Question

Consider this scenario and answer the following questions: Chlorine atoms resulting from decomposition of chlorofluoromethanes, such as \({\bf{CC}}{{\bf{l}}_{\bf{2}}}{{\bf{F}}_{\bf{2}}}\), catalyse the decomposition of ozone in the atmosphere. One simplified mechanism for the decomposition is:

\(\begin{aligned}{}{{\bf{O}}_{\bf{3}}}\overset{sunlight}{\rightarrow}{}{{\bf{O}}_{\bf{2}}}{\rm{ }} + {\rm{ }}{\bf{O}}\\{{\bf{O}}_{\bf{3}}}{\rm{ }} + {\rm{ }}{\bf{Cl}}\to {{\bf{O}}_{\bf{2}}}{\rm{ }} + {\rm{ }}{\bf{ClO}}\\{\bf{ClO}}{\rm{ }} + {\rm{ }}{\bf{O}}\to {\bf{Cl}}{\rm{ }} + {\rm{ }}{{\bf{O}}_{\bf{2}}}\end{aligned}\)

(a) Explain why chlorine atoms are catalysts in the gas-phase transformation:

\({\bf{2}}{{\bf{O}}_{\bf{3}}}\mathop {}\limits^{}\to {\bf{3}}{{\bf{O}}_{\bf{2}}}\)

(b) Nitric oxide is also involved in the decomposition of ozone by the mechanism: Is NO a catalyst for the decomposition? Explain your answer.

\(\begin{aligned}{}{{\bf{O}}_{\bf{3}}}\overset{sunlight}{\rightarrow}{\rm{ }}{{\bf{O}}_{\bf{2}}}{\rm{ }} + {\rm{ }}{\bf{O}}\\{{\bf{O}}_{\bf{3}}}{\rm{ }} + {\rm{ }}{\bf{NO}}\rightarrow {\bf{N}}{{\bf{O}}_{\bf{2}}}{\rm{ }} + {\rm{ }}{{\bf{O}}_{\bf{2}}}\\{\bf{N}}{{\bf{O}}_{\bf{2}}}{\rm{ }} + {\rm{ }}{\bf{O}}\rightarrow {\bf{NO}}{\rm{ }} + {\rm{ }}{{\bf{O}}_{\bf{2}}}\end{aligned}\)

Short answer

(a) Chlorine atoms are catalysts in gas-phase transformation of ozone because chlorine atoms act as homogeneous catalyst, which exists in the same gas-phase of the reactant. (b) Yes, nitric oxide NO, acts as a homogeneous catalyst for the decomposition of ozone.

Step-by-step solution

Definition of Catalyst, Homogeneous Catalyst, Heterogeneous Catalyst

Catalyst are substances which increases the rate of the reaction without getting used up by themselves while decreasing the activation energy of the reaction.

Homogeneous catalyst is one which exists in the same phase of the reactants, reacts directly with the reactants forming an intermediate substance which undergo decomposition with another reactant in one or many steps forming product and reproducing the catalyst.

Example, Earth’s ozone layer. Ozone is formed when oxygen molecule absorbs ultraviolet radiations.

\({\rm{3}}{{\rm{O}}_{\rm{2}}}{\rm{(g) }}\xrightarrow[no catalyst]{hv}{\rm{ 2}}{{\rm{O}}_{\rm{3}}}{\rm{(g)}}\)

Heterogeneous catalyst is one which exists in different phase (generally solid) than the reactants (which may be gas or liquid phase), provides an active surface for the reaction to takes place in at least four steps.

Example, Hydrogenation of ethylene with nickel as catalyst to form ethane.

\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{ }}\xrightarrow[{catalyst}]{{Ni}}{\text{ }}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{6}}}\)

Chlorine atoms are Catalysts in gas-phase Transformations  

Ozone is formed when oxygen molecule absorbs ultraviolet radiations.

\({\text{3}}{{\text{O}}_{\text{2}}}{\text{(g) }}\xrightarrow[{}]{{hv}}{\text{ 2}}{{\text{O}}_{\text{3}}}{\text{(g)}}\)

The triatomic ozone is highly unstable which undergoes reverse decomposition reaction to form diatomic oxygen with mechanism as shown below:

\(\begin{gathered} {{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ }}{{\text{O}}_{\text{2}}}{\text{ + O}} \hfill \\ {\text{O + }}{{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ 2}}{{\text{O}}_{\text{2}}} \hfill \\ \end{gathered} \)

This reaction is catalysed by chlorine by the following mechanism:

\(\begin{gathered} {{\mathbf{O}}_{\mathbf{3}}}\xrightarrow{{{\mathbf{sunlight}}{\text{ }}}}{{\mathbf{O}}_{\mathbf{2}}}{\text{ }} + {\text{ }}{\mathbf{O}} \hfill \\ {{\mathbf{O}}_{\mathbf{3}}}{\text{ }} + {\text{ }}{\mathbf{Cl}}\xrightarrow{{}}{{\mathbf{O}}_{\mathbf{2}}}{\text{ }} + {\text{ }}{\mathbf{ClO}} \hfill \\ {\mathbf{ClO}}{\text{ }} + {\text{ }}{\mathbf{O}}\xrightarrow{{}}{\mathbf{Cl}}{\text{ }} + {\text{ }}{{\mathbf{O}}_{\mathbf{2}}} \hfill \\ \end{gathered} \)

Chlorine atoms act as homogeneous catalyst, which exists in the same gas-phase of the reactant ozone.

These chlorine atoms are reproduced in the decomposition of ozone and are not completely used.

They increase the rate of the reaction.

It is demonstrated by scientists that single monoatomic chlorine can decompose thousands of ozone molecules.

Thus, chlorine atoms are catalysts in gas-phase transformation of ozone.

Decomposition of Ozone with Nitric Oxide as Catalyst  

Ozone is formed when oxygen molecule absorbs ultraviolet radiations.

\(\begin{gathered} {\text{3}}{{\text{O}}_{\text{2}}}{\text{(g) }}\xrightarrow{{{\text{hv}}}}{\text{ 2}}{{\text{O}}_{\text{3}}}{\text{(g)}} \hfill \\ {{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ }}{{\text{O}}_{\text{2}}}{\text{ + O}} \hfill \\ {\text{O + }}{{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ 2}}{{\text{O}}_{\text{2}}} \hfill \\ \end{gathered} \)

This formed triatomic ozone is highly unstable which undergoes reverse decomposition reaction to form diatomic oxygen with mechanism as shown below:

\(\begin{gathered}{{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ }}{{\text{O}}_{\text{2}}}{\text{ + O}} \hfill \\ {\text{O + }}{{\text{O}}_{\text{3}}}{\text{ }}\xrightarrow{{}}{\text{ 2}}{{\text{O}}_{\text{2}}} \hfill \\ \end{gathered} \)

This reaction is catalysed by nitric oxide (NO), which acts as a homogeneous catalyst. Where nitric oxide exists in the same gas-phase of the reactant ozone. The mechanism is:

\(\begin{gathered} {\text{NO(g) + }}{{\text{O}}_{\text{3}}}{\text{(g) }}\xrightarrow{{}}{\text{ N}}{{\text{O}}_{\text{2}}}{\text{(g) + }}{{\text{O}}_{\text{2}}}{\text{(g)}} \hfill \\ {{\text{O}}_{\text{3}}}{\text{(g) }}\xrightarrow{{}}{\text{ }}{{\text{O}}_{\text{2}}}{\text{(g) + O(g)}} \hfill \\ {\text{N}}{{\text{O}}_{\text{2}}}{\text{(g) + O(g) }}\xrightarrow{{}}{\text{ NO(g) + }}{{\text{O}}_{\text{2}}}{\text{(g)}} \hfill \\ \end{gathered} \)

As nitric oxide is reproduced in these reactions; and it is not used up completely it acts as catalyst by increasing the rate of reaction.

The overall reaction which is catalysed by nitric oxide, will be:

\({\text{3}}{{\text{O}}_{\text{2}}}{\text{(g) }}\xrightarrow[{NOcatalyst}]{{hv}}{\text{ 2}}{{\text{O}}_{\text{3}}}{\text{(g)}}\)

Thus, chlorine atoms are catalysts in the gas-phase transformations and nitric oxide also acts as catalysts in the decomposition of ozone.