. It involves understanding how different factors, like temperature or concentration, play a role in the rate at which reactions occur. The provided exercise offers an opportunity to delve into the kinetic analysis of ozone destruction. Here, we consider the elementary steps:

• Each step in the mechanism has its own rate law that depends solely on the reactants involved in that particular step.
• In a mechanism, the slowest step, often known as the rate-determining step, dictates the overall rate of the reaction.

By investigating chemical kinetics, scientists can make predictions about reaction behavior, optimize conditions for desired reactions, and understand the overall dynamics in complex reactions like the multi-step destruction of ozone. Hence, chemical kinetics not only broadens comprehension but also enhances our capability to innovate in chemical processes.