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.[2]
Use equations to describe the mechanism of ozone depletion catalysed by the \({\text{CC}}{{\text{l}}_{\text{2}}}{{\text{F}}_{\text{2}}}\) molecule.[3]
▶️Answer/Explanation
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.
Question
Chlorofluorocarbons, CFCs, deplete the ozone layer.
Chlorine atoms and nitrogen oxides react at the surface of ice particles in the arctic winter.
State the equations that represent the depletion of ozone in the stratosphere which is catalysed by chlorine free radicals.[2]
(i) Deduce the type of catalysis that occurs.
(ii) Outline why the depletion of ozone is greatest during the arctic spring.[3]
▶️Answer/Explanation
Markscheme
\({\text{Cl}} \bullet + {{\text{O}}_{\text{3}}} \to {\text{ClO}} \bullet + {{\text{O}}_{\text{2}}}\);
\({\text{ClO}} \bullet + {\text{O}} \bullet \to {\text{Cl}} \bullet + {{\text{O}}_{\text{2}}}\);
Radical symbols not required for mark.
(i) heterogeneous (catalysis);
(ii) ice particles melt releasing the pollutants;
light breaks the bonds producing radicals / Cl;
Examiners report
The equations for the depletion of ozone were correctly answered by few candidates. Instead of the catalysis being heterogeneous, some incorrectly identified heterolytic or homogeneous or surface catalysis.
Why in spring the depletion was greatest was not very well answered either.
Question
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.[2]
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.[3]
▶️Answer/Explanation
Markscheme
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;
\({\text{NO}} + {{\text{O}}_3} \to {\text{N}}{{\text{O}}_2} + {{\text{O}}_2}\);
\({\text{N}}{{\text{O}}_2} + {\text{O}} \to {\text{ NO}} + {{\text{O}}_2}\);
Examiners report
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.