IB DP Chemistry Reactivity 1.2 Energy cycles in reactions HL Paper 2- Exam Style Questions - New Syllabus
Question
Oxygen exists as two allotropes, diatomic oxygen, \( \mathrm{O_2} \), and ozone, \( \mathrm{O_3} \).
(a) (i) Draw a Lewis (electron dot) structure for ozone. [1]
(ii) Discuss the relative length of the two O–O bonds in ozone. [2]
(b) Explain why there are frequencies of UV light that will dissociate \( \mathrm{O_3} \) but not \( \mathrm{O_2} \). [2]
(c) Explain, using equations, how the presence of \( \mathrm{CCl_2F_2} \) results in a chain reaction that decreases the concentration of ozone in the stratosphere. [2]
▶️ Answer/Explanation
Markscheme
(a)(i)
Accept any correct pair of resonance (Lewis) structures for ozone. For example:
\( \displaystyle \;{:}\mathrm{O}-\mathrm{O}^{+}{=}\mathrm{O}^{-}{:}\quad\longleftrightarrow\quad {:}\mathrm{O}^{-}{=}\mathrm{O}^{+}-\mathrm{O}{:} \)
(Any combination of lines/dots/crosses to show electron pairs is acceptable; do not show “1.5” bonds.) A1
[1 mark]
Accept any correct pair of resonance (Lewis) structures for ozone. For example:
\( \displaystyle \;{:}\mathrm{O}-\mathrm{O}^{+}{=}\mathrm{O}^{-}{:}\quad\longleftrightarrow\quad {:}\mathrm{O}^{-}{=}\mathrm{O}^{+}-\mathrm{O}{:} \)
(Any combination of lines/dots/crosses to show electron pairs is acceptable; do not show “1.5” bonds.) A1
[1 mark]
(a)(ii)
Both O–O bonds are the same length due to resonance/delocalization over the three oxygen atoms. Their length lies between that of a single O–O bond and a double O=O bond (data booklet covalent bond lengths: \( \mathrm{O{-}O} \approx 148\ \mathrm{pm} \), \( \mathrm{O{=}O} \approx 121\ \mathrm{pm} \)). M1 A1
[2 marks]
Both O–O bonds are the same length due to resonance/delocalization over the three oxygen atoms. Their length lies between that of a single O–O bond and a double O=O bond (data booklet covalent bond lengths: \( \mathrm{O{-}O} \approx 148\ \mathrm{pm} \), \( \mathrm{O{=}O} \approx 121\ \mathrm{pm} \)). M1 A1
[2 marks]
(b)
The O–O bonding in \( \mathrm{O_3} \) has bond order \(1.5\) (weaker than the double bond in \( \mathrm{O_2} \)), so the energy required to break it is lower. Because photon energy is \( E = h f \) and \( c = \lambda f \), there are UV frequencies with enough energy to break \( \mathrm{O_3} \) but not enough to break the stronger \( \mathrm{O_2} \) bond. M1 A1
Supporting data (for context): average bond enthalpies (data booklet): \( D(\mathrm{O{=}O}) \approx 498\ \mathrm{kJ\,mol^{-1}} \); \( D(\mathrm{O{-}O}) \approx 146\ \mathrm{kJ\,mol^{-1}} \). The \( \mathrm{O_3} \) bond energy lies between these, so its threshold frequency is lower than for \( \mathrm{O_2} \).
[2 marks]
The O–O bonding in \( \mathrm{O_3} \) has bond order \(1.5\) (weaker than the double bond in \( \mathrm{O_2} \)), so the energy required to break it is lower. Because photon energy is \( E = h f \) and \( c = \lambda f \), there are UV frequencies with enough energy to break \( \mathrm{O_3} \) but not enough to break the stronger \( \mathrm{O_2} \) bond. M1 A1
Supporting data (for context): average bond enthalpies (data booklet): \( D(\mathrm{O{=}O}) \approx 498\ \mathrm{kJ\,mol^{-1}} \); \( D(\mathrm{O{-}O}) \approx 146\ \mathrm{kJ\,mol^{-1}} \). The \( \mathrm{O_3} \) bond energy lies between these, so its threshold frequency is lower than for \( \mathrm{O_2} \).
[2 marks]
(c)
Initiation by UV photodissociation of a CFC:
\( \mathrm{CCl_2F_2 \xrightarrow{UV} \cdot CClF_2 + Cl\cdot} \)
Propagation (catalytic ozone depletion):
\( \mathrm{Cl\cdot + O_3 \rightarrow O_2 + ClO\cdot} \)
\( \mathrm{ClO\cdot + O_3 \rightarrow 2\,O_2 + Cl\cdot} \)
(Net: \( \mathrm{2\,O_3 \rightarrow 3\,O_2} \); \( \mathrm{Cl\cdot} \) is regenerated and catalyses the chain.) M1 A1
[2 marks]
Initiation by UV photodissociation of a CFC:
\( \mathrm{CCl_2F_2 \xrightarrow{UV} \cdot CClF_2 + Cl\cdot} \)
Propagation (catalytic ozone depletion):
\( \mathrm{Cl\cdot + O_3 \rightarrow O_2 + ClO\cdot} \)
\( \mathrm{ClO\cdot + O_3 \rightarrow 2\,O_2 + Cl\cdot} \)
(Net: \( \mathrm{2\,O_3 \rightarrow 3\,O_2} \); \( \mathrm{Cl\cdot} \) is regenerated and catalyses the chain.) M1 A1
[2 marks]
Total Marks: 7