On a graph of product against time, steepness shows the rate and the final height shows the total product. A flat line means the reaction has finished.
This lesson is part of rates of reaction. The next lesson, comparing factors affecting rate, uses these readings to compare experiments.
What should I look at first on the graph?
Look at the axes, then at three parts of the curve. Check that time is on the horizontal axis and product on the vertical axis.
The start tells you the fastest rate, the middle shows the rate slowing, and the flat part shows the reaction has finished. Each part answers a different type of question.
Worked example: one curve in three sections
The table is original. It records gas collected when a metal reacts with acid.
| Time (s) | 0 | 20 | 40 | 60 | 80 | 100 |
|---|---|---|---|---|---|---|
| Gas (cm³) | 0 | 18 | 30 | 38 | 40 | 40 |
Start (0 to 20 s). 18 cm³ forms in 20 s, so the rate is 18 ÷ 20 = 0.9 cm³ per second. This is the steepest part.
Middle (40 to 60 s). 8 cm³ forms in 20 s, so the rate is 8 ÷ 20 = 0.4 cm³ per second. The curve has bent over because the acid and metal are being used up.
Plateau (80 to 100 s). The reading stays at 40 cm³, so the rate is 0. The reaction finished at about 80 s and made 40 cm³ in total.
The mistake that costs marks
The common slip is to read height instead of steepness. A taller curve looks more impressive, but the question about speed asks about the slope.
| Step | Wrong | Right |
|---|---|---|
| Read speed | The higher curve is faster | The steeper curve is faster |
| Read the plateau | The reaction is still going slowly | The reaction has stopped |
| Read total product | Use the steepest point | Use the final flat height |
| Wording | “It goes up more” | “It is steeper, so the rate is higher” |
The fix is to ask two questions separately: how steep is it, and how high does it end.
Two curves, same final height
Experiment A reaches 50 cm³ at 60 s. Experiment B also reaches 50 cm³ but takes 120 s. Both end at the same height, so the same amount of limiting reactant was used in each.
A is faster because it is steeper. Its average rate is 50 ÷ 60 = 0.83 cm³ per second, against 50 ÷ 120 = 0.42 cm³ per second for B. The graph evidence and fair-comparison lab lets you practise reading two curves side by side.
Check yourself
Curve C ends at 30 cm³ after 40 s. Curve D ends at 60 cm³ after 40 s. A student says D is twice as fast because it is twice as high. Is the claim supported?
Answer
Not fully. Both curves took 40 s to finish, so D has the higher average rate: 60 ÷ 40 = 1.5 cm³ per second against 30 ÷ 40 = 0.75 cm³ per second for C.
However, the heights differ because D made more product in total, which means D probably had more limiting reactant. Height compares the amount formed, and steepness compares the speed.
What to study next
Continue with comparing factors affecting rate, then test the whole cluster with the rates of reaction practice set.
If you want a teacher to go through your graph questions with you, see online one-to-one Science tuition.