Rate of Reaction – IGCSE Chemistry Definition
IGCSE Chemistry definition of rate of reaction: the speed at which reactants are converted to products. Covers five factors, measurement methods, and collision theory.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Rate of reaction is how fast a chemical reaction occurs — specifically, how quickly reactants are used up or products are formed. It is one of the most examined topics in the 0620 syllabus, appearing across all paper types. You must know the five factors affecting rate, explain each using collision theory, and describe methods for measuring rate.
The 0620 definition
Rate of reaction is the change in the amount (or concentration) of a reactant or product per unit time.
The five factors
| Factor | How it increases rate | Collision theory explanation |
|---|---|---|
| Temperature | Increase temperature | Particles move faster; more frequent collisions; more collisions exceed activation energy |
| Concentration | Increase concentration (or pressure for gases) | More particles per unit volume; more frequent collisions |
| Surface area | Use smaller pieces / powder | More particles exposed at the surface; more frequent collisions |
| Catalyst | Add a catalyst | Provides alternative pathway with lower activation energy; more successful collisions |
| Light | Expose to light (photochemical reactions only) | Provides energy to start the reaction |
Measuring rate of reaction
| Method | What is measured | Suitable for |
|---|---|---|
| Gas syringe | Volume of gas produced over time | Reactions producing gas (e.g. metal + acid) |
| Mass loss (on a balance) | Decrease in mass as gas escapes | Reactions where gas escapes (e.g. CaCO₃ + HCl) |
| Disappearing cross (clock reaction) | Time for precipitate to obscure a cross | Reactions producing a precipitate (e.g. Na₂S₂O₃ + HCl) |
| Colour change | Time for colour to change | Reactions involving coloured substances |
Interpreting rate graphs
On a graph of volume of gas (or mass of product) vs time:
- Steep curve = fast rate
- Gentle curve = slow rate
- Flat line = reaction has finished
- Higher temperature or concentration gives a steeper initial curve and finishes sooner
- Using more reactant gives the same rate initially but produces more product (higher final value)
Worked exam question
Marble chips (CaCO₃) react with dilute HCl. Describe and explain how using powdered CaCO₃ instead of large chips would affect the rate. (3)
Mark scheme
The rate increases / reaction is faster [1]; because the powder has a larger surface area (than the chips) [1]; so more particles of CaCO₃ are exposed to the acid / more frequent collisions between CaCO₃ and HCl particles per unit time [1]
Common exam mistakes
- Confusing rate with yield. A faster rate does not mean more product — it means the same amount of product is formed more quickly. The total product depends on the amount of reactant.
- Not linking the factor to collision theory. Stating “higher temperature increases rate” without explaining why (more energy, more successful collisions) loses the explanation marks.
- Writing “more collisions” without “per unit time” or “more frequent”. The frequency matters, not just the total number over the whole reaction.
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