| Date | May 2009 | Marks available | 3 | Reference code | 09M.3.hl.TZ1.E4 |
| Level | HL | Paper | 3 | Time zone | TZ1 |
| Command term | Describe | Question number | E4 | Adapted from | N/A |
Question
The ozone in the stratosphere protects us from harmful UV radiation. Above Australia there is an area of decreased ozone concentration that has led to an increase in the incidence of some skin cancers.
Explain how the dissociation of \({{\text{O}}_{\text{2}}}\) and \({{\text{O}}_{\text{3}}}\) is dependent on the wavelength of light.
Use equations to describe the mechanism of ozone depletion catalysed by the \({\text{CC}}{{\text{l}}_{\text{2}}}{{\text{F}}_{\text{2}}}\) molecule.
Markscheme
\(\lambda = 242{\text{ nm}}\) / wavelength lower/frequency higher for \({{\text{O}}_2} \to {\text{2O}} \bullet \) /oxygen dissociation;
\(\lambda = 330{\text{ nm}}\) / wavelength higher/frequency lower for \({{\text{O}}_3} \to {{\text{O}}_2} + {\text{O}} \bullet \) /ozone dissociation;
the bonding in \({{\text{O}}_{\text{2}}}\) is stronger than in \({{\text{O}}_{\text{3}}}\);
\({\text{CC}}{{\text{l}}_2}{{\text{F}}_2} \to {\text{CCl}}{{\text{F}}_2} + {\text{Cl}} \bullet \);
\({\text{Cl}} \bullet + {{\text{O}}_3} \to {\text{ClO}} \bullet + {{\text{O}}_2}\);
\({\text{ClO}} \bullet + {\text{O}} \bullet \to {{\text{O}}_2} + {\text{Cl}} \bullet \);
Accept other suitable equations including CFC or Cl radicals.
\( \bullet \) not necessary for mark but if + or – used instead penalize once only.
Examiners report
In part (a) only the more able candidates were able to answer correctly. Few candidates quoted specific wavelengths or mentioned shorter or longer wavelengths. Many scored the mark for stronger bond in the oxygen molecule.
In (b) many candidates scored 1 or 2 points here, but only the more able candidates scored 3 marks by giving the three equations in the markscheme, with a wide range of implausible reactions appearing.
