Can one curve give several rates?

Yes. Each stretch has its own average rate, found by dividing that stretch volume change by its time.

Does doubling concentration always double the rate?

Only when the rate is directly proportional to concentration; a real reaction of different order gives a different factor.

S4 Chemistry: Rate of Reaction (Paper 2)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

Paper 2 turns one set of readings into gradients: zero, forty, sixty-four, seventy-six and eighty cubic centimetres at zero, twenty, forty, sixty and eighty seconds give one point six per second over the first forty seconds, two point zero over the opening twenty, and only zero point six between forty and sixty, while a three gram mass loss over ninety seconds is one thirtieth per second. The middle items compare powder with chips, take two point zero to four point zero per second when the acid doubles, and cut thirty seconds to seven point five using the doubling rule for every ten kelvin. The challenge items explain catalysts and the assumptions behind gas collection, and the paper ends with the error of reading a plateau as a total.

Direct answer　Read a graph section by section: the gradient of each stretch is the rate there, and a plateau near zero is not a mistake.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S4 Chemistry: Rate of Reaction
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationGas volume readings are 0 cm3 at 0 s, 40 at 20 s, 64 at 40 s, 76 at 60 s and 80 at 80 s. Find the average rate from 0 to 40 s.
FoundationUsing the same readings, find the average rate over the first 20 s.
FoundationFrom the same readings, find the rate between 40 s and 60 s and compare it with the start.
FoundationA mass-loss experiment loses 3.0 g in total and levels off at 90 s. Find the average rate over the whole run.
IntermediateEqual masses of marble, one as powder and one as chips, go into acid of the same concentration. Which reacts faster and why?
IntermediateAcid at 0.10 mol dm-3 gives gas at 2.0 cm3/s. Estimate the rate with 0.20 mol dm-3 acid, all else equal.
IntermediateA rate roughly doubles for every 10 K rise. A reaction takes 30 s at 30 C; estimate the time at 50 C.
ChallengeExplain how a catalyst speeds a reaction up and whether it is used up.
ChallengeWhat assumptions underlie measuring a rate by collecting gas over water?
Error redoError repair: a student sees the curve flatten after 80 s and concludes the earlier rates were miscalculated. Explain the correct reading.

03　Answers and one-line notes

#	Answer	One-line note
1	1.6 cm3/s	Sixty-four cubic centimetres over forty seconds gives one point six cubic centimetres per second.
2	2.0 cm3/s	Forty cubic centimetres over twenty seconds gives two point zero per second, the fastest stretch of the run.
3	0.6 cm3/s, much slower than 2.0 cm3/s at the start.	Seventy-six minus sixty-four over twenty gives zero point six, far slower than the opening two point zero because less reactant remains.
4	1/30 g/s, about 0.033 g/s	Three point zero grams over ninety seconds is one thirtieth, about zero point zero three three grams per second.
5	The powder, because its larger surface area allows more collisions.	Powder exposes a far larger surface, so more particles meet the acid at once; the rate rises while the total gas stays the same.
6	About 4.0 cm3/s	Doubling the concentration roughly doubles the collision frequency, so two point zero becomes four point zero.
7	About 7.5 s	Twenty kelvin means two doublings, so the rate is four times larger and thirty seconds becomes seven point five.
8	It provides a route of lower activation energy and is not used up.	A lower activation energy turns more collisions into successful ones, and the catalyst keeps its mass and chemical identity.
9	The gas is insoluble, temperature and pressure stay constant, and nothing leaks.	If the gas dissolved or escaped, the measured volume would no longer track how far the reaction has gone.
10	A flat curve means the reaction has finished; 80 cm3 is the total volume of gas, not a rate.	The plateau has almost no slope, so the rate is near zero; eighty cubic centimetres is the total product, while the rate comes from the slope of the earlier stretch.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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