OCR GCSE Chemistry A: Controlling reactions (J248) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover sub-topic C5.2 Controlling reactions of the OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification (Version 4.0, August 2026): outcomes C5.2a–C5.2i, CM5.2i–CM5.2iv and the PAG C1, C7 and C8 rate practicals. No C5.2 statement is in bold, so all suit both tiers.

For a refresher beforehand, see the study guide or revision notes. The OCR GCSE Chemistry hub and J248 checklist show where this fits.

Questions

1. Calcium carbonate chips react with dilute hydrochloric acid, giving off carbon dioxide. Suggest two different measurements a student could make at regular time intervals to follow the rate of this reaction. [2]

2. Zinc granules react with dilute sulfuric acid. A student collects 48 cm³ of hydrogen in the first 64 s.

(a) Work out the mean rate of hydrogen production for this period, with its unit. [2] (b) Suggest why measuring the loss in mass of the flask would not be a good way to follow this reaction. [1]

3. For each change, state whether the rate of reaction increases, decreases or stays the same.

(a) The acid is cooled from 40 °C to 20 °C. [1] (b) The acid is diluted with an equal volume of water. [1] (c) The pressure of a mixture of two reacting gases is increased. [1] (d) A lump of solid is replaced by the same mass of powder. [1]

4. A student adds a piece of magnesium ribbon to excess dilute hydrochloric acid and collects the hydrogen in a gas syringe.

Time (s)	0	12	24	36	48	60	72	84
Volume of H₂ (cm³)	0	25	42	53	59	61	61	61

(a) State the time at which the reaction stopped. [1] (b) Calculate the mean rate over the first 36 s. Give your answer to 3 significant figures. [2] (c) A line drawn touching the curve at 24 s runs through (0 s, 17 cm³) and (48 s, 67 cm³). Use it to find the rate at 24 s, to 3 significant figures. [2] (d) Suggest why the curve becomes less steep over time. [2] (e) The student repeats the run with an identical piece of ribbon, this time in an equal volume of stronger acid. Describe how the new curve would compare with the first. [2]

5. A student times how long the same length of magnesium ribbon takes to react completely in hydrochloric acid at four temperatures.

Temperature (°C)	18	28	38	48
Time (s)	196	109	61	34

(a) Calculate 1/t for the runs at 18 °C and 48 °C. Give your answers to 3 significant figures. [2] (b) Use the times to calculate how many times faster the reaction is at 48 °C than at 18 °C. [1] (c) Explain, in terms of particles and collisions, why the rate increases with temperature. [3]

6. A small block of marble measures 12 mm × 6 mm × 2 mm.

(a) Work out the block’s surface area to volume ratio. [2] (b) The block is cut into 2 mm cubes. Calculate the new surface area to volume ratio. [1] (c) Explain why the 2 mm cubes react faster with acid than the block. [2]

7. Zinc reacts slowly with dilute sulfuric acid. A student runs three experiments with the same zinc and the same acid, collecting hydrogen for 60 s.

Experiment	Powder added	Hydrogen in 60 s (cm³)	Mass of powder left at end (g)
1	none	11	–
2	0.50 g copper	41	0.50
3	0.50 g magnesium	66	0.00

(a) Calculate the mean rate of hydrogen production in experiments 1 and 2. [2] (b) Explain how the results show that copper is a catalyst for this reaction. [2] (c) Explain why the results do not show that magnesium is a catalyst. [1]

8. For a reaction, the activation energy is 120 kJ/mol without a catalyst and 84 kJ/mol with a catalyst. The energy change of the reaction is −46 kJ/mol.

(a) Calculate how much the catalyst lowers the activation energy. [1] (b) State the energy change of the catalysed reaction. [1] (c) Explain how lowering the activation energy increases the rate. [2]

9. A student writes: “A catalyst takes part in the reaction, so it gets used up and you have to keep adding more.”

(a) Explain what is wrong with this statement. [1] (b) Explain why only a small amount of catalyst is needed. [1] (c) State what an enzyme is. [1]

10. Two gases react in a sealed container. Explain why increasing the pressure at constant temperature increases the rate of reaction, and state what happens to the energy of the collisions. [3]

11. A student varies the strength of hydrochloric acid added to marble chips to see how it alters the rate. She records the mass lost from the flask in the first 60 s.

Acid concentration (mol/dm³)	0.5	1.0	1.5	2.0
Mass lost in 60 s (g)	0.21	0.43	0.62	0.85

(a) Describe a method for this investigation. Include the apparatus, what is measured, and the variables that must be controlled. [4] (b) Calculate the mean rate in g/s for 0.5 mol/dm³ and for 2.0 mol/dm³ acid. Give your answers to 3 significant figures. [2] (c) The student says that the rate is directly proportional to the concentration. Use the data to evaluate this claim. [2]

Answers

1. Measure the volume of carbon dioxide collected, for example in a gas syringe [1]; measure the loss in mass of the flask on a balance [1]. [2] Examiner insight: Name the measurement, not just the apparatus: “mass lost at regular times” beats “use a balance”.

2. (a) 48 ÷ 64 [1] = 0.75 cm³/s [1] (b) Hydrogen has a very small mass, so the change in mass would be too small to measure accurately [1]. Examiner insight: Always attach a unit to a rate; for gas volumes it is cm³/s, not g/s.

3. (a) Decreases [1] (b) Decreases [1] (c) Increases [1] (d) Increases [1] Examiner insight: A vague word such as “changes” will not do; commit to increases or decreases.

4. (a) 60 s, the first time at which the volume stops changing [1]. (b) 53 ÷ 36 [1] = 1.47 cm³/s [1] (c) (67 − 17) ÷ (48 − 0) = 50 ÷ 48 [1] = 1.04 cm³/s [1] (d) The magnesium is being used up, so there are fewer reacting particles [1]; collisions become less frequent, so the rate falls [1]. (e) The curve is steeper at the start and levels off sooner [1]; it reaches the same final volume of 61 cm³, because the amount of magnesium (the limiting reactant) is the same [1]. Examiner insight: In (e), a higher plateau is likely to lose credit: the same limiting reactant makes the same volume of gas.

5. (a) 18 °C: 1 ÷ 196 = 0.00510 s⁻¹ [1]; 48 °C: 1 ÷ 34 = 0.0294 s⁻¹ [1] (b) 196 ÷ 34 = 5.76 times faster (about 5.8) [1] (c) Warmer particles move faster, so they meet more often [1]. A larger share of those collisions carry at least the activation energy [1]. Successful collisions therefore happen more often each second [1]. Examiner insight: Collision frequency alone is incomplete for temperature; the activation energy point completes the answer.

6. (a) Surface area = 2 × (72 + 12 + 24) = 216 mm²; volume = 12 × 6 × 2 = 144 mm³ [1]. Ratio = 216 : 144 = 1.5 : 1 [1] (b) 6 × 3 × 1 = 18 cubes, each 24 mm², total 432 mm²; volume still 144 mm³, so ratio 3 : 1 [1] (c) Cutting raises the surface area to volume ratio, exposing more marble particles to the acid [1]; collisions with acid particles are more frequent, so the rate is higher [1]. Examiner insight: Show area and volume separately before the ratio so your method stays visible.

7. (a) Experiment 1: 11 ÷ 60 = 0.183 cm³/s [1]; experiment 2: 41 ÷ 60 = 0.683 cm³/s [1] (b) Hydrogen is produced faster with copper present (41 cm³ compared with 11 cm³ in 60 s) [1], and the same mass of copper, 0.50 g, is left at the end, so it has not been used up [1]. (c) All the magnesium was used up (0.00 g left), so it reacted with the acid rather than acting as a catalyst [1]. Examiner insight: Give two separate pieces of evidence, faster rate and unchanged mass, and quote the figures.

8. (a) 120 − 84 = 36 kJ/mol [1] (b) −46 kJ/mol, unchanged by the catalyst [1] (c) At the same temperature, more collisions have at least the new, lower activation energy [1], so successful collisions are more frequent [1]. Examiner insight: Avoid saying the catalyst “gives particles more energy”; the particles’ energies are the same, and the barrier is lower.

9. (a) A catalyst is not used up: the same mass, chemically unchanged, is left at the end, so there is no need to keep adding it [1]. (b) Because it is not used up, the same catalyst particles can speed up the reaction again and again [1]. (c) An enzyme is a catalyst in a biological system [1]. Examiner insight: Saying only “catalysts speed up reactions” does not address what is wrong with the statement.

10. Raising the pressure pushes the gas particles closer together [1], so they meet more often and successful collisions happen more often each second [1]. Collision energy stays as it was, since the particles are no hotter [1]. [3] Examiner insight: Keep pressure and temperature explanations apart; “particles have more energy” is wrong for a pressure change.

11. (a) Put weighed marble chips of one size and a measured volume of acid in a flask on a balance, with a cotton wool plug [1]. Record the mass every 10 s for at least 60 s [1]. Repeat with each acid concentration [1]. Control: volume of acid, mass and size of marble chips, and temperature [1]. (b) 0.5 mol/dm³: 0.21 ÷ 60 = 0.00350 g/s [1]; 2.0 mol/dm³: 0.85 ÷ 60 = 0.0142 g/s [1] (c) Doubling the concentration roughly doubles the mass lost: 0.21 g to 0.43 g (0.5 to 1.0 mol/dm³) and 0.43 g to 0.85 g (1.0 to 2.0 mol/dm³) [1]. Rate ÷ concentration is close to 0.007 every time, so the data support the claim, though repeats would make it more secure [1]. Examiner insight: “Evaluate” needs a judgement backed by numbers, not just “the mass goes up as concentration goes up”.

Where marks are usually lost

• Giving rates without units, or using g/s for gas volumes.
• Taking tangent points from the curve instead of the tangent line.
• Drawing a faster curve with a higher final volume when the limiting reactant has not changed.
• Writing “more collisions” with no reference to time.
• Leaving activation energy out of temperature, or using energy arguments for concentration or pressure.
• Calling a substance a catalyst because the rate rose, without checking its mass.

Next steps

• Recap with the Controlling reactions revision notes.
• Re-read the Controlling reactions study guide.
• Record C5.2 as complete on the J248 checklist and find the next unit to work on via the OCR GCSE Chemistry hub.
• Take a free 10-minute diagnostic.
• Book a free trial class.

Official syllabus

OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification, Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Sub-topic C5.2 Controlling reactions, learning outcomes C5.2a–C5.2i, mathematical learning outcomes CM5.2i–CM5.2iv and the linked practical suggestions (PAG C1, PAG C7, PAG C8).