(a)     Ozone decomposition can proceed photochemically. Describe, using chemical equations, the two-step mechanism of photochemical decomposition of ozone in the Earth’s stratosphere.

Step 1:

Step 2:

(b)     Ozone decomposition can also be catalysed by ozone-depleting substances such as chlorofluorocarbons, CFCs. State two alternatives to CFCs.

(a)     Step 1:

${{\text{O}}_{\text{3}}} + hv \to {{\text{O}}_{\text{2}}} + {\text{O}} \bullet {\text{ }}$;

Step 2:

${\text{O}} \bullet  + {{\text{O}}_3} \to {\text{2}}{{\text{O}}_2}$;

Dots and radicals are not required for the mark.

Accept more detailed mechanisms ($O \bullet  + {O_2} + M \to {O_3} + M*$, etc.).

(b)     hydrocarbons

(per)fluorocarbons

hydrofluorocarbons / HFCs

hydrochlorofluorocarbons / HCFCs

ammonia/ ${\text{N}}{{\text{H}}_{\text{3}}}$ / sulfur dioxide/ ${\text{S}}{{\text{O}}_2}$

nitrogen/ ${{\text{N}}_2}$ / argon/Ar

Any two correct answers scores [1].

Only a small proportion of candidates could correctly quote the two equations for the photochemical decomposition of ozone, though many could correctly identify alternatives to CFCs.