Chemistry 0620 · syllabus 6.2
Cambridge IGCSE Chemistry 0620 — Rate of reaction topical past papers
How concentration, gas pressure, surface area, temperature and catalysts change the rate of a reaction, how rates are measured and graphed, and collision theory.
- Questions
- 476
- Years
- 2009–2026
- With mark scheme
- 98%
- Questions 2022–2026
- 166
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Rate of reaction covers
The rate of a reaction is how quickly reactants are used up or products made. It goes up when you increase the concentration of a solution, the pressure of a gas, the surface area of a solid or the temperature, and when you add a catalyst, which speeds a reaction up and is unchanged at the end; enzymes are biological catalysts. Rates are followed by measuring something against time: the loss in mass as a gas escapes from a flask on a balance, or the volume of gas collected in a gas syringe.
Results are plotted as mass or volume against time. The curve is steepest at the start, when the reactants are most concentrated, and levels off when the limiting reactant is used up. A faster reaction gives a steeper start, but the final amount of product depends only on the amount of limiting reactant, not on how quickly it reacts. Core candidates describe these effects and interpret the data; on the Extended route you also draw a tangent to a curve to find the rate at a particular moment.
Extended candidates explain each effect with collision theory. Particles react only when they collide with at least the activation energy. More particles in the same volume, from a higher concentration or pressure, or more exposed particles, from a larger surface area, mean more frequent collisions; at a higher temperature particles move faster, so they collide more often and a greater proportion of the collisions have enough energy to react. A catalyst provides a pathway with a lower activation energy. You also evaluate methods, for example why a gas syringe gives better results than collecting over water for a gas that dissolves in water. Rates of reaction are a standard context in the practical papers.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 6.2 Rate of reaction
What you should be able to do
- Describe how concentration, pressure, surface area, temperature and catalysts affect the rate of a reaction
- Describe how to follow a reaction by loss of mass on a balance or by the volume of gas in a gas syringe
- Interpret graphs of volume or mass against time, including how the limiting reactant fixes the final value
- Calculate a mean rate over a time interval, in units such as cm³/s or g/s
- Explain each effect with collision theory: particles per unit volume, collision frequency, kinetic energy and activation energy (Extended)
- State that a catalyst lowers the activation energy, and evaluate methods of measuring rate (Extended)
Key ideas
- Mean rate = change in amount of product or reactant ÷ time taken, e.g. in cm³/s
- The rate at one instant is the gradient of the tangent to the curve at that time (Extended)
- Only collisions with energy equal to or greater than Ea lead to reaction
- Higher temperature: faster particles, more frequent collisions and a greater proportion of collisions with energy at least Ea
- A catalyst gives an alternative pathway with a lower Ea, increasing the rate without being used up
How Rate of reaction has been examined
From 2022 to 2026, Rate of reaction was on Paper 6 · Alternative to Practical in 10 of 14 exam series, worth about 26% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 46 | 4% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 49 | 4% | 13 of 14 |
| Paper 3 · Theory (Core) | 29 | 10% | 12 of 14 |
| Paper 4 · Theory (Extended) | 18 | 10% | 11 of 14 |
| Paper 6 · Alternative to Practical | 24 | 26% | 10 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: The Speed of a Reaction.
A worked example
Magnesium ribbon reacts with an excess of dilute hydrochloric acid, and the hydrogen is collected in a gas syringe. The volumes at 0, 10, 20, 30, 40, 50 and 60 s are 0, 24, 40, 50, 55, 56 and 56 cm³. Calculate the mean rate over the first 20 s and over the first 50 s, explain why the rate falls, and predict the final volume if half the mass of magnesium is used.
- First 20 s: 40 cm³ ÷ 20 s = 2.0 cm³/s
- First 50 s: 56 cm³ ÷ 50 s = 1.12 cm³/s, which is 1.1 cm³/s to two significant figures
- The rate falls because the magnesium is used up and the acid becomes more dilute, so there are fewer collisions per second
- The acid is in excess, so magnesium is the limiting reactant and the volume of hydrogen is proportional to its mass
- Half the mass of magnesium gives half the volume: 56 ÷ 2 = 28 cm³
Answer: 2.0 cm³/s over the first 20 s and 1.1 cm³/s over the first 50 s; the rate falls as reactants are used up; the final volume would be 28 cm³
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Saying a catalyst gives more product — it only makes the reaction faster; the final volume or mass is set by the limiting reactant
- Explaining the effect of concentration as just 'more collisions' — the mark needs more particles per unit volume and so more frequent collisions, collisions per second
- Explaining temperature only by more frequent collisions — the larger effect is that a greater proportion of collisions have at least the activation energy
- Drawing a faster reaction's curve finishing higher when the same amounts are used — it should be steeper at the start but level off at the same final value
- Saying the rate falls during a reaction because the particles slow down — it falls because reactants are used up, so there are fewer collisions per second
Exam technique
- When comparing two curves, describe the gradient at the start and the final value separately; each can carry a mark
- Use 'frequency of collisions' or 'collisions per second' rather than just 'more collisions'
- For a mean rate, read both values carefully from the graph or table, show the subtraction and give units
- In method questions, name the apparatus (gas syringe, balance, stop-watch), what is measured and how often
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.