Question

The discovery that chlorofluorocarbons initiate a radical -chain reaction that catalytically destroys ozone won the 1995Nobel prize in chemistry Given the initiation step below which of the following reactions is the most likely to serve as one of the propagation steps?
$CC{l}_2{F}_2+hv{\to }^{\ast }CCl{F}_2{+}^{\ast }Cl$

• A. ${}^{\ast }Cl{+}^{\ast }Cl\to C{l}_2$
• B. ${}^{\ast }Cl+O_3{\to }^{\ast }OCl+O_2$
• C. ${}^{\ast }CCl{F}_2{+}^{\ast }Cl\to CC{l}_2{F}_2$
• D. $2^{\ast }CCl{F}_2+2O_3\to 2CO{F}_2+C{l}_2+2O_2$

Answer 1

The correct option is B ${}^{\ast }Cl+O_3{\to }^{\ast }OCl+O_2$

The ultraviolet light, as it hits the atmosphere, makes CFCs break up to form free radicals like this:

$CC{l}_2{F}_2{\to }^{\ast }CCl{F}_2+C{l}^{\ast }$

Chlorine free radicals will then go on to react with the ozone in the stratosphere: $O_3+C{l}^{\ast }\to Cl{O}^{\ast }+O_2$ (Propagation step)

The chlorine oxide molecule $\left(ClO\right)$ is very reactive, and it then proceeds to react with ozone to make two oxygen molecules and a $C{l}^{\ast }$ free radical:

$Cl{O}^{\ast }+O_3\to 2O_2+C{l}^{\ast }$

That $C{l}^{\ast }$ free radical will then go on and react with another ozone molecule, resulting in a chain reaction between the last two equations that theoretically never stops, due to their unreactive nature and thus their inability to be easily removed from the atmosphere. The chlorine atoms aren’t used up, which means they are free to carry on breaking down the ozone.