Chlorofluoromethanes (CFMs) are carbon compounds of chlorine and fluorine and
are also known as Freons. Examples are Freon-11
\(\left(\mathrm{CFCl}_{3}\right)\) and Freon-12 \(\left(\mathrm{CF}_{2}
\mathrm{Cl}_{2}\right),\) which were used as aerosol propellants. Freons have
also been used in refrigeration and air-conditioning systems. In 1995 Mario
Molina, F. Sherwood Rowland, and Paul Crutzen were awarded the Nobel Prize
mainly for demonstrating how these and other CFMs contribute to the "ozone
hole" that develops at the end of the Antarctic winter. In other parts of the
world, reactions such as those shown below occur in the upper atmosphere where
ozone protects the earth's inhabitants from harmful ultraviolet radiation. In
the stratosphere CFMs absorb high-energy radiation from the sun and split off
chlorine atoms that hasten the decomposition of ozone, \(\mathrm{O}_{3}\).
Possible reactions are
$$
\begin{array}{ll}
\mathrm{O}_{3}(g)+\mathrm{Cl}(g) \longrightarrow
\mathrm{O}_{2}(g)+\mathrm{ClO}(g) & \Delta H^{\circ}=-126 \mathrm{~kJ} \\
\mathrm{ClO}(g)+\mathrm{O}(g) \longrightarrow \mathrm{Cl}(g)+\mathrm{O}_{2}(g)
& \Delta H^{\circ}=-268 \mathrm{~kJ} \\
\mathrm{O}_{3}(g)+\mathrm{O}(g) \longrightarrow 2 \mathrm{O}_{2}(g) & \Delta
H^{\circ}=?
\end{array}
$$
The \(\mathrm{O}\) atoms in the second equation come from the breaking apart of
\(\mathrm{O}_{2}\) molecules caused by ultraviolet radiation from the sun. Use
the first two equations to calculate the value of \(\Delta H^{\circ}\) (in
kilojoules) for the last equation, the net reaction for the removal of
\(\mathrm{O}_{3}\) from the atmosphere.
