| Date | May 2010 | Marks available | 3 | Reference code | 10M.3.sl.TZ2.E3 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Describe and State | Question number | E3 | Adapted from | N/A |
Question
The ozone layer protects living organisms from dangerous UV radiation. In the Earth’s stratosphere, ozone is photochemically formed from oxygen by the following two-step process.
\[\begin{array}{*{20}{l}} {{{\text{O}}_2}\xrightarrow{{{\text{UV light}}}}2{\text{O}} \bullet } \\ {{{\text{O}}_2} + {\text{O}} \bullet \to {{\text{O}}_3}} \end{array}\]
(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.
Markscheme
(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].
Examiners report
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.
