| Date | May 2014 | Marks available | 2 | Reference code | 14M.3.sl.TZ1.17 |
| Level | SL | Paper | 3 | Time zone | TZ1 |
| Command term | Describe | Question number | 17 | Adapted from | N/A |
Question
Ozone prevents UV radiation emitted from the Sun reaching the surface of the Earth.
Describe, using chemical equations, the two-step mechanism of photochemical decomposition of ozone in the Earth’s stratosphere.
Step 1:
Step 2:
Markscheme
Step 1:
\({{\text{O}}_3}{\text{(g)}}\xrightarrow{{{\text{UV/}}hf{\text{/}}hv}}{{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}}\);
UV not needed for mark (since given in question).
Step 2:
\({{\text{O}}_3}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {\text{2}}{{\text{O}}_2}{\text{(g)}}\);
Ignore state symbols.
Allow O instead of \(O \bullet \) if consistent throughout.
Examiners report
Most of the candidates were able to score part marks for (a); marks were not gained either due to incorrectly balanced equations or incorrect Step 2 equation. In part (b) comparing \({{\text{C}}_{\text{2}}}{{\text{F}}_{\text{6}}}\) with CFCs was challenging with many not really knowing why \({{\text{C}}_{\text{2}}}{{\text{F}}_{\text{6}}}\) is used. Candidates demonstrated a poor understanding of reactivity and the strength of the fluorine to carbon bond. Many referred to CFCs as having longer lifetime and being more stable. Most candidates were able to score only one mark for this question.
