CCEA A Level Chemistry
CCEA A Level Chemistry 2.10 Equilibrium Exam Questions
Free CCEA A Level Chemistry revision for 2.10 Equilibrium: what you need to know, common mistakes and exam-style questions with answers. Covers dynamic equilibrium, homogeneous and heterogeneous systems, Le Chatelier effects, Kc expressions and the Haber and Contact processes.
Practise 2.10 Equilibrium with past papers
Once you have learnt this topic, find the 2.10 Equilibrium questions in real CCEA A Level Chemistry past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
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About CCEA A Level Chemistry 2.10 Equilibrium
Section 2.10 of Unit AS 2 introduces chemical equilibrium. You need to describe dynamic equilibrium in a closed system, distinguish homogeneous and heterogeneous equilibria, and predict qualitatively how temperature, pressure, concentration and a catalyst affect the position of equilibrium. You must write Kc expressions with units and interpret what the size of Kc tells you. CCEA uses the Haber process for ammonia and the Contact process for sulfuric acid to discuss the compromise between yield and rate in industry. Quantitative calculation of Kc from equilibrium amounts follows in A2 1 (4.4), and acid-base equilibria build on these ideas.
What you need to know
- Dynamic equilibrium In a closed system the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products remain constant.
- Changing conditions Increasing temperature shifts equilibrium in the endothermic direction. Increasing pressure shifts it towards fewer gas moles. Adding a reactant shifts it to the right. A catalyst does not change the position, but equilibrium is reached faster.
- Kc For aA + bB in equilibrium with cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b. Work out units by cancelling concentration terms. A large Kc means equilibrium lies to the right. Only temperature changes the value of Kc.
- Haber process N2 + 3H2 in equilibrium with 2NH3, delta H negative. An iron catalyst, a moderate temperature (about 450 C) and high pressure (about 200 atm) are used as a compromise between yield, rate and cost.
- Contact process 2SO2 + O2 in equilibrium with 2SO3, delta H negative. A vanadium(V) oxide catalyst, about 450 C and a pressure close to atmospheric are used, because conversion is already high.
Common mistakes in 2.10 Equilibrium
- Saying a catalyst increases yield. It speeds up both directions equally and does not shift equilibrium.
- Saying equilibrium means equal concentrations. It means rates are equal and concentrations are constant.
- Including solids in a Kc expression. CCEA limits Kc to homogeneous systems, so all species should be in the same phase.
- Saying pressure changes Kc. Only temperature changes Kc; pressure changes the position for gaseous reactions.
- Explaining the Haber temperature only in terms of yield. A lower temperature would give a higher yield but too slow a rate.
Try three 2.10 Equilibrium exam-style questions
1. Write the Kc expression and units for N2(g) + 3H2(g) in equilibrium with 2NH3(g). (2 marks)
Show the mark scheme answer
Kc = [NH3]^2 / [N2][H2]^3; units dm6 mol-2.
2. Explain the effect of increasing the temperature on the yield of ammonia in the Haber process. (2 marks)
Show the mark scheme answer
Forward reaction is exothermic; equilibrium shifts in the endothermic (backward) direction so yield decreases.
3. Explain why the Contact process is run at a pressure only slightly above atmospheric. (2 marks)
Show the mark scheme answer
Equilibrium conversion is already very high at low pressure; higher pressure would increase costs and safety risks for little gain.
2.10 Equilibrium FAQs
What does CCEA A Level Chemistry 2.10 Equilibrium cover?
It covers dynamic equilibrium, the effect of changing conditions, Kc expressions and units, and the Haber and Contact processes.
Which unit is 2.10 Equilibrium examined in?
It is examined in Unit AS 2: Further Physical and Inorganic Chemistry and an Introduction to Organic Chemistry, a 1 hour 30 minute paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit AS 3.
How do I work out Kc units?
Substitute mol dm-3 for every concentration in the expression, cancel, and simplify. If all terms cancel, Kc has no units.
