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Date	May 2021	Marks available	2	Reference code	21M.2.hl.TZ1.7
Level	HL	Paper	2	Time zone	TZ1
Command term	Explain	Question number	7	Adapted from	N/A

Question

Oxygen exists as two allotropes, diatomic oxygen, O₂, and ozone, O₃.

Draw a Lewis (electron dot) structure for ozone.

[1]

a(i).

Discuss the relative length of the two O−O bonds in ozone.

[2]

a(ii).

Explain why there are frequencies of UV light that will dissociate O₃ but not O₂.

[2]

Explain, using equations, how the presence of ${CCl}_{2} F_{2}$ results in a chain reaction that decreases the concentration of ozone in the stratosphere.

[2]

Markscheme

✔

Accept any combination of lines, dots or crosses to represent electrons.

Do not accept structures that represent 1.5 bonds.

a(i).

both equal ✔

delocalization/resonance ✔

Accept bond length between 121 and 148 pm/ that of single O−O bond and double O=O bond for M1.

a(ii).

bond in O₃ is weaker
OR
O₃ bond order 1.5/< 2 ✔

Do not accept bond in O₃ is longer for M1.

lower frequency/longer wavelength «UV light» has enough energy to break the O–O bond in O₃ «but not that in O₂» ✔

Accept “lower frequency/longer wavelength «UV light» has lower energy”.

${CCl}_{2} F_{2} {(g) →∙CClF}_{2} (g) Cl ∙ (g)$ ✔

${Cl•(g)+O}_{3} {(g)→O}_{2} (g)+ClO•(g)$
AND
${ClO∙(g)+O}_{3} {(g)→2O}_{2} (g)+Cl ∙ (g)$ ✔

Do not penalize missing radical.

Accept:for M2:
${Cl∙(g) + O}_{3} {(g) → O}_{2} (g) + ClO ∙ (g)$
AND
${ClO∙(g) + O(g) → O}_{2} (g) + Cl ∙ (g)$

Examiners report

[N/A]

a(i).

[N/A]

a(ii).

[N/A]

Syllabus sections

Core » Topic 2: Atomic structure » 2.2 Electron configuration

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