Who this is for: Cambridge IGCSE Chemistry (0620) students who want rate of reaction — collision theory, factors affecting rate and graph interpretation — to become a reliable source of marks instead of a list of factors they cannot explain.
What query it owns: how to understand and revise rate of reaction in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the rate of reaction revision-guide angle, while Tutopiya’s Rate of Reaction subtopic page owns the learning resource and the free Rate of Reaction quiz owns the practice.

Rate of reaction is one of the most frequently tested topics in the Chemical Reactions unit of Cambridge IGCSE Chemistry (0620). Examiners expect you to explain how concentration, temperature, surface area, pressure and catalysts affect rate using collision theory, interpret rate graphs and describe practical methods for measuring rate. This guide covers the syllabus content, the explain chains that earn marks, and the question types that appear every year.

Key takeaways

• Rate of reaction measures how quickly reactants are used up or products are formed.
• Collision theory: particles must collide with sufficient energy (≥ activation energy) and the correct orientation.
• Increasing rate means more frequent successful collisions per second.
• Catalysts speed up reactions by providing an alternative pathway with lower activation energy — they are not used up.

What is rate of reaction in Cambridge IGCSE Chemistry?

Rate of reaction is the speed at which a chemical reaction occurs. In Cambridge IGCSE Chemistry it is explained using collision theory: reactant particles must collide with enough energy to overcome the activation energy barrier. Factors such as concentration, temperature, pressure (gases), surface area and catalysts change the frequency or success of collisions. You must also know how to measure rate practically and interpret graphs of volume of gas or mass lost against time.

You can read the full explanation, worked examples and notes on Tutopiya’s Rate of Reaction subtopic page before you attempt questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Collision theory	Collisions need energy + orientation	”Explain why increasing temperature increases rate.”
Activation energy	Minimum energy for a successful collision	”State the effect of a catalyst on activation energy.”
Concentration	More particles → more collisions	”Explain effect of dilute vs concentrated acid.”
Surface area	More exposed particles → more collisions	”Explain why powder reacts faster than lump.”
Catalyst	Lowers activation energy; not used up	”Describe the effect of a catalyst.”

Factors affecting rate: the explain chain examiners want

Every “explain” question needs the full chain: factor → particles → collisions → rate.

Factor	Effect on particles	Effect on rate
Higher concentration	More particles per unit volume	More frequent collisions → faster rate
Higher temperature	Particles move faster, more energy	More collisions with energy ≥ Eₐ → faster rate
Larger surface area	More particles exposed at surface	More frequent collisions → faster rate
Higher pressure (gas)	Particles closer together	More frequent collisions → faster rate
Catalyst	Alternative pathway, lower Eₐ	More successful collisions → faster rate

How to interpret rate graphs — step by step

1. Steeper initial gradient = faster initial rate.
2. Graph levels off when a limiting reactant is used up.
3. Same final volume/mass but different curves = same total product, different rate.
4. Higher curve reaching same level faster = faster rate, same amount of product.

Once you have worked through a few, test yourself with the free Rate of Reaction quiz — it tells you fast whether the explain chains have actually stuck.

Rate of reaction in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical rate stem
Explain	Full collision theory chain	”Explain why powdered zinc reacts faster.”
Describe	Practical method step by step	”Describe how to measure the rate of a gas-producing reaction.”
State	Short factual answer	”State two factors that affect rate of reaction.”
Compare	Two conditions side by side	”Compare the rate when temperature is doubled.”
Sketch / Draw	Rate graph with labels	”Sketch a graph of volume of gas against time.”

Worked exam-style stems (how to answer the wording)

1. “Explain why increasing the concentration of hydrochloric acid increases the rate of reaction with magnesium.” Higher concentration → more acid particles per unit volume → more frequent collisions between H⁺ ions and magnesium → more successful collisions per second → faster rate. Mark-scheme reward: more particles + more frequent/successful collisions.
2. “Explain how a catalyst increases the rate of reaction.” A catalyst provides an alternative pathway with lower activation energy → a greater proportion of collisions have energy ≥ Eₐ → more successful collisions per second → faster rate. Catalyst is not used up. Reward: lower Eₐ + more successful collisions + not used up.
3. “Describe how you would measure the rate of reaction between marble chips and acid.” Measure volume of CO₂ collected using a gas syringe at regular time intervals; plot volume against time; steeper gradient = faster rate. Reward: named method + time intervals + graph interpretation.

When you can recognise the wording instantly, work the full set on the Chemical Reactions topical past paper questions and the Rate of Reaction quiz to lock the method in.

How rate of reaction connects to the rest of the syllabus

Rate of reaction links to Chemical Energetics (activation energy) and Reversible Reactions and Equilibrium (forward and reverse rates). When you are ready to mix topics, the Cambridge IGCSE Chemistry resource hub links every Chemical Reactions subtopic.

Common mistakes students make

• Listing factors without explaining collision theory (no marks for a bare list on “Explain”).
• Saying catalysts are used up in the reaction.
• Confusing surface area with concentration (powder vs lump is surface area, not concentration).
• Reading rate graphs backwards — steeper means faster, not slower.
• Forgetting that pressure only affects gaseous reactants.

When you need more support

If rate of reaction questions keep costing marks — especially explain chains and graph interpretation — work through the Chemical Reactions topical past paper questions and the Rate of Reaction quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

Is rate of reaction hard in Cambridge IGCSE Chemistry? The concepts are logical, but marks are lost when students list factors without the collision theory explain chain or misread rate graphs.

What is collision theory in simple terms? Particles must collide with enough energy and the correct orientation for a reaction to occur. More successful collisions per second means a faster rate.

Do catalysts get used up? No. Catalysts speed up reactions by lowering activation energy and are chemically unchanged at the end.

How do I revise rate of reaction effectively? Learn the explain chain for each factor, practise graph questions, then take the Rate of Reaction quiz.

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