Why convert volume into moles?

The molar volume of 24 dm3 per mole links gas volume to amount, which then compares directly with the equation coefficients.

Heating or pressure, which matters more?

Heating usually matters more because it raises both collision frequency and the fraction of successful collisions.

S4 Chemistry: Rate of Reaction (Paper 3)

2026-10-31

Tutor Prep Guide · Secondary Chemistry

Paper 3 joins rate to mole calculations and collision theory: zero point zero five zero mole of magnesium gives one point two cubic decimetres of hydrogen at twenty-four per mole, a zero point one zero molar solution used at zero point zero zero two zero per second lasts fifty seconds, zero point three zero mole at zero point zero one five per second takes twenty seconds, and ninety-six cubic centimetres is zero point zero zero four mole for one times ten to the minus four per second. The middle items compare initial rates in one and two molar acid at about two to one, turn zero point six six grams of carbon dioxide into zero point zero one five mole, and use thirty-second halvings for a total of sixty. The challenge items contrast heating with pressurising and take forty cubic centimetres of zero point two zero molar acid to its moles of hydrogen, closing with the error of confusing cubic decimetres with cubic centimetres.

Direct answer　Convert volume and mass into moles before comparing rates, and keep the units consistent so cubic decimetres never become cubic centimetres.

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)

FoundationMg + 2HCl gives MgCl2 + H2. Find the volume of hydrogen from 0.050 mol of magnesium at 24 dm3 per mole.
FoundationA reaction runs at 0.0020 mol dm-3 s-1 from a concentration of 0.10 mol dm-3. How long to reach zero?
Foundation0.30 mol of reactant is used at 0.015 mol per second. Find the time taken.
Foundation96 cm3 of gas is collected in 40 s. Give the rate in mol per second at 24 dm3 per mole.
IntermediateEqual masses of magnesium go into 1.0 and 2.0 mol dm-3 acid. Estimate the ratio of the initial rates and explain.
IntermediateA reaction loses 0.66 g of carbon dioxide, molar mass 44 g per mole. Find the amount in moles.
IntermediateA reactant falls from 4.0 g to 2.0 g in 30 s. If each halving takes the same time, how long to reach 1.0 g?
ChallengeWhy does raising the temperature usually speed a reaction up more than raising the pressure?
Challenge40 cm3 of 0.20 mol dm-3 hydrochloric acid reacts with excess magnesium. Find the moles of acid and of hydrogen.
Error redoError repair: a student treats 24 dm3 per mole as 24 cm3 per mole and gets 1.2 cm3 for 0.050 mol. Give the correct volume.

03　Answers and one-line notes

#	Answer	One-line note
1	1.2 dm3	One mole of magnesium gives one mole of hydrogen, so zero point zero five times twenty-four is one point two cubic decimetres.
2	50 s	Zero point one zero divided by zero point zero zero two zero gives fifty seconds.
3	20 s	Zero point three zero mole divided by zero point zero one five mole per second is twenty seconds.
4	1.0 x 10-4 mol/s	Ninety-six cubic centimetres is zero point zero zero four mole, and dividing by forty seconds gives one times ten to the minus four per second.
5	About 2 to 1, because the more concentrated acid gives more frequent collisions.	With everything else equal the rate is proportional to concentration, and two point zero is twice one point zero.
6	0.015 mol	Zero point six six grams divided by forty-four grams per mole is zero point zero one five mole.
7	About another 30 s, 60 s in total.	Each halving takes the same time, so four to two and two to one take thirty seconds each.
8	Heating raises both collision frequency and the fraction of collisions above the activation energy; pressure mainly raises frequency.	The fraction of successful collisions depends on the energy distribution, which heating shifts directly, so the effect is larger.
9	0.0080 mol HCl and 0.0040 mol H2	Zero point two zero times zero point zero four zero is zero point zero zero eight zero mole of acid, and two moles give one mole of hydrogen.
10	1.2 dm3, which is 1200 cm3	Twenty-four cubic decimetres is twenty-four thousand cubic centimetres, so the answer is twelve hundred cubic centimetres.

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.