Overview
What you'll learn
State the conditions for an effective collision. Particles must collide, with enough energy (≥ Eₐ) and in the right orientation.
Measure a rate. Use continuous monitoring (gradient = rate) or time-to-fixed-point methods.
Apply the five factors. Particle size, concentration, pressure, temperature, catalyst — each explained through the same collision-frequency chain.
Describe enzymes. Biological catalysts that are specific, selective and sensitive (they denature at high temperature or the wrong pH).
Tutor's Insight
"Reasoning, not recall."
Almost every rate question is the same logic: relate the change to the number of particles, to the frequency of effective collisions, to the rate. Learn the chain once and reuse it for any of the five factors. This is where it shows up — Paper 2 wants you to explain in terms of collisions, and the practical paper asks you to read gradient = rate and use time-to-fixed-point methods. Master the chain and the whole topic collapses into one idea.
10.1 Collision Theory & Measuring Rate
10.1 Collision Theory & Measuring Rate
What this subtopic asks of you
- State the conditions for an effective collision: particles must collide, with energy ≥ the activation energy (Eₐ), and in the correct orientation.
- Describe how to follow the rate of a reaction, both by continuous monitoring of a changing property and by timing a change to a fixed point.
- Interpret rate graphs — read the gradient of the tangent as the rate at that moment, and know why the graph is steepest at the start.
10.1 Collision Theory
For a reaction to happen, particles must collide effectively
Requirement 01
Collide
Particles must physically meet. No contact, no reaction. The more collisions per second, the faster the rate.
Requirement 02
With enough energy
The collision energy must be at least the activation energy (Eₐ). Below that, the particles bounce apart unchanged.
Requirement 03
In the right orientation
The reacting groups must line up correctly — like hitting a key into a lock the right way around.
The answer-template chain. More particles per volume → more collisions per second → faster rate. Or: higher proportion of particles with E ≥ Eₐ → more effective collisions per second → faster rate. Pick the chain that matches the factor and you can answer any rate question.
10.1 Measuring Rate
Read it off a graph, or time a change
Method 01
Continuous monitoring
Record a property (volume of gas, mass of the flask, conductivity) every few seconds and plot it against time. Rate at any moment = gradient of the tangent; average rate = total change ÷ total time. The graph is steepest at the start and flattens as reactants run out.
Method 02
Time to a fixed point
Pick a specific event — e.g. a cross drawn under the flask becoming invisible when a precipitate forms (Na₂S₂O₃ + HCl). Measure the time taken. Rate ∝ 1 / time, so halving the time doubles the rate. Keep the depth of solution the same in each trial.
A steeper slope at the start means a faster reaction.
Free Notes · O-Level Pure Chemistry
Read the full chapter
The five factors — surface area, concentration, pressure, temperature and catalysts — plus enzymes and 4 worked exam questions.
10.2 The Five Factors, Catalysts & Enzymes
10.2 The Five Factors, Catalysts & Enzymes
What this subtopic asks of you
- Explain, in terms of collision frequency and effective collisions, how particle size, concentration, pressure and temperature change the rate of a reaction.
- Describe the action of a catalyst as providing an alternative pathway with a lower activation energy, and know it is unchanged in mass and does not alter ΔH.
- Describe enzymes as biological catalysts that are specific, selective and sensitive to temperature and pH.
10.2 Five Factors · 01
Smaller pieces, bigger surface
What changes
Particle size of a solid
Crushing a lump into a powder gives the same mass much more exposed surface for collisions.
Why rate ↑
More sites for collision
More surface = more solid particles exposed to the liquid/gas particles = more collisions per second. Faster rate.
Only matters when one reactant is a solid. If both reactants are aqueous or gaseous, 'surface area' doesn't apply — particles are already free to mix. e.g. CaCO₃ powder vs marble chips with HCl — powder reacts visibly faster.
10.2 Five Factors · 02 & 03
More particles per volume, more collisions
Factor 02 — Concentration
For aqueous reactants
Higher concentration = more solute particles in the same volume of solution. More particles → more collisions per second → faster rate.
Factor 03 — Pressure
For gaseous reactants
Higher pressure squeezes the gas into a smaller volume — same particles, less space. Same collision-frequency argument → faster rate. No effect if no reactant is gaseous.
Look out for
Why a reaction slows down and finally stops. As reactants are used up, their concentration falls — and the rate falls with it. The reaction stops when the limiting reactant runs out, not because anything 'wears out'. This is the standard answer to "why does Mg + HCl finish?"
10.2 Five Factors · 04
Heat them up — they move and they hit harder
Effect 01
More frequent collisions
Higher temperature gives the particles more kinetic energy. They move faster and collide more often.
Effect 02
More effective collisions
More importantly, a larger fraction of particles now have enough energy to reach Eₐ — so many more collisions are effective.
Every 10 °C rise roughly doubles the rate. It's not a law — it depends on the reaction — but it captures how strongly temperature affects rate. The Eₐ-fraction effect dominates over the speed-of-motion effect.
10.2 Five Factors · 05
A catalyst opens a shorter road
What it is
A substance that speeds the reaction
Speeds the reaction by providing an alternative pathway with a lower Eₐ. More collisions now have enough energy.
What it doesn't do
Doesn't change ΔH
The energies of reactants and products are unchanged. A catalyst lowers the hump on the energy profile — it does not lower the step.
Stays unchanged
Not used up
A catalyst is chemically unchanged at the end of the reaction. You can recover the same mass you started with. Small amount, big effect.
Where you'll meet them
Three named catalysts the syllabus expects: Fe — the Haber process (N₂ + 3H₂ → 2NH₃); Ni — hydrogenation of vegetable oils; Pt / Rh / Pd — catalytic converters in car exhausts.
10.2 Biological Catalysts
Enzymes — the body's own catalysts
Specific
One enzyme, one reaction
Each enzyme is shaped to fit only one substrate — like a key in a single lock. e.g. amylase only digests starch; pepsin only digests protein.
Selective
Speeds one reaction at a time
Even when the substrate is in a mixture, an enzyme picks it out and catalyses only its reaction. That is why digestion can happen with many enzymes side by side in the gut.
Sensitive
Denature outside optimum
Too hot (above ~40 °C in the body) or the wrong pH → the enzyme loses its shape (denatures) and stops working. Cold slows enzymes but does not denature them.
Denaturing is permanent — a denatured enzyme no longer fits its substrate. Cold is different: it only slows the enzyme, and warming it back up restores activity.
Practice
Exam-style questions
Question 01 · AMKSS 2024 Prelim
The Question
MCQ
When pressure doesn't matter
Which reaction is pressure least likely to affect the speed of?
Worked Answer
C
- Pressure changes the concentration of gases — not solids or liquids.
- A: CO₂ is a gaseous reactant → pressure affects rate.
- B: ethene is a gas → pressure affects rate.
- D: two gaseous reactants on the left → pressure affects rate.
- C: sodium (solid) + water (liquid) — no gaseous reactant, so changing pressure does almost nothing. Answer C.
Question 02 · AMKSS 2024 Prelim
The Question
MCQ
Why a reaction stops
When excess magnesium ribbon is added to dilute hydrochloric acid, the reaction soon becomes slower and finally stops. Which statement best explains this?
Worked Answer
A
- Magnesium is in excess — it does not run out. Rules out B.
- Magnesium doesn't form an insoluble layer with dilute HCl (it dissolves cleanly). Rules out C.
- The reaction is exothermic — the mixture warms up, not cools. Rules out D.
- As HCl is used up, [H⁺] falls, so there are fewer collisions per second and the rate falls. When [H⁺] = 0, the reaction stops. Answer A.
Question 03 · CCHM 2024 Prelim
The Question
MCQ
What lowers activation energy?
Which change in the conditions of the Haber process results in a decrease in activation energy?
Worked Answer
D — using a catalyst
- A: more concentration → more collisions → faster rate, but Eₐ is unchanged.
- B: higher temperature → more particles reach Eₐ → faster rate, but Eₐ itself is unchanged.
- C: higher pressure → more collisions for gases → faster rate, Eₐ unchanged.
- D: a catalyst provides an alternative pathway with a lower Eₐ. Answer D.
Question 04
The Question
Structured
Marble chips and acid
Excess marble chips (calcium carbonate) react with dilute hydrochloric acid, producing carbon dioxide:
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)
(a) Explain, in terms of collisions, why crushing the chips into a powder speeds up the reaction. (b) State and explain the effect on the rate of warming the acid from 20 °C to 30 °C.
Worked Answer
Surface area + temperature
- (a) A powder has a much larger total surface area than a few lumps of the same mass.
- More surface means more solid particles are exposed to the acid particles, so there are more collisions per second.
- More effective collisions per second → faster rate.
- (b) The rate increases (roughly doubles for a 10 °C rise). Warmer particles have more kinetic energy, so they collide more often, and a larger fraction have energy ≥ Eₐ — more effective collisions per second.
Frequently Asked Questions
Rate of Reactions — FAQ
What is Rate of Reactions in the O-Level Chemistry syllabus (6092)?
Rate of Reactions (Topic 10 of Syllabus 6092) is about how fast reactions go and why. It covers collision theory — the idea that particles must collide with enough energy and the right orientation to react — how to measure a rate, and the five factors that change rate: particle size (surface area), concentration, pressure, temperature and catalysts. It also covers enzymes as biological catalysts.
What are the conditions for an effective collision?
For a reaction to happen, particles must (1) collide — physically meet, (2) collide with enough energy — at least the activation energy (Eₐ), and (3) collide in the right orientation, so the reacting groups line up correctly. A collision that meets all three is an effective collision. The more effective collisions per second, the faster the rate.
How do you measure the rate of a reaction?
Two main ways. Continuous monitoring: record a changing property (volume of gas, mass of the flask, or conductivity) every few seconds and plot it against time — the gradient of the tangent gives the rate at any moment, and the graph is steepest at the start. Time to a fixed point: time how long a specific change takes (e.g. a cross under the flask disappearing when a precipitate forms). Here rate ∝ 1 / time, so halving the time doubles the rate.
What are the five factors that affect the rate of a reaction?
Particle size (surface area) — smaller pieces of a solid expose more surface; concentration — more solute particles per volume of solution; pressure — squeezing a gas into a smaller volume; temperature — faster and more energetic collisions; and a catalyst — an alternative pathway with a lower activation energy. Each works through the same chain: more effective collisions per second means a faster rate.
How does a catalyst speed up a reaction, and does it change ΔH?
A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy (Eₐ), so a larger fraction of collisions are effective. It does not change ΔH — the energies of the reactants and products are unchanged — it only lowers the hump on the energy profile, not the step. A catalyst is not used up and can be recovered chemically unchanged. Named examples: Fe (Haber process), Ni (hydrogenation) and Pt/Rh/Pd (catalytic converters).
Why does a reaction slow down and eventually stop?
As reactants are used up, their concentration falls, so there are fewer collisions per second and the rate falls with it. The reaction stops when the limiting reactant runs out — not because anything 'wears out'. For example, when excess magnesium is added to dilute HCl, the acid is limiting: as [H⁺] falls the rate drops, and when [H⁺] reaches zero the reaction stops.
O-Level Pure Chemistry · Syllabus 6092 · Topic 10 of 12 · © 2026 Overmugged. For personal study use only.