OCR GCSE Chemistry A: Controlling reactions (J248) – Revision Notes

These notes condense sub-topic C5.2 Controlling reactions of the OCR GCSE (9–1) Chemistry A (Gateway Science) J248 specification (Version 4.0, August 2026): learning outcomes C5.2a–C5.2i, mathematical learning outcomes CM5.2i–CM5.2iv and the rate practicals linked to PAG C1, PAG C7 and PAG C8. None of C5.2 is in bold type, so all of it is for both tiers. The specification’s assessment overview places Topic C5 in Paper 2 (Foundation) and Paper 4 (Higher).

The Controlling reactions study guide has the full explanations. To test yourself under pressure, use the Controlling reactions practice questions. The OCR GCSE Chemistry hub and J248 checklist show the rest of the course, and the free diagnostics take 10 minutes.

Core definitions

Term	Meaning
Rate of reaction	The speed at which reactants are consumed or products made
Mean rate	Change in amount ÷ time taken, over an interval
Activation energy	Lowest energy a collision must have for the particles to react
Collision theory	Particles react only when they collide with at least the activation energy
Surface area to volume ratio	Total surface area ÷ volume of a solid; bigger for smaller pieces
Catalyst	A substance that speeds up a reaction and is not used up
Enzyme	A catalyst in a biological system

C5.2a and CM5.2i: measuring rate

Method	What you record	Best for	Rate unit
Gas syringe, or upturned measuring cylinder or burette over water	Gas volume at set times	Any reaction giving a gas	cm³/s
Flask on a balance, cotton wool plug	Mass at set times	Reactions giving a heavy gas such as CO₂	g/s
Disappearing cross	Time for a cross to vanish under a cloudy mixture	Thiosulfate + acid (sulfur precipitate)	1/t in s⁻¹
Time to disappear	Time for a fixed piece of solid to react	Magnesium ribbon in acid	1/t in s⁻¹

Method in steps: mean rate

1. Find the change (cm³ or g) over the interval.
2. Divide by the time in seconds.
3. Give the unit.

Worked reminder. 105 cm³ of gas in 70 s → 105 ÷ 70 = 1.5 cm³/s.

Mass loss is poor for hydrogen-producing reactions: hydrogen is so light that the mass barely changes.

Fair test checklist. One independent variable; control the volume and concentration of acid, mass and size of solid, temperature; repeat, ignore anomalies, average.

C5.2b, CM5.2ii and CM5.2iii: rate graphs

• Product–time curves start steep and level off. Steeper = faster.
• Flat = stopped, because a reactant has run out.
• Same final height = same amount of limiting reactant, whatever the rate.
• A faster run with the same reactant amounts reaches the same plateau sooner.
• Doubling the limiting reactant doubles the plateau height.

Method in steps: rate from a tangent

1. Rule a line that just grazes the curve at the time given.
2. Extend it and choose two points far apart on the tangent.
3. Gradient = (change in y) ÷ (change in x), with units.

Worked reminder. A tangent passes through (10 s, 28 cm³) and (50 s, 76 cm³). Gradient = (76 − 28) ÷ (50 − 10) = 48 ÷ 40 = 1.2 cm³/s.

1/t. For a fixed amount of change, rate ∝ 1/t. A run lasting 80 s has 1/t = 0.0125 s⁻¹; one lasting 20 s has 1/t = 0.050 s⁻¹, so it is 4 times as fast. The topic does not ask for orders of reaction.

C5.2c and C5.2d: what changes the rate, and why

Factor increased	Rate	Collision frequency	Collision energy
Temperature	Up	Up	Up: more collisions reach the activation energy
Concentration	Up	Up	No change
Pressure (gases)	Up	Up	No change
Surface area	Up	Up	No change

Model sentence for temperature. “Heated particles move faster. They meet more often, and a larger share of those meetings carry at least the activation energy, so successful collisions happen more often each second.”

Model sentence for concentration or pressure. “Reacting particles are packed more closely, so they meet more often and successful collisions happen more often each second.”

C5.2e and CM5.2iv: surface area to volume ratio

Method in steps

1. Surface area of one piece = sum of the areas of its faces (cube: 6 × side²).
2. Volume of one piece (cube: side³).
3. Multiply both by the number of pieces if you cut it up; the total volume does not change.
4. Ratio = surface area ÷ volume.

Worked reminder. A 5 mm cube: 150 mm² ÷ 125 mm³ = 1.2 : 1. Cut into 1 mm cubes: 125 × 6 = 750 mm² ÷ 125 mm³ = 6 : 1. The smaller the pieces, the bigger the ratio, the more frequent the collisions, the faster the reaction.

C5.2f, C5.2g and C5.2h: catalysts

Characteristics. A catalyst:

• increases the rate;
• is not used up, and has the same mass and chemical make-up at the end;
• does not change the amount of product or the overall energy change;
• is needed only in small amounts;
• usually works for one reaction (or type of reaction) only.

Identifying one. Faster and recovered with unchanged mass. A substance that speeds things up but loses mass is reacting, so it is not acting as a catalyst.

Examples from the practical suggestions. Manganese(IV) oxide for the decomposition of hydrogen peroxide; copper powder for zinc with dilute sulfuric acid.

How it works. It provides an alternative pathway with a lower activation energy. At the same temperature, a bigger proportion of collisions now have enough energy, so successful collisions are more frequent.

Reaction profile with a catalyst. Same reactant level, same product level, lower hump. Activation energy is drawn from the reactant line up to the peak.

C5.2i: enzymes

Enzymes are biological catalysts: they speed up reactions in living things and are not used up.

Must-know distinctions

• Mean rate (over an interval) vs rate at a moment (tangent gradient).
• Reaction stopped (flat line) vs reaction slow (shallow but rising).
• Frequency of collisions (concentration, pressure, surface area) vs frequency and energy (temperature).
• Lowering activation energy (catalyst) vs giving particles more energy (heating).
• Catalyst (unchanged mass) vs reactant (used up).
• Surface area (goes up when cut) vs volume (stays the same when cut).

Quick self-test

1. 90 cm³ of oxygen forms in 75 s. Calculate the mean rate.
2. A flask loses 0.66 g in 150 s. Calculate the mean rate.
3. The cross vanishes after 45 s. Calculate 1/t to 3 significant figures.
4. Calculate the surface area to volume ratio of an 8 mm cube.
5. A tangent passes through (0 s, 9 cm³) and (30 s, 57 cm³). Calculate its gradient.
6. Why does the gradient of a gas volume curve fall with time?
7. Name two factors that change only the frequency of collisions.
8. Explain why raising the pressure speeds up a gas reaction.
9. Give two pieces of evidence that a powder is a catalyst.
10. How does a catalyst change a reaction profile?
11. What is an enzyme?
12. Run X takes 30 s and run Y takes 90 s for the same change. How many times faster is X?

Answers

1. 90 ÷ 75 = 1.2 cm³/s.
2. 0.66 ÷ 150 = 0.0044 g/s.
3. 1 ÷ 45 = 0.0222 s⁻¹.
4. Surface area 6 × 64 = 384 mm²; volume 512 mm³; ratio 0.75 : 1.
5. (57 − 9) ÷ (30 − 0) = 1.6 cm³/s.
6. Reactants are used up, so particles are less concentrated and collisions become less frequent.
7. Any two of: concentration, pressure, surface area.
8. The gas particles are pushed closer together, so they collide more often.
9. The rate increases, and the same mass of powder is recovered at the end.
10. The activation energy hump is lower; the reactant and product levels do not change.
11. A catalyst in a biological system.
12. 90 ÷ 30 = 3 times faster (1/t is three times bigger).

Where marks are usually lost

• Leaving the unit off a rate, or writing cm³ when the data were masses.
• Reading tangent points from the curve rather than the tangent line, or using two points too close together.
• Saying a flat line means a “constant rate”; it means the reaction has finished.
• Drawing a faster curve that ends at a higher volume when the amount of limiting reactant has not changed.
• Writing “more collisions” with no reference to time (“more often”, “each second”).
• Explaining temperature only through faster particles, with no mention of activation energy.
• Claiming a catalyst energises the particles or gets used up.
• Calling a substance a catalyst without the evidence that its mass is unchanged.
• Treating a longer time as a faster rate when using 1/t.
• In surface area questions, forgetting to multiply by the number of small pieces.

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).