Lesson 3 of 3
Read slope and final amount separately
A plateau means product formation has stopped, not that nothing happened.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
I can interpret rate data.Syllabus
Syllabus K326 / K328, 10(b). Interpret rate data
Read slope and final amount separately
A plateau means product formation has stopped, not that nothing happened.
Make a guess. You are not marked.
Magnesium ribbon (in excess) reacts with 50 cm3 of dilute hydrochloric acid, and the gas volume is plotted against time. The experiment is repeated with 50 cm3 of acid at half the concentration. How does the new curve compare?
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Less steep, and it levels off at half the volume
Fewer acid particles per cm3 means fewer collisions per second. The acid is the limiting reactant and there are half as many moles of it, so half as much gas forms.
Same yield, different rates
The faster experiment reaches 44 cubic centimetres earlier; both finish at the same volume.
The gradient of a product-volume graph gives rate. A steeper initial slope means faster initial reaction. The curve becomes less steep as reactants are consumed. A horizontal line means no further gas is collected; a reactant may have been exhausted. A tangent estimates the instantaneous rate at a point; a line joining two measured points gives the average over that interval.
For a mass-loss graph, the mass decreases, so the slope is negative. Use the magnitude of the decrease per time for a positive reaction-rate value. Compare final gas volumes only at the same temperature and pressure. A leak can reduce apparent volume without changing the actual chemical yield.
Worked exampleDiagnose a changed curve
A repeat reaches its plateau sooner but at the same final volume. What can be concluded?
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- It formed the measured product more quickly.
- The same final amount formed under the stated gas conditions.
- A rate factor such as temperature, concentration or surface area may explain it; the curve alone does not identify which.
Faster rate, same final amount. Do not claim that a catalyst or temperature change is uniquely proved.
Check your understanding
A lower final volume of gas is recorded after a bung leaks. Does that prove less product formed?
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No. Some gas probably escaped before it could be collected.
A leak changes the measurement, not the chemistry. Separate apparatus faults from chemical explanations before drawing a conclusion.
Exam-style question
Magnesium ribbon reacts with excess dilute hydrochloric acid. The volume of hydrogen is recorded every 30 s.
Time / s: 0, 30, 60, 90, 120, 150, 180
Volume / cm3: 0, 20, 34, 43, 48, 50, 50
(a) Plot the results and draw a suitable line. [2]
(b) Use your graph to find the volume at 45 s. [1]
(c) State the time at which the reaction stopped, and explain why it stopped. [1]
(d) The experiment is repeated with the same mass of magnesium and acid that is twice as concentrated, still in excess. Sketch the new curve on the same axes and explain its shape. [2] [6]
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Check your understanding
Excess zinc reacts with 50 cm3 of dilute sulfuric acid, so the acid is limiting. It is repeated with 50 cm3 of acid of half the concentration. How does the new volume-time curve compare?
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It starts less steeply and levels off at half the volume
Fewer acid particles per unit volume means fewer effective collisions, so the starting gradient is lower. Half the concentration in the same volume is half the moles of the limiting acid, so half the hydrogen forms.