A checklist of the errors that most often cost marks in the Form 4 Rate of Reaction chapter, from stopping short of 'effective collisions' to claiming a catalyst is used up, with the correct version for each.
This page lists the mistakes we see most often in the Rate of Reaction chapter. For each one, read what students write, why it loses marks, and the version the marking scheme accepts. Fix these and you protect the easy marks.
Collision theory (7.3)
1. Stopping at “more collisions”. Students write “the particles collide more often, so the rate increases” and stop. Why it loses marks: the syllabus wants the link through effective collisions. Correct version: “…so the frequency of effective collisions increases, and the rate increases.” Make this the ending of every factor explanation.
2. An incomplete definition of an effective collision. Students say only “particles that collide react”. Why it loses marks: an effective collision has two conditions. Correct version: a collision is effective only if the particles have energy equal to or greater than the activation energy and collide in the correct orientation.
3. Temperature explained as only “particles move faster”. Why it loses marks: speed alone is the smaller effect and misses the key idea. Correct version: at a higher temperature particles move faster and a larger fraction have energy above the activation energy, so effective collisions are more frequent.
4. Confusing “activation energy” with “energy released”. Why it loses marks: activation energy is an input barrier, not the energy given out. Correct version: the activation energy is the minimum energy colliding particles must have for a collision to be effective.
Factors (7.2)
5. Saying pressure speeds up a reaction in solution. Students apply “increase the pressure” to acid–metal reactions. Why it loses marks: pressure only affects reactions involving gases. Correct version: for solutions, change the concentration; keep pressure for gas-phase reactions.
6. Confusing concentration with amount. Students think adding more of the same dilute acid speeds the reaction. Why it loses marks: it is the concentration, not the total amount, that sets the collision frequency. Correct version: use a more concentrated solution to increase the rate; a larger volume of the same concentration does not raise the initial rate.
7. Treating “surface area” as “more product”. Students say powdered marble gives more gas than lumps. Why it loses marks: surface area changes the rate, not the amount. Correct version: the same mass of marble gives the same volume of gas; the powder just reaches it faster.
Catalysts (7.4)
8. Saying a catalyst is “used up”. Why it loses marks: a catalyst is recovered unchanged. Correct version: a catalyst remains chemically unchanged in mass and composition at the end and can be reused.
9. Saying a catalyst “lowers the activation energy of the reaction”. Why it loses marks: it does not alter the original path. Correct version: a catalyst provides an alternative path with a lower activation energy.
10. Claiming a catalyst increases the amount of product. Why it loses marks: a catalyst changes only the speed. Correct version: the catalyst makes the same amount of product form faster; the final yield is unchanged.
Rate, calculations and graphs (7.1, 7.5)
11. Getting rate ∝ 1 ÷ time backwards. Students say “the time is shorter, so the rate is slower”. Why it loses marks: it reverses the relationship. Correct version: a shorter time means a faster rate, because rate is proportional to 1 ÷ time.
12. Reading the height of a curve as the rate. Why it loses marks: the height is the quantity produced, not the rate. Correct version: the gradient at a point is the rate; the instantaneous rate needs the gradient of a tangent.
13. Leaving the unit off a rate. Students write “the rate is 2”. Why it loses marks: a rate without a unit cannot score. Correct version: write 2 cm³ s⁻¹ (or g s⁻¹, mol dm⁻³ s⁻¹), matching the quantity measured.
14. Saying the rate is constant, or fastest at the end. Why it loses marks: the rate falls as reactants are used. Correct version: the rate is fastest at the start (steepest gradient) and falls to zero at the plateau.
15. Ignoring the mole ratio in a gas calculation. Students multiply the wrong number of moles by the molar volume. Why it loses marks: the balanced equation fixes the ratio. Correct version: use the mole ratio from the equation, then volume = moles of gas × molar volume, with Molar volume of a gas is 24 dm3 mol−1 at room conditions.
How to use this list
Go through your last few answers and check each one against these fifteen points. The single most valuable habit is finishing every factor explanation with “…so the frequency of effective collisions increases, and the rate increases”, and keeping “how fast” (the gradient) separate from “how much” (the plateau). A one-to-one teacher can mark a set of your explanations and show you exactly which of these slips is costing you marks, so you can drill it before the SPM Chemistry papers. Correct these fifteen and the chapter becomes one of the most reliable sources of marks in Form 4, because the same reasoning is tested again and again with only the context changed.
A few more slips to avoid
16. Describing the thiosulfate experiment loosely. Students say “the solution turns cloudy so the reaction is fast”. Why it loses marks: you must link the observation to a measured time. Correct version: time how long the mark under the flask takes to disappear, and note that a shorter time means a higher rate because rate is proportional to 1 ÷ time.
17. Forgetting orientation in the collision definition. Students give only the energy condition. Why it loses marks: both conditions carry the mark in a full definition. Correct version: state energy ≥ activation energy and the correct orientation together.
18. Mixing up the manipulated and responding variables in a rate experiment. Why it loses marks: the fair-test marks depend on naming them correctly. Correct version: the factor you change (for example temperature or concentration) is the manipulated variable; the measured quantity (volume of gas or time taken) is the responding variable, with everything else controlled.
Work through the full eighteen points once, then re-mark two of your own past answers. Most students find that just two or three of these slips account for nearly all their lost marks in this chapter, so fixing those first gives the fastest improvement.
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