Two reactions can produce the same final volume of gas and reach it at different speeds. Compare gas volumes at the same elapsed time, or compare the times needed to collect the same volume. A final reading by itself tells you how much gas was collected by that point.
SEAB’s 2027 SEC G3 Chemistry syllabus (K324) includes interpreting rate-of-reaction experiments. The worked data below let you describe the early rate and the eventual gas yield separately.
- Compare both experiments over the same interval.
- Use volume change divided by time for an average collection rate.
- Check for a plateau before treating the last value as the final yield.
Worked example | Two curves reach 60 cm³
In a fictional gas-collection experiment, Setup A records 0, 30, 50 and 60 cm³ at 0, 10, 20 and 30 seconds. Setup B records 0, 15, 30 and 45 cm³ at those same times. Both record 60 cm³ at 40 and 50 seconds.
| Elapsed time (s) | Setup A (cm³) | Setup B (cm³) |
|---|---|---|
| 0 | 0 | 0 |
| 10 | 30 | 15 |
| 20 | 50 | 30 |
| 30 | 60 | 45 |
| 40 | 60 | 60 |
| 50 | 60 | 60 |
Over the first ten seconds, A collects 30 ÷ 10 = 3.0 cm³/s and B collects 15 ÷ 10 = 1.5 cm³/s. These are interval averages. The readings show that A collects gas twice as fast on average during those first ten seconds.
Both eventually record 60 cm³. A reaches 30 cm³ at ten seconds, and B reaches it at twenty seconds. That second comparison also shows A reaching a fixed volume sooner.
Read the last point in context
If measurements ended at 30 seconds, the recorded volumes would be 60 and 45 cm³. B was still gaining volume in the preceding interval. Those readings would establish how much gas had been collected by 30 seconds and leave B’s eventual yield open.
The extra readings support a plateau in both setups over the observed period. If asked why the rate slows, use information about the reactants and conditions. A plateau alone does not identify which reactant ran out or rule out an apparatus problem.
Choose the interval before calculating
From ten to twenty seconds, A collects 50 − 30 = 20 cm³ and B collects 30 − 15 = 15 cm³. The interval rates are 2.0 and 1.5 cm³/s. The claim that A was twice as fast on average applies to the first interval only.
Ask a study partner to show where each number comes from. One person can name the interval and the other can compute the change. Then exchange roles and compare both results in a sentence with units.
Frequently asked questions
Is an average over ten seconds the initial rate?
It estimates the early rate over that interval. An instantaneous initial rate would come from the gradient at the start if the question provides a suitable graph.
What does a horizontal segment show?
The measured gas volume stayed constant during that interval. The setup and question determine how far you can explain the cause.
Official information
SEAB’s 2027 SEC G3 Chemistry syllabus (K324), Section 10 Rate of Reactions. Checked 9 November 2026.