| Date | November 2014 | Marks available | 3 | Reference code | 14N.3.sl.TZ0.20 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Describe | Question number | 20 | Adapted from | N/A |
Question
The ozone layer protects us by absorbing ultraviolet (UV) radiation from the Sun during its natural formation and depletion.
Describe, using equations, the formation and depletion of ozone in the stratosphere by natural processes.
Formation:
Depletion:
Markscheme
Formation:
\({{\text{O}}_2}{\text{(g)}}\xrightarrow{{{\text{UV(light)/}}hv{\text{/}}hf}}{\text{2O}} \bullet {\text{(g)}}\);
\({{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {{\text{O}}_3}{\text{(g)}}\);
Depletion:
\({{\text{O}}_3}{\text{(g)}}\xrightarrow{{{\text{UV(light)/}}hv{\text{/}}hf}}{{\text{O}}_2}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}}\);
\({{\text{O}}_3}{\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {\text{2}}{{\text{O}}_2}{\text{(g)}}\);
Allow representation of radical without \( \bullet \) if consistent throughout mechanism.
UV(light)/hv/hf can be represented above arrow or mentioned in accompanying description in words.
Penalize omission of UV (light)/hv/hf once only.
Ignore state symbols.
Examiners report
This question related to the formation and depletion of ozone in the stratosphere by natural processes. The most common errors in (a) were failure to mention the necessity for UV light to form the oxygen radicals and inconsistent use of the dot in equations to represent radicals.
