O-LevelChemistryChemistry of the Environment5070/21 · May/June 2015 · Q311 Marks

Ozone molecules are continually being broken down and formed in the upper atmosphere.

(a)(i)The equation shows one way in which ozone is formed in the upper atmosphere. O2 + O → O3 ΔH = −392kJ/mol Explain, in terms of bond breaking and bond forming, why this reaction is exothermic.[2]
(a)(ii)When one mole of oxygen molecules reacts, 392kJ of energy is released. Calculate the amount of energy released when 48.0 g of oxygen molecules react. energy released =[2]
(b)Name a pollutant that depletes ozone in the upper atmosphere.[1]
(c)(i)Ozone molecules decompose into oxygen molecules in a reversible reaction. 2O3(g) ⇌ 3O2(g) ΔH = +143kJ/mol The reaction reaches an equilibrium if carried out in a closed system. The reaction is studied at a temperature of 400 °C. Describe and explain what happens to the position of equilibrium if the pressure is increased.[2]
(c)(ii)The reaction is studied at 25 atmospheres pressure. Describe and explain what happens to the position of equilibrium if the temperature is decreased.[2]
(c)(iii)Describe and explain what will happen to the rate of the reaction if the temperature is decreased.[2]

Mark scheme

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

[1] (a) (i) Bond breaking absorbs energy and bond making releases energy/bond breaking is endothermic and bond making is exothermic (1) Less energy absorbed than released/more energy released than absorbed/endothermic energy change is less than exothermic energy change/exothermic energy change is more than endothermic energy change (1)
[1] (a) (ii) Moles of oxygen = 1.5 (1) Energy released = 588 (1)
[1] (b) CFC/oxides of nitrogen/nitric oxide (1)
[1] (c) (i) Moves to the left/moves to reactants/moves to ozone/backward reaction favoured (1) More moles (of gas) on right/fewer moles (of gas) on left/more molecules on right/more volume (of gas) on right (1)
[1] (c) (ii) Moves to the left/moves to reactants/moves to ozone/backward reaction favoured (1) (Forward) reaction is endothermic/reverse reaction is exothermic (1)
[1] (c) (iii) Reaction is slower because particles are moving slower/rate decreases because particles have less energy (1) There are fewer successful collisions/fewer particles have energy above the activation energy (1)

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