Practise 30 real questions on Equilibria from 2021 to 2026. See the answer and explanation as soon as you answer.
Equilibria is one of the 18 main topics in our A Level Chemistry (Cambridge (CAIE) 9701) question bank, and about 5% of its real questions come from it. Tayyari Karo has 30 real questions on this topic from 2021 to 2026, from Paper 1, Paper 2, Paper 4; 23 are multiple choice and marked instantly, and the rest are structured questions with a model answer. By difficulty: 17 hard, 13 medium.
Work through the questions below, then open the explanation for each one. Redo the ones you missed a few days later.
These are real past-paper questions. Think of your answer first, then open the explanation.
2NO2(g) <=> 2NO(g) + O2(g). When 0.75 mol of NO2 is heated in a vessel of volume 9.80 dm3 at 500 C, 7.04 g of oxygen is found in the equilibrium mixture. What is the value of Kc at this temperature?
Answer: B. 0.0452
O2 = 0.22 mol, NO = 0.44 mol, NO2 = 0.31 mol. Kc = [NO]^2[O2]/[NO2]^2 = (0.44^2 x 0.22)/(0.31^2 x 9.80) = 0.0452 mol dm-3.
Aqueous solution X contains substance HQ which behaves as a weak acid: HQ(aq) ⇌ H+(aq) + Q⁻(aq). The soluble salt NaQ is added to X. What happens to [H+(aq)] and the pH in X?
Answer: B. [H+] decreases, pH increases
Adding Q⁻ (common ion) shifts the equilibrium left, decreasing [H+(aq)] and so increasing the pH.
Hb(aq) + 4O2(aq) ⇌ Hb(O2)4(aq). When the equilibrium concentration of O2 is 7.6 × 10⁻⁶ mol dm⁻³, the equilibrium concentrations of Hb and Hb(O2)4 are equal. What is the numerical value of Kc?
Answer: A. 3.0 × 10²⁰
Kc = [Hb(O2)4]/([Hb][O2]⁴); since [Hb]=[Hb(O2)4], Kc = 1/[O2]⁴ = 1/(7.6×10⁻⁶)⁴ ≈ 3.0×10²⁰.
When 0.20 mol of hydrogen gas and 0.15 mol of iodine gas are heated at 723 K until equilibrium is established, the equilibrium mixture contains 0.26 mol of hydrogen iodide. H2(g) + I2(g) ⇌ 2HI(g). What is the correct expression for the equilibrium constant Kc?
Answer: C. (0.26)²/(0.07×0.02)
At equilibrium [HI]=0.26; H2 and I2 each decrease by 0.13 mol (half of HI formed), so [H2]=0.20−0.13=0.07 and [I2]=0.15−0.13=0.02, giving Kc = (0.26)²/(0.07×0.02).
The Contact process takes place at a pressure between 100 000 Pa and 200 000 Pa. A catalyst is used. Which statement is correct?
Answer: C. The equilibrium yield of the reaction is very high under the conditions used.
2SO2 + O2 ⇌ 2SO3 has an equilibrium yield of around 96-99% under Contact process conditions; V2O5 (not iron) is the catalyst and catalysts don't shift equilibrium position.
When an equimolar mixture of H2 and I2 react, H2(g) + I2(g) ⇌ 2HI(g), the mole fraction of HI in the final mixture is x. What is the equilibrium constant, Kp, for the reaction?
Answer: A. x²/(1−x)²
Starting equimolar (1:1) with total moles conserved, mole fractions of H2 and I2 are each (1−x)/2; Kp = x²/[((1−x)/2)²×2/2]... simplifying with equal partial pressures cancelling gives Kp = x²/(1−x)².
| Session | Paper 1 | Paper 2 | Paper 4 |
|---|---|---|---|
| May/June 2026 | 3 | – | – |
| May/June 2025 | 2 | – | 1 |
| October/November 2025 | 2 | – | – |
| May/June 2024 | 2 | 1 | – |
| October/November 2024 | 2 | – | – |
| May/June 2023 | 2 | – | 1 |
| October/November 2023 | 2 | – | 1 |
| May/June 2022 | 2 | – | – |
| October/November 2022 | 2 | – | 1 |
| May/June 2021 | 2 | – | 1 |
| October/November 2021 | 2 | – | 1 |
30 real A Level Chemistry (9701) questions on Equilibria from 2021 to 2026, from Paper 1, Paper 2, Paper 4. Each one shows the year, session and paper it came from.
Yes. Every question shows the correct answer and a short explanation the moment you answer.
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