O-LevelChemistryChemistry of the Environment5070/21 · Oct/Nov 2015 · Q36 Marks

A layer of ozone is present in the stratosphere about 30 km above the Earth's surface.

(a)(i)Compounds with formulae such as CCl3F and C2ClF5 are responsible for the depletion of ozone. Give the general name for these compounds.[1]
(a)(ii)Explain, in terms of human health, why it is important that the ozone in the stratosphere does not become too depleted.[2]
(b)(i)In the stratosphere, ozone, O3, is broken down to oxygen by photochemical reactions. What is meant by the term photochemical reaction?[1]
(b)(ii)Construct an equation for the breakdown of ozone molecules to oxygen molecules.[1]
(c)Ozone oxidises Fe2+ ions to Fe3+ ions. Complete the ionic equation for this reaction. Fe2+ + .... H+ + O3 → .... Fe3+ + H2O + O2[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2015). The mark or mark type is shown in brackets.

[1] A3 (a) (i) chlorofluorocarbons/CFCs (1)
[2] (ii) ozone absorbs uv (radiation) (1) too much uv increases incidence of skin cancer/cataracts etc. (1)
[1] (b) (i) reaction catalysed by light/light involved in breakdown of chemicals (1)
[1] (ii) 2O3 → 3O2 (1)
[1] (c) 2Fe2+ + 2H+ + O3 → 2Fe3+ + H2O + O2 (1)

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