Explain how the bonding in ${{\text{O}}_{\text{2}}}$ and ${{\text{O}}_{\text{3}}}$ affects the wavelengths of UV light they absorb.

The chemical balance of the stratosphere is disrupted by the presence of chlorofluorocarbons (CFCs) and other ozone-depleting compounds. Describe, using equations, how CFCs contribute to ozone depletion.

${{\text{O}}_{\text{2}}}$ double bond and ${{\text{O}}_{\text{3}}}$ bond order 1.5/between single and double bond;

${{\text{O}}_{\text{2}}}$ stronger/shorter / O₃ weaker/longer bond;

${{\text{O}}_{\text{2}}}$ absorbs shorter $\lambda$ (242 nm) / ${{\text{O}}_{\text{3}}}$ absorbs longer $\lambda$ (330 nm);

Initiation

${\text{CC}}{{\text{l}}_2}{{\text{F}}_2}{\text{(g)}}\xrightarrow{{{\text{UV(light)/}}hv{\text{/}}hf}}{\text{ }} \bullet {\text{CCl}}{{\text{F}}_2}{\text{(g)}} + {\text{Cl}} \bullet {\text{(g)}}$;

Propagation

${\text{Cl}} \bullet {\text{(g)}} + {{\text{O}}_3}{\text{(g)}} \to {\text{ClO}} \bullet {\text{(g)}} + {{\text{O}}_2}{\text{(g)}}$;

${\text{ClO}} \bullet {\text{(g)}} + {\text{O}} \bullet {\text{(g)}} \to {{\text{O}}_2}{\text{(g)}} + {\text{Cl}} \bullet {\text{(g)}}$;

Accept for 2^nd propagation step

${\text{ClO}} \bullet {\text{(g)}} + {{\text{O}}_3}{\text{(g)}} + {\text{Cl}} \bullet {\text{(g)}} + {\text{2}}{{\text{O}}_2}{\text{(g)}}$.

Allow representation of the radical without $\bullet$ if consistent throughout the mechanism.

Allow UV light to be represented above the arrow or mentioned in accompanying description as words.

Ignore state symbols.

Names of processes (initiation and propagation) not required.

There was a reasonable appreciation of the effect of ${{\text{O}}_{\text{2}}}$ and ${{\text{O}}_{\text{3}}}$ on UV light but the distinction between bond order 2 in ${{\text{O}}_{\text{2}}}$ and 1.5 in ${{\text{O}}_{\text{3}}}$ (or similar) was often not made. There were some good answers to (b) and most were consistent about the use of dots ${\text{(}} \bullet {\text{)}}$ to signify radicals. It is worth noting, however, that the question includes the words “using equations”. Candidates who did not use equations did not score any marks.

There was a reasonable appreciation of the effect of ${{\text{O}}_{\text{2}}}$ and ${{\text{O}}_{\text{3}}}$ on UV light but the distinction between bond order 2 in ${{\text{O}}_{\text{2}}}$ and 1.5 in ${{\text{O}}_{\text{3}}}$ (or similar) was often not made. There were some good answers to (b) and most were consistent about the use of dots ${\text{(}} \bullet {\text{)}}$ to signify radicals. It is worth noting, however, that the question includes the words “using equations”. Candidates who did not use equations did not score any marks.