The following reactions take place in the ozone layer by the absorption of ultraviolet light.

$$\begin{array}{*{20}{l}} {\text{I}}&{{{\text{O}}_3} \to {{\text{O}}_2} + {\text{O}} \bullet } \\ {{\text{II}}}&{{{\text{O}}_2} \to {\text{O}} \bullet + {\text{O}} \bullet } \end{array}$$

State and explain, by reference to the bonding, which of the reactions, I or II, requires a shorter wavelength.

CFCs and ${\text{N}}{{\text{O}}_{\text{x}}}$ are pollutants responsible for the depletion of the ozone layer. Discuss the role of ${\text{N}}{{\text{O}}_{\text{x}}}$ in this process and include equations for a stepwise mechanism.

reaction II (requires a shorter wavelength);

${{\text{O}}_{\text{2}}}$ has double bond/bond order 2 and ${{\text{O}}_{\text{3}}}$ intermediate between double and single bonds/bond order of ${\text{1}}\frac{1}{2}$;

Do not accept stronger/weaker bonding without justification for the second marking point.

NO/${\text{N}}{{\text{O}}_{\text{x}}}$/${\text{N}}{{\text{O}}_{\text{2}}}$ act as catalysts;

${\text{NO}} + {{\text{O}}_3} \to {\text{N}}{{\text{O}}_2} + {{\text{O}}_2}$;

${\text{N}}{{\text{O}}_2} + {\text{O}} \to {\text{ NO}} + {{\text{O}}_2}$;

In (a) most candidates stated the reaction and identified the double bond in oxygen, but very few mentioned the bond order of 1 1/2 for the ozone molecule.

In (b), very few candidates answered correctly to score the three marks and did not recognise the role of ${\text{N}}{{\text{O}}_{\text{x}}}$ as a catalyst.