Overview
What you’ll learn
- Choose and describe a method to measure a reaction rate, and calculate mean rates from data.
- Explain every rate factor — temperature, concentration, pressure, surface area, catalyst — with collision theory.
- Read rate graphs, including drawing tangents for instantaneous rates.
- Describe the concentration required practical with its variables and evaluation.
- Explain why industry invests in catalysts and enzymes.
Specification checklist (AQA 8462 · Edexcel 1CH0 · OCR Gateway J248)
Statements are paraphrased from the shared DfE subject content and grouped by tier. Foundation content is studied by every student; Higher statements are examined on Higher-tier papers only, and statements badged Chemistry only are examined in separate (triple) Chemistry, not Combined Science. QuizQuip is not affiliated with AQA, Pearson Edexcel or OCR.
Foundation — every student
- 12.1Describe practical methods for measuring the rate of a reaction: collecting gas in a syringe, measuring mass loss on a balance, and timing the appearance of a precipitate that obscures a mark.
- 12.2Calculate the mean rate from measured quantities, and find the rate at a specific time from the gradient of a tangent on a rate graph.
- 12.3Explain rates using collision theory: reactions occur when particles collide with at least the activation energy, and rate depends on collision frequency and energy.
- 12.4Explain the effects of temperature, concentration/pressure, and surface area on rate using collision theory.
- 12.5Describe catalysts as substances that speed reactions without being used up, and explain that they lower the activation energy by providing an alternative pathway.
- 12.6Describe the required practical: investigating how concentration affects the rate (magnesium with hydrochloric acid collecting hydrogen, or sodium thiosulfate with hydrochloric acid obscuring a cross), including variables, method and evaluation.
- 12.7Interpret rate graphs: steeper gradient means faster rate, the graph flattens as reactants are used up, and tangents give instantaneous rates.
- 12.8Explain why catalysts are important industrially (less energy, lower cost, fewer emissions) and why enzymes are biological catalysts.
Key points
- 1Rate = how much product forms (or reactant is used) per unit time; units like cm³/s or g/s.
- 2Collision theory: react only when particles collide with energy ≥ the activation energy.
- 3Hotter → particles move faster → more frequent AND more energetic collisions → faster.
- 4More concentrated/pressurised → more particles per volume → more frequent collisions → faster.
- 5Finer powder → bigger surface area → more exposed particles → more frequent collisions → faster.
- 6Catalysts speed reactions without being used up by lowering the activation energy (different route).
- 7Rate graphs: steeper = faster; the curve flattens as reactants run out; tangent gives the rate at one instant.
- 8Practical: magnesium + acid (gas syringe) or sodium thiosulfate + HCl (disappearing cross) — one variable at a time.
Notes · 1 of 5
How to measure a rate
Follow a product appearing or a reactant disappearing — three standard methods.
| Method | How it works | Best for |
|---|---|---|
| Gas syringe | Gas product fills the syringe; read volume against time | Reactions making a gas (metal + acid, carbonate + acid) |
| Mass loss on a balance | Gas escaping from an open flask lowers the mass; plot mass against time | Gas-making reactions; heavier gases give clearer readings |
| Disappearing cross | A precipitate clouds the mixture; time how long a cross under the flask stays visible | Sodium thiosulfate + HCl (sulfur precipitate) |
The mean rate is quantity changed ÷ time taken: 48 cm³ of hydrogen collected in 2.0 minutes is a mean rate of 24 cm³/min. The disappearing-cross method has an honest weakness — the endpoint is judged by eye, so different observers stop the clock at slightly different cloudiness.
Notes · 2 of 5
Collision theory and the rate factors
Collide more often, collide harder — every factor funnels into one of those two.
Particles must collide, and collide with at least the activation energy, for a reaction to happen. That is collision theory, and it explains every rate factor in two moves: the factor changes how often particles collide, or how energetic the collisions are — usually both. Temperature: hotter particles move faster, colliding more often and — crucially — a larger fraction clears the activation-energy bar, so temperature hikes give dramatic speed-ups. Concentration (or pressure for gases): more particles crowded into the same volume collide more often. Surface area: powder exposes far more particles at the surface than one lump, so more collisions can happen per second.
Notes · 3 of 5
Catalysts: the shortcut providers
Same start, same finish, lower hump — and never used up.
A catalyst speeds a reaction without being consumed — it emerges ready to work again. It works by offering an alternative pathway with a lower activation energy, so a far larger fraction of collisions succeeds. Industry loves catalysts: they cut the temperature needed (lower energy bills, less fossil fuel burned) and raise output per hour — iron in the Haber process, platinum in car converters, enzymes in washing powder and brewing.
Notes · 4 of 5
Rate graphs and tangents
Steep early, flat late: the graph tells the reaction’s whole life story.
A graph of product volume against time starts steep (plenty of reactant, fast rate) and flattens as reactants are consumed (fewer particles, fewer collisions) until it goes horizontal — reaction over. Two graphs let you compare experiments: the steeper the initial gradient, the faster the reaction; all curves finish at the same height only if the same amounts of reactants were used — a more concentrated acid finishes faster and at the same volume; more acid finishes faster and higher.
Notes · 5 of 5
Required practical: concentration vs rate
One variable, fair test, and a graph that proves the trend.
- 1Gas-syringe version
Drop equal masses of magnesium ribbon into hydrochloric acid of different concentrations; record hydrogen volume every 10 seconds. Keep mass, length of ribbon, acid volume and temperature constant.
- 2Disappearing-cross version
Mix sodium thiosulfate of different concentrations with fixed dilute HCl over a cross; time how long until the cross disappears. Keep volumes and total concentration constant where possible.
- 3Conclusion
Higher concentration → steeper initial gradient / shorter time — because more particles per volume collide more often.
- 4Evaluation
Syringe: gas may leak past the plunger; cross: the endpoint is subjective — use the same observer and the same cross size, or a light sensor for precision.
Variables: independent = acid concentration; dependent = volume of gas per time (or time taken); controls = temperature, mass/surface area of metal, volumes, stirring.
Revise
Key terms
The words examiners expect you to use precisely.
- Rate of reaction
- How much product forms or reactant is used per unit time.
- Collision theory
- Reactions occur when particles collide with at least the activation energy.
- Activation energy
- The minimum energy a collision needs for reaction.
- Catalyst
- A substance that speeds a reaction without being used up, by lowering the activation energy.
- Surface area to volume ratio
- Powders have a high ratio, exposing more particles to collide.
- Tangent
- A straight line touching a curve at one point; its gradient gives the rate at that instant.
Show 6 more key termsShow fewer key terms
- Mean rate
- Total change ÷ total time.
- Gas syringe
- Apparatus that measures the volume of gas produced against time.
- Precipitate
- An insoluble solid formed in solution; used to time reactions by cloudiness.
- Enzyme
- A biological catalyst, specific to its substrate.
- Fair test
- Changing one variable while controlling all others.
- Concordant repeats
- Repeat timings close enough to trust the mean.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying higher temperature “gives particles more energy to collide harder” and stopping there.
Half the story — frequency matters too.
Instead: Higher temperature: particles move faster, colliding MORE OFTEN and with MORE ENERGY — both raise the rate.
Mistake: Explaining concentration via “more energy per collision”.
Concentration does not change particle speed.
Instead: Concentration increases the NUMBER of particles per volume → more frequent collisions, same energy.
Show 10 more mistakesShow fewer mistakes
Mistake: Saying a catalyst gets used up by the end.
That would make it a reactant.
Instead: Catalysts are unchanged in mass and can be recovered — that is the test for one.
Mistake: Reading the rate from the final flat part of the graph.
Flat means the reaction has finished — rate zero.
Instead: Rate is the gradient; steepest at the start, zero when flat.
Mistake: Comparing two rate graphs with different reactant amounts and expecting equal final volumes.
More reactant makes more product — the curves end at different heights.
Instead: Equal final volume means equal reactant amounts; concentration changes speed, not yield.
Mistake: Using powder and ribbon in the same experiment “for variety”.
Surface area is itself a rate variable — it must be controlled.
Instead: Change one variable (concentration); control surface area, temperature, volumes.
Mistake: Drawing the tangent as a short stroke.
Short triangles amplify drawing errors.
Instead: Draw a long tangent and use a large gradient triangle.
Mistake: Writing rate units without time, like “48 cm³”.
A quantity is not a rate.
Instead: Rate units divide by time: cm³/s, g/min, mol/s.
Mistake: Saying higher pressure slows gases because particles are “squeezed”.
Squeezing packs particles closer.
Instead: Higher pressure = same particles in less volume = more frequent collisions = faster.
Mistake: Judging the disappearing-cross endpoint differently in each repeat.
The method’s main weakness — subjective endpoint.
Instead: Same observer, same cross, same solution depth — or replace the eye with a light sensor.
Mistake: Forgetting that enzymes are catalysts too.
Biological catalysts obey the same activation-energy logic.
Instead: Enzymes lower activation energy in living processes — washing powders, brewing, digestion.
Mistake: Claiming concentration affects the total product made.
Concentration changes speed; amount of reactant changes yield.
Instead: Same moles of reactant → same final volume, just reached faster.
Check yourself
Quick quiz
30 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Calculate mean rate
The mean rate of a reaction that collects 60 cm³ of gas in 40 s is
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Practise
Topical questions
18 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this chapter, in three tiers, each with a model answer, mark scheme and examiner tip. Higher-tier and Chemistry-only questions are tagged.
Practise topical questionsExam practice
Practice paper
A 30-mark paper in the style of a GCSE paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 12 Practice Paper
30 marks · 40 min · 5 pages
10 multiple-choice questions and structured questions with answer lines. Chapter 12: measuring rates, collision theory, temperature/concentration/pressure/surface area, catalysts and enzymes, rate graphs and tangents, the concentration practical.
Mark scheme
PROChapter 12 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
How is this chapter examined?
Paper 1
1 h 45 · 100 marks
Chapters 1–11: atoms, bonding, quantitative, chemical and energy changes
Paper 2
1 h 45 · 100 marks
Chapters 12–18: rates, organic, analysis, atmosphere, resources
Tiers
Foundation (grades 1–5) · Higher (grades 4–9)
Higher assumes all Foundation content; a periodic table is printed
Want a full paper? Try the Paper 1-style (Foundation), Paper 1-style (Higher), Paper 2-style (Foundation), Paper 2-style (Higher), Required practicals practice sets.
Questions
Frequently asked questions
How do I answer every “explain the rate” question?
Two-step template: name what changed (frequency or energy of collisions), then link to rate. More particles per volume → more frequent collisions. Hotter → faster AND more energetic collisions. Surface area → more exposed particles. Catalyst → lower activation energy.
Why do concentration graphs finish at the same height but temperature graphs might not?
Concentration changes speed only — same reactant amounts, same final volume. Amount of reactant changes the final volume. Temperature changes speed only too — but if temperature changes during an experiment it also affects side reactions. Keep the comparison simple: change speed → same height, different steepness.
Which practical is this?
AQA required practical 5: investigate how concentration changes the rate — gas syringe with magnesium/acid or the disappearing cross with sodium thiosulfate. Edexcel and OCR Gateway test the same skills in their core practicals/PAGs.
What units should rates have?
Whatever you measured per unit time: cm³/s, g/s, mol/s, or 1/s for reciprocal-time methods like the disappearing cross. Always state the unit.
Does QuizQuip have any connection to the exam boards?
No. AQA, Pearson Edexcel and OCR are named only to identify the specifications this content supports. QuizQuip is not affiliated with, endorsed by, or connected to AQA, Pearson Edexcel or OCR, and no past-paper or specimen material is reproduced.
How do enzymes fit into GCSE rates?
As biological catalysts: they lower activation energy for specific reactions — washing powders at 30 °C, brewing, digestion. Same collision-theory logic, living systems.
Up next
Chapter 13: Reversible reactions, equilibrium and industry