Lesson 1 of 5
Explain a rate change using collisions
Changing a condition changes how often particles react.
O-Level 6092 (2026) / SEC G3 K324 (2027)
I can explain rate factors.Syllabus
Syllabus K324, 10(a). Explain rate factors
Explain a rate change using collisions
Changing a condition changes how often particles react.
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Marble chips react with excess dilute hydrochloric acid. The experiment is repeated with the same mass of marble as a fine powder. What changes?
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Faster reaction, same total volume of gas
Powder exposes more surface, so acid particles collide with it more often. The mass of marble is unchanged, so the total carbon dioxide is unchanged.
Reaction rate measures how quickly a reactant is used up or a product forms. Reacting particles must collide, and collisions must have enough energy to lead to reaction. An explanation should connect the changed condition to particle behaviour, then to the number of successful collisions per unit time.
| Change | Particle explanation | Important control |
|---|---|---|
| Higher solution concentration | More reacting particles per unit volume; more frequent collisions | Keep total solution volume and temperature controlled |
| Higher gas pressure at fixed temperature | Gas particles are closer together; more collisions per unit volume per second | Applies to gaseous reactants, not simply pressing on a solid |
| Smaller solid pieces, same total mass | Greater exposed surface area offers more places for collisions | Grinding does not create more total moles of solid |
| Higher temperature | Particles gain kinetic energy and move faster. A greater proportion of particles have energy equal to or greater than the activation energy (the minimum energy needed to react), so the frequency of effective collisions increases | Do not explain temperature solely as an increase in concentration |
A larger lump and equal-mass powder can produce the same final amount of gas while reacting at different speeds. In contrast, adding more of a limiting reactant can change the final amount. Keep the question "how fast?" separate from "how much?".
Check your understanding
Why does powdered calcium carbonate react faster than the same mass of chips in excess acid?
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The powder has more surface area, so there are more collisions with acid each second.
Acid particles can only hit the exposed surface. Powder exposes far more surface, so effective collisions are more frequent. The final volume of CO2 is the same, because the mass of carbonate is the same.
Exam-style question
Magnesium ribbon reacts with dilute hydrochloric acid. The experiment is repeated with more concentrated acid, keeping everything else the same. Explain, in terms of particles, why the reaction is faster. [2]
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Exam-style question
Calcium carbonate chips react with dilute hydrochloric acid. The experiment is repeated at a higher temperature, keeping everything else the same. Explain, in terms of particles, why the reaction is faster. [3]
Saved on this browser only. Anyone using this browser can see it.