Explain how the stratospheric ozone layer serves as a protective shield for living organisms on Earth.

Hydrochlorofluorocarbons (HCFCs) such as chlorodifluoromethane (${CHClF}_2$) have been used as transitional replacements for CFCs. Describe how a molecule of chlorodifluoromethane forms chlorine atoms in the upper atmosphere.

Write two equations to show how chlorine atoms (${Cl}^{\bullet }$) catalyse the decomposition of ozone ($O_3$).

HCFCs can be synthesised in the laboratory via the free-radical substitution of fluoroalkanes. In the mechanism to make chlorodifluoromethane (${CHClF}_2$) from difluoromethane (${CH}_2{F}_2$), two incomplete steps are shown below.

Complete each step in the mechanism:

$......................................................\to {}^{\bullet }{CHF}_2+HCl$ ${}^{\bullet }{CHF}_2+{Cl}_2\to ......................................................$

Give the name of the type of step represented by both of these equations.