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Explain a rate change using collisions
Changing a condition changes how often particles react.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Explain a rate change using collisions
Changing a condition changes how often particles react.
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 move faster and collide more energetically; more successful collisions | 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 understandingWhy does powdered calcium carbonate react faster than equal-mass chips in excess acid?Think it through, then reveal the answer
The powder exposes more surface area, so acid particles can collide with more carbonate surface per second. The total amount of carbonate, and hence final carbon dioxide amount, is unchanged.