OCR A Level Chemistry A: Reaction rates (H432) – Revision Notes

These revision notes condense section 3.2.2, Reaction rates, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. All outcomes from 3.2.2(a) to (g) are included; the section is part of Module 3 (Periodic table and energy), and H432 is the full A level. For step-by-step teaching, go back to the Reaction rates study guide.

For exam-style work, use the Reaction rates practice questions. Browse the course via the H432 chemistry hub, track outcomes with the checklist you can print, and take the free diagnostics for a quick whole-course test.

Key terms

Term	Meaning
Rate of reaction	Speed at which reactants are consumed or products made: quantity change ÷ time
Activation energy, Ea	Minimum energy required for a reaction to take place (3.2.1(c))
Catalyst	Increases rate without being used up by the overall reaction; gives an alternative route with lower Ea
Homogeneous catalyst	Same physical state as the reactants
Heterogeneous catalyst	Different physical state from the reactants
Boltzmann distribution	Spread of molecular energies in a sample of gas at one temperature

3.2.2(a): Collision frequency

• Reaction needs a collision with energy of at least Ea.
• Higher concentration means more particles per dm³, so more frequent collisions and a higher rate.
• Higher gas pressure at constant temperature means the same molecules in a smaller volume, so a higher gas concentration and more frequent collisions.
• Concentration and pressure change the frequency of collisions, not the energy of each collision.
• As a reaction runs, reactant concentration falls, collisions get less frequent and the rate drops. That is why quantity-time curves level off.

3.2.2(b): Rate from a graph

Method in steps: instantaneous rate

1. Put a ruler against the curve at the time you need, touching it at one point.
2. Extend the line over most of the graph paper.
3. Choose two well-separated points on the line, not on the curve.
4. Gradient = change in y ÷ change in x.
5. Give the rate as a positive number with units from the axes.

Units at a glance

Quantity on the y-axis	Rate units
Concentration / mol dm⁻³	mol dm⁻³ s⁻¹
Gas volume / cm³	cm³ s⁻¹
Mass / g	g s⁻¹
Amount / mol	mol s⁻¹

Converting to moles

• Gas volume: amount (mol) = volume (cm³) ÷ 24 000, using 24.0 dm³ mol⁻¹ at RTP.
• Mass loss: amount (mol) = mass lost (g) ÷ Mr of the escaping gas.
• Do the conversion on the rate itself: 1.2 cm³ s⁻¹ ÷ 24 000 = 5.0 × 10⁻⁵ mol s⁻¹.

Small worked reminder

A reactant falls from 0.240 to 0.195 mol dm⁻³ between 30 s and 60 s. Mean rate = 0.045 ÷ 30 = 1.5 × 10⁻³ mol dm⁻³ s⁻¹.

Mean rate over an interval and rate at an instant are different. The tangent at t = 0 gives the initial rate, which is usually the largest rate on the curve.

3.2.2(c): How a catalyst works

• It provides a different route with a lower activation energy.
• Overall it is not consumed. It may react in one step and be regenerated in a later step.
• On an enthalpy profile, reactant and product levels are fixed, so ΔH is unchanged. Only the hump is lower.
• Reverse Ea = forward Ea − ΔH. For an exothermic reaction (ΔH negative), the reverse Ea is the bigger one.
• The catalyst lowers the forward and reverse Ea by the same amount.
• Details of how any specific catalyst works are not required.

Small worked reminder

An endothermic reaction has ΔH = +25 kJ mol⁻¹. Its uncatalysed Ea is 130 kJ mol⁻¹ and its catalysed Ea is 85 kJ mol⁻¹.

• The catalyst lowers Ea by 130 − 85 = 45 kJ mol⁻¹.
• Reverse Ea, uncatalysed: 130 − 25 = 105 kJ mol⁻¹.
• Reverse Ea, catalysed: 85 − 25 = 60 kJ mol⁻¹, also 45 kJ mol⁻¹ lower.
• Here the products sit above the reactants, so the reverse hump is the smaller one.

Enthalpy profile checklist

• Axes: enthalpy (y) and progress of reaction (x).
• Reactant and product lines at the correct relative heights.
• Two humps, catalysed lower than uncatalysed.
• Arrows from reactants up to each peak, labelled Ea and Ea(cat).
• Arrow from reactants to products labelled ΔH, pointing down for exothermic.

3.2.2(d): Catalyst types and sustainability

Type	Example	Phases
Homogeneous	H⁺ in esterification in solution	Liquid catalyst, liquid reactants
Homogeneous	Cl radicals in ozone breakdown	All gases
Heterogeneous	Fe in ammonia production	Solid catalyst, gaseous reactants
Heterogeneous	Ni in hydrogenation of alkenes	Solid catalyst, fluid reactants passing over it

Why catalysts matter

• Industrial processes reach a useful rate at a lower temperature.
• Less energy is needed, so less fossil fuel is burned.
• That means lower CO₂ emissions and lower costs, so improved sustainability.
• Against this: some catalysts are toxic, so they need careful handling, recovery and disposal (HSW9,10).

A good evaluation answer gives both sides. State the benefit chain in full, then the drawback, then a judgement, for example that the saving is worth having only if the toxic catalyst is kept out of the environment.

3.2.2(e): Measuring rates (PAG9)

Measure	Apparatus	Use when
Mass lost	Conical flask with cotton wool plug on a balance	A heavy gas, such as CO₂, escapes
Gas volume	Gas syringe; or an upturned burette or measuring cylinder filled with water	A gas forms; avoid water if the gas dissolves readily
Time	Stopwatch or timer	Timing a fixed change; 1/t is proportional to mean rate

• PAG9 is continuous monitoring, with measurement of time and software for processing data.
• Control temperature with a water bath. Keep volumes, concentrations and solid mass and surface area fixed apart from the one you change.
• Start timing as reactants mix.

3.2.2(f): The Boltzmann distribution

Rules for the curve:

• y-axis: number of molecules. x-axis: energy.
• Starts at the origin.
• One peak, at the most probable energy.
• Long tail at high energy that never meets the x-axis.
• Total area represents every molecule in the sample.
• Region beyond Ea: molecules able to react on collision.

3.2.2(g): Temperature and catalysts

Must-know distinction: what moves?

Change	Curve	Ea line	Area beyond Ea
Higher temperature	Peak lower and further right; same area	Stays put	Larger
Add a catalyst	Unchanged	Moves left (lower)	Larger
Higher concentration or pressure	Same shape (same temperature)	Stays put	Same proportion; collisions just more frequent

Model answer pattern

1. Draw or describe the curve(s), with Ea marked.
2. Say what moves (curve or Ea line).
3. State that a greater proportion of molecules has energy of at least Ea.
4. Link to more successful collisions per second, so a higher rate.

Quick self-test

1. A gas syringe collects 45 cm³ in the first 25 s. Calculate the mean rate.
2. A reactant falls from 0.150 to 0.105 mol dm⁻³ in 90 s. Calculate the mean rate.
3. A flask loses 0.176 g of CO₂ (Mr 44.0) in 40 s. Calculate the mean rate in mol s⁻¹.
4. A tangent passes through (0 s, 0.80 mol dm⁻³) and (200 s, 0.20 mol dm⁻³). Calculate the rate.
5. A reaction has forward Ea = 95 kJ mol⁻¹ and ΔH = −40 kJ mol⁻¹. Calculate the reverse Ea.
6. Solid V₂O₅ speeds up the reaction of SO₂(g) with O₂(g). Classify the catalyst.
7. What happens to the total area under a Boltzmann curve when the gas is heated?
8. Why does the Boltzmann curve start at the origin?
9. The pressure of a gas mixture is tripled at constant temperature. What happens to its concentration?
10. State the units of rate taken from a graph of mass against time.
11. Why is mass loss a poor way to follow a reaction that gives hydrogen?

Answers

1. 45 ÷ 25 = 1.8 cm³ s⁻¹
2. 0.045 ÷ 90 = 5.0 × 10⁻⁴ mol dm⁻³ s⁻¹
3. 0.176 ÷ 44.0 = 0.004 00 mol; 0.004 00 ÷ 40 = 1.0 × 10⁻⁴ mol s⁻¹
4. 0.60 ÷ 200 = 3.0 × 10⁻³ mol dm⁻³ s⁻¹
5. 95 − (−40) = 135 kJ mol⁻¹
6. Heterogeneous: solid catalyst, gaseous reactants
7. It stays the same; the number of molecules is unchanged
8. No molecules have zero energy
9. It triples
10. g s⁻¹
11. Hydrogen has a very small Mr, so the mass change is too small to measure precisely

Where marks are usually lost

• Writing that higher concentration makes collisions “more energetic” or “harder”.
• Saying “more collisions” without “per second” or “more frequent”.
• Taking a gradient from a chord between two table points when a tangent at one time was asked for.
• Reading tangent points off the curve instead of off the drawn line.
• Leaving units off, or mixing cm³ and dm³ when converting gas volumes to moles.
• Drawing the hotter Boltzmann curve with a higher peak, or with a larger area.
• Shifting the Boltzmann curve to show a catalyst instead of moving Ea.
• Stating that a catalyst changes ΔH or is used up.
• Classifying a catalyst without stating the physical states involved.
• Listing sustainability benefits without the chain: lower temperature, less fuel burned, less CO₂.

Official syllabus

Source: section 3.2.2 Reaction rates, learning outcomes (a) to (g), in the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, published by OCR.