How do I read an instantaneous rate from a graph?

Draw the tangent at that moment; its gradient is the rate, while the height only gives the concentration.

Does a flat curve mean the reaction is over?

No. The concentration is low so collisions are rare, but the reaction still continues slowly.

S5 Chemistry Rates of Reaction, Paper 2: measurement methods, rate graphs and temperature effects

2026-11-02

Ten graded items from gas syringes through concentration-time gradients and temperature steps to rate comparisons, each with an answer.

Focus　Read the tangent, not the height, on a rate graph; count ten-degree steps carefully; a flattening curve only means slower.

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: S5 Chemistry: Rates of Reaction
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

Foundation48 cm^3 of hydrogen is collected in 60 s. What is the average rate?
FoundationWhich apparatus best measures the volume of gas given off over time?
FoundationAs a rule of thumb, how does the rate change for every 10 C rise?
FoundationWhy does raising the pressure speed up a gaseous reaction?
IntermediateA concentration-time graph shows the reactant falling from 0.80 to 0.40 mol dm^-3 in the first 20 s. Find the average rate.
IntermediateIf the rate doubles for every 10 C rise, how many times faster is the reaction at 50 C than at 30 C?
IntermediateWhat does it mean when the tangent to a concentration-time curve becomes flatter?
Challenge0.60 mol of reactant falls to 0.15 mol in 45 s. Find the average rate in mol s^-1.
ChallengeRun A gives 40 cm^3 in 80 s and run B gives 40 cm^3 in 20 s. How many times faster is B?
Error redoError repair: a student claims a flat curve means the reaction has stopped. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	0.80 cm^3 s^-1	Rate is volume over time, so 48 divided by 60 gives 0.80.
2	A gas syringe read at fixed time intervals	A gas syringe gives a direct reading of the accumulated volume, so with a timer you get volume against time.
3	It roughly doubles	Heating raises particle energy so collisions are more frequent and more effective, roughly doubling the rate.
4	The particle concentration rises, so collisions are more frequent	Compressing the gas raises the number of particles per unit volume, so collisions become more frequent.
5	0.020 mol dm^-3 s^-1	The concentration falls by 0.40 over 20 s, which divides to 0.020.
6	Four times	A rise of 20 C is two ten-degree steps, and two squared is four.
7	The rate is slowing down but the reaction continues	The tangent gradient is the instantaneous rate, so flattening means a slower rate, not a stopped reaction.
8	0.0100 mol s^-1	0.45 mol is used up and divides by 45 s to give 0.0100.
9	Four times	Run A gives 0.50 and run B 2.0 cm^3 per second, so B is four times faster.
10	The rate is slow at low concentration, but the reaction still proceeds	With little reactant left collisions are rare, so the rate is close to zero but the reaction still continues.

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.