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Date May 2011 Marks available 3 Reference code 11M.3.hl.TZ1.E4
Level HL Paper 3 Time zone TZ1
Command term Discuss and State Question number E4 Adapted from N/A

Question

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

IO3O2+OIIO2O+O

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

[2]
a.

CFCs and NOx are pollutants responsible for the depletion of the ozone layer. Discuss the role of NOx in this process and include equations for a stepwise mechanism.

[3]
b.

Markscheme

reaction II (requires a shorter wavelength);

O2 has double bond/bond order 2 and O3 intermediate between double and single bonds/bond order of 112;

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

a.

NO/NOx/NO2 act as catalysts;

NO+O3NO2+O2;

NO2+O NO+O2;

b.

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.

a.

In (b), very few candidates answered correctly to score the three marks and did not recognise the role of NOx as a catalyst.

b.

Syllabus sections

Additional higher level (AHL) » Topic 14: Chemical bonding and structure » 14.1 Covalent bonding and electron domain and molecular geometries
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