CIE A Level Chemistry
CIE A Level Chemistry Equilibria Exam Questions
Free CIE A Level Chemistry exam questions for Equilibria, with a mark scheme for every pack. Covers dynamic equilibrium, Le Chatelier's principle, Kc and Kp, industrial processes, Bronsted-Lowry acids, pH, Ka, buffers, Ksp and partition coefficients.
Equilibria exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Chemistry Equilibria
Equilibria in Cambridge International 9701 covers reversible reactions and acid-base chemistry. At AS you describe dynamic equilibrium, apply Le Chatelier's principle, calculate Kc and Kp, and discuss conditions in the Haber process and Contact process. You also meet Bronsted-Lowry acids and bases and the difference between strong and weak acids. The A Level extension adds quantitative acid-base work: pH, Ka, pKa and Kw, buffer calculations, titration curves and indicators, solubility product Ksp, the common ion effect and partition coefficients. It links to kinetics, energetics and organic acids. Examiners regularly set Kc and pH calculations and ask for precise explanations of how buffers work.
What you need to know
- Dynamic equilibrium In a closed system the forward and reverse reactions occur at the same rate, so concentrations stay constant. Le Chatelier's principle predicts the shift when concentration, pressure or temperature changes; a catalyst does not change the position.
- Kc and Kp Kc uses equilibrium concentrations and Kp uses partial pressures (mole fraction x total pressure). Only temperature changes the value of K; an exothermic forward reaction has a smaller K at higher temperature.
- Industrial processes The Haber process (iron catalyst, about 450 °C, 200 atm) and Contact process (V2O5 catalyst) use compromise conditions balancing yield, rate and cost.
- Acids and bases A Bronsted-Lowry acid is a proton donor and a base a proton acceptor. Strong acids fully dissociate in water and weak acids partially dissociate.
- pH, Ka and Kw (A Level) pH = -log[H+]. For a weak acid, [H+] = square root of (Ka x [HA]). Kw = [H+][OH-] = 1.00 x 10^-14 mol2 dm-6 at 298 K.
- Buffers (A Level) A buffer resists changes in pH when small amounts of acid or alkali are added, for example ethanoic acid and sodium ethanoate. [H+] = Ka x [HA]/[A-]. The HCO3-/H2CO3 system controls blood pH.
- Ksp and partition coefficients (A Level) Ksp is the product of ion concentrations in a saturated solution, each raised to its stoichiometric power. A common ion lowers solubility. The partition coefficient is the ratio of concentrations of a solute in two immiscible solvents at equilibrium.
Common mistakes in Equilibria
- Saying a catalyst increases the equilibrium yield; it only increases the rate at which equilibrium is reached.
- Including solids or water as a solvent in a Kc expression for a heterogeneous or aqueous equilibrium.
- Thinking pressure changes alter Kp; only temperature changes the value of an equilibrium constant.
- Using the strong acid method for a weak acid pH; you must use Ka and the square root expression.
- Explaining a buffer without equations; show that added H+ reacts with A- and added OH- reacts with HA, keeping the ratio almost constant.
Try three Equilibria exam-style questions
1. Calculate the pH of 0.100 mol dm-3 ethanoic acid. Ka = 1.74 x 10^-5 mol dm-3. (2 marks)
Show the mark scheme answer
[H+] = square root of (1.74 x 10^-5 x 0.100) = 1.32 x 10^-3 mol dm-3; pH = 2.88.
2. State and explain the effect of increasing pressure on the equilibrium N2 + 3H2 ⇌ 2NH3. (2 marks)
Show the mark scheme answer
Position shifts to the right (more NH3); fewer moles of gas on the right, so the system opposes the increase in pressure.
3. Explain how a mixture of ethanoic acid and sodium ethanoate acts as a buffer. (3 marks)
Show the mark scheme answer
Large reservoir of CH3COOH and CH3COO-; added H+ reacts with CH3COO- to form CH3COOH; added OH- reacts with CH3COOH to form CH3COO- and water, so [H+] and pH stay almost constant.
Equilibria FAQs
What does Cambridge AS & A Level Chemistry Equilibria cover?
It covers dynamic equilibrium, Le Chatelier's principle, Kc and Kp and acid-base theory at AS, plus pH, Ka, buffers, Ksp and partition coefficients at A Level.
Which parts of Equilibria are A Level only?
pH calculations with Ka and Kw, buffers, titration curves and indicators, Ksp, the common ion effect and partition coefficients are A Level content.
How do I set out a Kc calculation?
Use an ICE table with initial, change and equilibrium amounts, divide by volume to get concentrations, then substitute into the Kc expression and work out the units.
