O-LevelChemistryRates of Reaction5070/22 · May/June 2023 · Q59 Marks
Carbon reacts with steam to make carbon monoxide and hydrogen. This reaction is reversible. The forward reaction absorbs thermal energy. C(s) + H₂O(g) ⇌ CO(g) + H₂(g)
- (a)(i)An equilibrium mixture is formed when the reversible reaction happens in a closed system. Explain why the reversible reaction must be in a closed system for an equilibrium mixture to be formed.[1]
- (a)(ii)Predict what happens to the position of equilibrium when the temperature is decreased and the pressure remains constant. Explain your answer. prediction explanation[2]
- (a)(iii)Predict what happens to the position of equilibrium when the pressure is decreased and the temperature remains constant. Explain your answer. prediction explanation[2]
- (b)Predict what happens to the rate of the backward reaction when the temperature is decreased and the pressure remains constant. Explain your answer. prediction explanation[2]
- (c)Predict what happens to the rate of the backward reaction when the pressure is increased and the temperature remains constant. Explain your answer. prediction explanation[2]
Mark scheme
Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.
- 5(a)(i) so reactants cannot escape / so products cannot escape (1)
- 5(a)(ii) (position of equilibrium) moves to the left (1)
- because the (forward) reaction is endothermic / to minimise temperature decrease by releasing energy / because the backward reaction is exothermic (1)
- 5(a)(iii) (position of equilibrium) moves to the right (1)
- because more moles of gas on product side / less moles of gas on reactant side (1)
- 5(b) rate decreases because
- particles move slower / particles have less kinetic energy (1)
- less successful collisions / fewer collisions (involve particles) with equal or more than activation energy / less effective collisions (1)
- 5(c) rate increases because
- more particles per unit volume (1)
- greater collision frequency (1)
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