Topic 3 of 5
Read slope and final amount separately
A plateau means product formation has stopped, not that nothing happened.
O-Level 6092 (2026) / SEC G3 K324 (2027)
Read slope and final amount separately
A plateau means product formation has stopped, not that nothing happened.
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 example
Diagnose a changed curve
A repeat reaches its plateau sooner but at the same final volume. What can be concluded?
- 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.