Comparing rates
In a typical rates experiment, the mass or volume of product is measured at regular time intervals. The results are usually recorded in a table. For example:
| Time / min | Volume of gas produced / cm3 |
| 0 | 0 |
| 1 | 34 |
| 2 | 42 |
| 3 | 48 |
| 4 | 50 |
| 5 | 50 |
| Time / min | 0 |
|---|---|
| Volume of gas produced / cm3 | 0 |
| Time / min | 1 |
|---|---|
| Volume of gas produced / cm3 | 34 |
| Time / min | 2 |
|---|---|
| Volume of gas produced / cm3 | 42 |
| Time / min | 3 |
|---|---|
| Volume of gas produced / cm3 | 48 |
| Time / min | 4 |
|---|---|
| Volume of gas produced / cm3 | 50 |
| Time / min | 5 |
|---|---|
| Volume of gas produced / cm3 | 50 |
These results show that the reaction had finished by four minutes. No more gas was produced after that point.
However, the rate decreased during the reaction as the volume of gas produced decreased as the time increased.
Graphs
The rate of reaction can be analysed by plotting a graph of amount of product against time. The graph below shows this for two reactions.
Compared to the slow reaction, the graph line for the faster reaction:
- has a steeper gradient at the start
- becomes horizontal sooner (gas stops being produced sooner so the reaction finishes sooner) meaning that the rate of reaction is greater.