| Date | May 2009 | Marks available | 2 | Reference code | 09M.3.hl.TZ2.E2 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | State | Question number | E2 | Adapted from | N/A |
Question
Chlorofluorocarbons, CFCs, deplete the ozone layer.
Chlorine atoms and nitrogen oxides react at the surface of ice particles in the arctic winter.
State the equations that represent the depletion of ozone in the stratosphere which is catalysed by chlorine free radicals.
(i) Deduce the type of catalysis that occurs.
(ii) Outline why the depletion of ozone is greatest during the arctic spring.
Markscheme
\({\text{Cl}} \bullet + {{\text{O}}_{\text{3}}} \to {\text{ClO}} \bullet + {{\text{O}}_{\text{2}}}\);
\({\text{ClO}} \bullet + {\text{O}} \bullet \to {\text{Cl}} \bullet + {{\text{O}}_{\text{2}}}\);
Radical symbols not required for mark.
(i) heterogeneous (catalysis);
(ii) ice particles melt releasing the pollutants;
light breaks the bonds producing radicals / Cl;
Examiners report
The equations for the depletion of ozone were correctly answered by few candidates. Instead of the catalysis being heterogeneous, some incorrectly identified heterolytic or homogeneous or surface catalysis.
Why in spring the depletion was greatest was not very well answered either.
