Question

In 1995, Mario Molina, Paul Crutzen, and F. Sherwood Rowland shared the Nobel Prize in chemistry for their work on atmospheric chemistry. One of several reaction sequences proposed for the role of chlorine in the decomposition of stratospheric ozone (we’ll see another sequence in Chapter 16) is

$\begin{array}{l}\mathbf{\left(}\mathbf{1}\mathbf{\right)}\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{3}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\\ \mathbf{\left(}\mathbf{2}\mathbf{\right)}\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }{\mathbf{Cl}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\\ \mathbf{\left(}\mathbf{3}\mathbf{\right)}{\mathbf{Cl}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\stackrel{\mathbf{light}}{\mathbf{\to }}\mathbf{2}\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\end{array}$

Over the tropics, O atoms are more common in the stratosphere:

$\mathbf{\left(}\mathbf{4}\mathbf{\right)}\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{O}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

(a) Which, if any, of these are oxidation-reduction reactions?

(b) Write an overall equation combining reactions 1–3.

Short answer

1. Equation (1), (3) and (4) undergoes oxidation-reduction reaction.
2. The overall equation is,

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

Step-by-step solution

(a) Determination of oxidation-reduction reaction

The oxidation states for equation (1) are shown below,

$\stackrel{\mathbf{0}}{\mathbf{Cl}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\stackrel{\mathbf{0}}{\mathbf{O}}}_{\mathbf{3}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\stackrel{\mathbf{0}}{\mathbf{O}}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

From the equation, it can be concluded that oxidation state of chlorine increases from 0 to +1 and the oxidation state of oxygen decreases from 0 to -1. Thus, the equation (1) undergoes oxidation-reduction reaction.

The oxidation states for equation (2) are shown below,

$\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }{\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}}_{\mathbf{2}}{\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

From the equation, it can be concluded that there is no change in the oxidation states of reactants and products. Thus, the equation (2) does not undergoes oxidation-reduction reaction.

The oxidation states for equation (3) are shown below,

${\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}}_{\mathbf{2}}{\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\stackrel{\mathbf{light}}{\mathbf{\to }}\mathbf{+}\mathbf{2}\stackrel{\mathbf{0}}{\mathbf{Cl}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }{\stackrel{\mathbf{0}}{\mathbf{O}}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

From the equation, it can be concluded that oxidation state of oxygen increases from -1 to 0 and the oxidation state of chlorine decreases from +1 to 0. Thus, the equation (3) undergoes oxidation-reduction reaction.

The oxidation states for equation (4) are shown below,

$\stackrel{\mathbf{+}\mathbf{1}}{\mathbf{Cl}}\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\stackrel{\mathbf{-}\mathbf{1}}{\mathbf{O}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\stackrel{\mathbf{0}}{\mathbf{Cl}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\stackrel{\mathbf{0}}{\mathbf{O}}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}$

From the equation, it can be concluded that oxidation state of oxygen increases from -1 to 0 and the oxidation state of chlorine decreases from +1 to 0. Thus, the equation (4) undergoes oxidation-reduction reaction.

Overall equation

The overall equation is,

$\begin{array}{l}\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{3}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\\ \mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }{\mathbf{Cl}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\\ {\mathbf{Cl}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\stackrel{\mathbf{light}}{\mathbf{\to }}\mathbf{2}\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\\ \mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\mathbf{\_}\\ \mathbf{ClO}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}{\mathbf{O}}_{\mathbf{3}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{\to }\mathbf{Cl}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{+}\mathbf{2}{\mathbf{O}}_{\mathbf{2}}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\end{array}$