Question

A proposed two-step mechanism for the destruction of ozone in the upper atmosphere is

(a) What is the overall balanced equation for the ozone destruction reaction?

(b) Which species is a catalyst?

(c) Which species is an intermediate?

(d) What is the rate law derived from this mechanism if the first step in the mechanism is slow and the second step is fast?

(e) One of the concerns about the use of Freons is that they will migrate to the upper atmosphere, where chlorine atoms can be generated by the reaction

$\underset{\mathbf{\text{Feron-12}}}{{\mathbf{\text{CCl}}}_{\mathbf{\text{2}}}{\mathbf{\text{F}}}_{\mathbf{\text{2}}}}\stackrel{\mathbf{\text{hv}}}{\mathbf{\to }}{\mathbf{\text{CF}}}_{\mathbf{\text{2}}}\mathbf{\text{Cl}}\mathbf{+}\mathbf{\text{Cl}}$

Chlorine atoms also can act as a catalyst for the destruction of ozone. The first step of a proposed mechanism for chlorine-catalysed ozone destruction is

$\mathbf{\text{Cl}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{\text{O}}}_{\mathbf{\text{3}}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\mathbf{\text{ClO}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{\text{O}}}_{\mathbf{\text{2}}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\text{Slow}}$

Assuming a two-step mechanism, propose the second step in the mechanism and give the overall balanced equation.

Short answer

(a) The overall balanced equation for the ozone destruction reaction is ${\text{O}}_3+\left[\text{O}\right]\to 2{\text{O}}_2$.

(b) The species which is catalyst is$\text{NO}$.

(c) The species which is intermediate is ${\text{NO}}_2$.

(d) The rate law derived from the mechanism if the first step in the mechanism is slow and the second step is fast is $\text{Rate}=\text{k}\left[{\text{O}}_{\text{3}}\right]\left[\text{NO}\right]$.

(e) The second step in the mechanism is${\text{ClO}}_{\left(g\right)}+{\left[\text{O}\right]}_{\left(g\right)}\to {{\text{O}}_{\text{2}}}_{\left(g\right)}+{\text{Cl}}_{\left(g\right)}$ and the overall balanced equation is ${{\text{O}}_{\text{3}}}_{\left(g\right)}+{\left[\text{O}\right]}_{\left(g\right)}\to {{\text{2O}}_{\text{2}}}_{\left(g\right)}$.

Step-by-step solution

Concept Introduction:

In chemistry, a catalyst is any material that speeds up a process without being consumed. Enzymes, which are catalysts found in nature, are used to speed up a number of important metabolic processes.

An equation is said to be balanced when the total charge and number of atoms for each element in the reaction are the same for both reactants and products.

(a) Overall balanced equation:

Chemical reactions can be divided into elementary reactions and multi-step reactions based on how many stages are involved in them. The basic step in a chemical reaction that shows how a reactant completely transforms into a product in a multi-step reaction is referred to as the elementary step.

Hence the steps of the given reaction can be written as,

$$\begin{gathered} {\text{O}}_{\text{3}}+\text{NO}\to {\text{NO}}_{\text{2}}+{\text{O}}_{\text{2}} \\ {\text{NO}}_{\text{2}}+\left[\text{O}\right]\to \text{NO}+{\text{O}}_{\text{2}} \\ {\text{O}}_{\text{3}}+\left[\text{O}\right]\to {\text{2O}}_{\text{2}}\left(\text{overallreaction}\right) \end{gathered}$$

Therefore, the overall reaction is ${\text{O}}_3+\left[\text{O}\right]\to 2{\text{O}}_2$.

(b) Catalyst Species:

A catalyst can be defined as the substance which enhance the rate of reaction and again regenerate at the end of reaction.

$$\begin{gathered} {\text{O}}_{\text{3}}+\text{NO}\to {\text{NO}}_{\text{2}}+{\text{O}}_{\text{2}} \\ {\text{NO}}_{\text{2}}+\left[\text{O}\right]\to \text{NO}+{\text{O}}_{\text{2}} \\ {\text{O}}_{\text{3}}+\left[\text{O}\right]\to {\text{2O}}_{\text{2}}\left(\text{overallreaction}\right) \end{gathered}$$

In the given reaction$\text{NO}$ involves in step-1 and again regenerate in step-2 therefore it is not part of overall reaction. Hence,$\text{NO}$ must be catalyst for the given reaction.

Therefore, the catalyst is $\text{NO}$.

(c) Intermediate Species:

The species which is formed during the reaction and will convert to product at the end of reaction is called as intermediate.

$$\begin{gathered} {\text{O}}_{\text{3}}+\text{NO}\to {\text{NO}}_{\text{2}}+{\text{O}}_{\text{2}}\left(\text{slow}\right) \\ {\text{NO}}_{\text{2}}+\left[\text{O}\right]\to \text{NO}+{\text{O}}_{\text{2}}\left(\text{fast}\right) \\ {\text{O}}_{\text{3}}+\left[\text{O}\right]\to {\text{2O}}_{\text{2}}\left(\text{overallreaction}\right) \end{gathered}$$

In the given reaction ${\text{NO}}_2$molecules form in step-1 and again consume in step-2 therefore it is not part of overall reaction. Hence,${\text{NO}}_2$ must be intermediate for the given reaction.

Therefore, the intermediate is ${\text{NO}}_2$.

(d) Evaluation of Law Rate:

The reaction is,

$$\begin{gathered} {\text{O}}_{\text{3}}+\text{NO}\to {\text{NO}}_{\text{2}}+{\text{O}}_{\text{2}}\left(\text{slow}\right) \\ {\text{NO}}_{\text{2}}+\left[\text{O}\right]\to \text{NO}+{\text{O}}_{\text{2}}\left(\text{fast}\right) \\ {\text{O}}_{\text{3}}+\left[\text{O}\right]\to {\text{2O}}_{\text{2}}\left(\text{overallreaction}\right) \end{gathered}$$

In the given reaction, the step-1 is slow step and step-2 is fast step therefore the rate law will be determined with the help of step-1. In step-1; ${\text{O}}_3$and $\text{NO}$take part therefore the rate law can be written as,

$\text{Rate}=\text{k}\left[{\text{O}}_{\text{3}}\right]\left[\text{NO}\right]$

Hence, the rate law is $\text{Rate}=\text{k}\left[{\text{O}}_{\text{3}}\right]\left[\text{NO}\right]$.

(e) Overall Balanced Equation:

The destruction of ozone with$\text{NO}$ is given below.

$$\begin{gathered} {\text{O}}_{\text{3}}+\text{NO}\to {\text{NO}}_{\text{2}}{\text{+O}}_{\text{2}}\left(\text{slow}\right) \\ {\text{NO}}_{\text{2}}+\left[\text{O}\right]\to {\text{NO}}_{\text{+}}{\text{O}}_{\text{2}}\left(\text{fast}\right) \\ {\text{O}}_{\text{3}}+\left[\text{O}\right]\to {\text{2O}}_{\text{2}}\left(\text{overallreaction}\right) \end{gathered}$$

Freons can form active $\text{Cl}$atoms in the presence of light which can further react with ozone to form oxygen gas. The mechanism can be written as,

$$\begin{gathered} {{\text{O}}_{\text{3}}}_{\left(g\right)}+{\text{Cl}}_{\left(g\right)}\to {{\text{O}}_{\text{2}}}_{\left(g\right)}+{\text{ClO}}_{\left(g\right)}\text{(slow)} \\ {\text{ClO}}_{\left(g\right)}+{\left[\text{O}\right]}_{\left(g\right)}\to {{\text{O}}_{\text{2}}}_{\left(g\right)}+{\text{Cl}}_{\left(g\right)}\text{(fast)} \\ {{\text{O}}_{\text{3}}}_{\left(g\right)}+{\left[\text{O}\right]}_{\left(g\right)}\to {{\text{2O}}_{\text{2}}}_{\left(g\right)}\text{(overallreaction)} \end{gathered}$$

Hence, the balanced equation is ${{\text{O}}_{\text{3}}}_{\left(g\right)}+{\left[\text{O}\right]}_{\left(g\right)}\to {{\text{2O}}_{\text{2}}}_{\left(g\right)}$.