These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. All substances named with letters (G, H, J, MX, X, Y) are fictional, and all data are invented. Every system is closed and at constant temperature unless a question says otherwise.
Question 1 (multiple choice · foundation)
A reversible reaction in a sealed flask has reached equilibrium. Which statement is true?
- (A) The concentrations of the reactants and the products are equal.
- (B) The forward and reverse reactions have both stopped.
- (C) The concentrations of all species are constant, and the forward and reverse reactions continue at equal rates.
- (D) The forward reaction has finished, and only the reverse reaction is now taking place.
Answer and explanation
Answer: (C). At equilibrium every concentration stays the same, because the forward and reverse processes go on at the same rate.
- (A) Concentrations are constant, not equal. Equal values would be a coincidence.
- (B) Equilibrium is dynamic. Both reactions continue; only the net change is zero.
- (D) If only the reverse reaction occurred, the reactant concentrations would rise, so the system would not be at equilibrium.
Question 2 (multiple choice · core)
A sealed glass bottle is half filled with water and kept at 25 °C. For the first hour the water level falls very slightly. After that the level stays the same for days. Which statement best explains the steady water level?
- (A) The air above the water is full, so no more molecules can evaporate.
- (B) Molecules evaporate from the liquid at the same rate as vapour molecules condense into it.
- (C) Evaporation stops once the temperature of the water is constant.
- (D) All of the water that can evaporate has already evaporated.
Answer and explanation
Answer: (B). Evaporation and condensation are opposite processes. At first evaporation is faster, so the level falls. As vapour builds up, condensation speeds up until the two rates are equal. Then there is no net change in the level, although molecules keep moving both ways.
- (A) and (D) describe evaporation as stopping. It does not stop: it is balanced by condensation.
- (C) The temperature was constant from the start, yet the level fell for an hour. A constant temperature does not stop evaporation.
Question 3 (multiple choice · core)
The fictional reaction G(aq) ⇌ H(aq) is started with [G] = 0.800 M and no H. The table shows the results.
| Time (min) | 0 | 1 | 2 | 4 | 6 | 8 |
|---|---|---|---|---|---|---|
| [G] (M) | 0.800 | 0.465 | 0.364 | 0.324 | 0.320 | 0.320 |
| [H] (M) | 0.000 | 0.335 | 0.436 | 0.476 | 0.480 | 0.480 |
Which conclusion is best supported by the data?
- (A) Equilibrium is first reached between 0 and 1 min, when [H] becomes greater than zero.
- (B) Equilibrium is reached at 2 min, because [H] is now greater than [G].
- (C) By 6 min the system is at equilibrium, and G is still changing into H and H into G at equal rates.
- (D) By 6 min the reaction has stopped, because 0.320 M of G cannot react.
Answer and explanation
Answer: (C). [G] and [H] are still changing at 4 min but are the same at 6 and 8 min. That is the sign of equilibrium, and equilibrium is dynamic.
- (A) H appearing shows the forward reaction has started, not that equilibrium is reached.
- (B) Which concentration is larger does not decide equilibrium. Both values change again between 2 and 4 min.
- (D) The remaining G can react, and does. It is turned into H as fast as H turns back into G.
Question 4 (calculation · core)
A fictional gas J dimerises in a sealed rigid flask: 2J(g) ⇌ J₂(g). The flask starts with J at 0.900 atm and no J₂. The total pressure falls and then stays steady at 0.620 atm.
(a) Explain why a steady total pressure shows that the system has reached equilibrium. (b) Calculate the partial pressures of J and J₂ at equilibrium. (c) What percentage of the J has been converted to J₂?
Worked solution
(a) Each time two J molecules make one J₂, the number of gas molecules falls, so the total pressure falls. When the total pressure is steady, the amounts of J and J₂ have stopped changing. The forward and reverse rates are equal.
(b)
- Let x atm of J₂ form. Then J falls by 2x: P_J = 0.900 − 2x and P_J₂ = x.
- Total: (0.900 − 2x) + x = 0.900 − x = 0.620, so x = 0.280.
- P_J₂ = 0.280 atm and P_J = 0.900 − 0.560 = 0.340 atm.
(c) J used = 2 × 0.280 = 0.560 atm out of 0.900 atm, so 62.2% has been converted.
Suggested mark points (4): 1 for linking constant total pressure to constant amounts (equilibrium); 1 for the 2 : 1 relationship (J falls by 2x); 1 for both partial pressures; 1 for 62.2%.
Common error: writing P_J = 0.900 − x. That ignores the coefficient 2. With it, the total pressure would stay at 0.900 atm and could never fall to 0.620 atm.
Question 5 (constructed response · core)
A beaker holds a saturated solution of the fictional salt MX in water, with some undissolved solid MX at the bottom. The beaker is covered and kept at constant temperature. A student adds 0.50 g of solid MX in which the X atoms are a radioactive isotope. After a day the student finds:
- the concentration of MX in the solution has not changed;
- the mass of undissolved solid is 0.50 g more than before the addition;
- the solution is now radioactive.
(a) Write the equation for the equilibrium between solid MX and its ions, M⁺ and X⁻. (b) Explain why the concentration of the solution did not change. (c) Explain how the radioactive solution shows that this equilibrium is dynamic.
Worked solution and suggested Marlbridge rubric
(a) MX(s) ⇌ M⁺(aq) + X⁻(aq)
(b) The solution was already saturated, so it was at equilibrium: dissolving and precipitating were happening at equal rates. At a fixed temperature a saturated solution holds a fixed concentration of MX, however much solid is present. The extra 0.50 g simply joins the solid already there, so overall it stays undissolved.
(c) Radioactive X⁻ ions are now in solution, so some of the labelled solid must have dissolved. Because the concentration and the mass of solid did not change overall, an equal amount of ions must have precipitated at the same time. Dissolving and precipitating are both still going on, at equal rates. That is a dynamic equilibrium.
| Point | What earns it |
|---|---|
| 1 | Correct equation with states and the double arrow |
| 1 | Links the unchanged concentration to the solution already being at equilibrium (saturated) |
| 1 | Radioactivity in solution means labelled solid dissolved |
| 1 | No net change means an equal amount precipitated: both processes continue at equal rates |
Do not award the last point for “the reaction is still happening” without saying that both directions occur at equal rates.
Question 6 (constructed response · core)
Use the data for G(aq) ⇌ H(aq) in Question 3. The forward rate is 0.576 M min⁻¹ at the start, and both rates are 0.230 M min⁻¹ (to 3 significant figures) from about 6 min onwards.
(a) State the reverse rate at time zero, and explain your answer. (b) Describe, in words or with a sketch, how the forward and reverse rates change from 0 to 8 min. (c) Explain why [G] stays at 0.320 M after 6 min even though G is still reacting.
Worked solution and suggested Marlbridge rubric
(a) The reverse rate is zero at time zero, because no H is present to turn back into G.
(b) Sketch: rate on the vertical axis, time (min) on the horizontal axis. The forward rate starts at 0.576 M min⁻¹ and curves downwards, steeply at first, because [G] is falling. The reverse rate starts at 0 and curves upwards because [H] is rising. The two curves meet at about 0.230 M min⁻¹ by about 6 min and then continue together as one flat line. Neither line reaches zero.
(c) G is used up by the forward reaction at 0.230 M min⁻¹ and made by the reverse reaction at 0.230 M min⁻¹. The two rates cancel, so there is no net change in [G].
| Point | What earns it |
|---|---|
| 1 | Reverse rate zero at the start, with the reason (no H) |
| 1 | Forward rate decreasing and reverse rate increasing, each linked to the change in concentration |
| 1 | Lines meet at the same value and stay level (not at zero) |
| 1 | Equal rates of using and making G explain the constant [G] |
Accept a sketch with labelled axes in place of the description in (b).
Question 7 (evaluation · stretch)
A student mixes equal volumes of two solutions so that, at the start, [X³⁺] = [Y⁺] = 0.0200 M in the mixture. Electrons transfer between them:
X³⁺(aq) + Y⁺(aq) ⇌ X²⁺(aq) + Y²⁺(aq)
Only Y²⁺ is coloured. The colour deepens for 10 minutes, then stays the same. From the colour, the student finds that [Y²⁺] is 0.0150 M after 10 minutes and still 0.0150 M after an hour. The student writes: “The colour has stopped changing, so the reaction has gone to completion and stopped.”
Evaluate both parts of the claim, using the data.
Model answer and suggested Marlbridge rubric
“Gone to completion” is wrong. The equation is 1 : 1 : 1 : 1. If the reaction were complete, all 0.0200 M of X³⁺ would have reacted, giving [Y²⁺] = 0.0200 M. It is only 0.0150 M, so 75% has reacted, and 0.0200 − 0.0150 = 0.0050 M each of X³⁺ and Y⁺ remain. Reactants and products are all present: this is an equilibrium mixture.
“Stopped” is wrong too. A constant colour shows that [Y²⁺] is constant. That means Y²⁺ is made and used up at equal rates, not that nothing is happening. Electron transfer continues in both directions at equal rates, with no net change.
| Point | What earns it |
|---|---|
| 1 | Uses the 1 : 1 ratio to show complete reaction would give 0.0200 M of Y²⁺ |
| 1 | States that 0.0050 M of each reactant remains, so all species are present |
| 1 | Explains that constant colour means constant concentration, not no reaction |
| 1 | States that forward and reverse electron transfer continue at equal rates |
How did you do?
- Q1, Q2 or Q3 wrong: re-read “What you see when a system reaches equilibrium” and “Equilibrium is dynamic” in the study guide.
- Q4 wrong: check how you used the coefficients; compare with the stoichiometry check under Figure 1 of the study guide.
- Q5, Q6 or Q7 incomplete: your answer needs the why: both directions continue, at equal rates.
Then tick off the topic checklist.