When two rate graphs are drawn together, steepness compares the rates and the final level compares the amount of product. A comparison is fair only when a single variable differs between the two experiments.
This lesson is part of SPM Chemistry rate of reaction. It uses the gradient skills from calculating average and instantaneous rates from graphs.
What does each feature of the graph tell you?
Read three features in order: the starting gradient, the time to level off, and the final level.
| Feature | What it tells you |
|---|---|
| Steeper start | A higher initial rate |
| Levels off sooner | The reaction finishes in less time |
| Higher plateau | More product, so more limiting reactant |
Worked example: a speed question and an amount question
An original set of experiments uses 0.12 g of magnesium ribbon, which is 0.12 ÷ 24 = 0.005 mol of Mg. Take the molar volume as 24 dm³/mol at room conditions, so 1 mol of gas is 24 000 cm³.
Experiment I uses 20 cm³ of 1.0 mol/dm³ hydrochloric acid, which is 0.020 mol of HCl. Mg + 2HCl → MgCl₂ + H₂ needs 0.010 mol HCl for 0.005 mol Mg, so acid is in excess. Hydrogen formed = 0.005 mol = 0.005 × 24 000 = 120 cm³.
Experiment II uses the same Mg with 20 cm³ of 2.0 mol/dm³ acid. Acid is again in excess, so the final volume is still 120 cm³. The curve is steeper at the start, but the plateau is the same.
Experiment III uses the same Mg with 10 cm³ of 0.5 mol/dm³ acid, which is 0.0050 mol HCl. This is only half of the 0.010 mol needed, so acid is now the limiting reactant. Hydrogen formed = 0.0025 mol = 60 cm³, so the plateau is lower.
The lesson: changing the concentration changed the rate in II, but changing the amount of limiting reactant changed the final level in III.
What makes the test fair?
A fair comparison changes one thing and keeps the rest constant. For a magnesium and acid experiment, control the mass and size of magnesium, the volume of acid and the temperature.
| Design | Verdict |
|---|---|
| Change acid concentration only, keep Mg mass, acid volume and temperature | Fair. Any difference is caused by concentration. |
| Change concentration and temperature together | Unfair. The cause of a faster curve cannot be identified. |
The mistake that costs marks
A common answer says “Experiment II has a higher curve, so more product was formed”. Check the plateau before saying so. If both curves reach 120 cm³, the amount of product is the same and only the rate differs.
Another slip is to compare the curves at one time reading and declare a winner. Compare the initial gradients and the final levels instead.
Check yourself
Experiment A uses 0.24 g of Mg with excess acid. Experiment B uses 0.12 g of Mg with the same excess acid, at the same temperature. Predict the final hydrogen volume in each, and say whether the comparison is fair for studying the effect of acid concentration.
Answer
A: 0.24 ÷ 24 = 0.010 mol Mg, so 0.010 mol H₂ = 240 cm³.
B: 0.12 ÷ 24 = 0.005 mol Mg, so 0.005 mol H₂ = 120 cm³.
The acid concentration is identical, so this pair tests the effect of magnesium mass, not acid concentration. It is not a fair comparison for acid concentration.
What to study next
Fair comparisons lead to the energy picture in interpreting activation-energy diagrams. Test the full set with the rate of reaction practice set.
The graph evidence and fair-comparison lab helps you practise the checks above. If you want a teacher to go through your own experiment write-ups, see online one-to-one Chemistry tuition.