Why must the unit be written?

A rate is a change divided by a time, so 0.02 g/s and 1.2 g/min describe the same event with different numbers.

Does a flat curve mean the rate was wrong?

No. A flat curve means the reaction is nearly finished, so the slope is close to zero and the final height is the total product.

S4 Chemistry: Rate of Reaction (Paper 1)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

Paper 1 drills the measurement and expression of rate: two grams of magnesium leave zero point eight grams after sixty seconds, so one point two grams over sixty seconds gives zero point zero two grams per second; forty-eight cubic centimetres in twenty-four seconds is two per second; two hundred and fifty cubic centimetres in fifty seconds becomes three hundred per minute; and zero point two zero mole used from a zero point five zero mole sample in one hundred seconds gives zero point zero zero two zero mole per second. The middle items recover a concentration change from a rate, convert three point six grams per minute into zero point zero six grams per second, and read the meaning of a steep opening slope on a volume against time graph. The challenge items contrast a catalyst with a temperature rise, and the paper closes with the error of dividing time by mass.

Direct answer　Rate is always change divided by time: name the quantity on top, then check the units.

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)

FoundationTwo grams of magnesium ribbon react with dilute hydrochloric acid and 0.8 g is left after 60 s. Find the average rate.
FoundationA reaction collects 48 cm3 of gas in 24 s. Work out the average rate.
Foundation250 cm3 of carbon dioxide is collected in 50 s. Give the rate in cm3 per minute.
FoundationA reaction uses 0.20 mol of a 0.50 mol sample in 100 s. Give the average rate in mol per second.
IntermediateA reaction runs at 0.05 mol dm-3 s-1. How much does the concentration fall in 10 s?
IntermediateA balloon loses 3.6 g of gas per minute. Express this in grams per second.
IntermediateThe volume against time curve is steepest at the beginning. What does this tell you about the rate?
ChallengeState the effect of adding a catalyst on the rate and on the final amount of product.
ChallengeRaising the temperature by 20 K speeds a reaction up markedly. Explain why.
Error redoError repair: a student divides 60 by 1.2 and writes 0.2 g/s for a 1.2 g mass loss over 60 s. Give the correct rate.

03　Answers and one-line notes

#	Answer	One-line note
1	0.02 g/s	The mass falls from two point zero to zero point eight grams, so twelve grams over sixty seconds is zero point zero two grams per second.
2	2 cm3/s	Forty-eight cubic centimetres divided by twenty-four seconds gives two cubic centimetres per second.
3	300 cm3/min	Two hundred and fifty divided by fifty gives five per second, and five times sixty is three hundred per minute.
4	0.0020 mol/s	Zero point two zero mole divided by one hundred seconds is zero point zero zero two zero mole per second.
5	0.50 mol dm-3	Rate times time gives zero point zero five times ten, which is zero point five zero mole per cubic decimetre.
6	0.06 g/s	Three point six grams per minute divided by sixty seconds is zero point zero six grams per second.
7	The rate is fastest at first because the reactant concentration is highest.	A steeper slope means a faster rate, so the rate is highest at the start when the reactant concentration is greatest and then falls away.
8	The rate increases; the final amount of product is unchanged.	A catalyst opens a route of lower activation energy, so the reaction is faster while the final amount of product stays the same.
9	Particles have more kinetic energy, so collisions are more frequent and more of them exceed the activation energy.	Heating raises the average kinetic energy, so collisions are more frequent and a larger fraction of them exceed the activation energy.
10	0.02 g/s	The student divided the time by the mass; the mass must be divided by the time, giving zero point zero two grams per second.

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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Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.