Calculating and comparing rates
Calculating rates
In a typical rates experiment, the The amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). or The volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml. of A substance formed in a chemical reaction. is measured at regular time intervals. The results are usually recorded in a suitable table.
| Time (mins) | Volume of gas produced (cm3) |
| 0 | 0 |
| 1 | 34 |
| 2 | 42 |
| 3 | 48 |
| 4 | 50 |
| 5 | 50 |
| Time (mins) | 0 |
|---|---|
| Volume of gas produced (cm3) | 0 |
| Time (mins) | 1 |
|---|---|
| Volume of gas produced (cm3) | 34 |
| Time (mins) | 2 |
|---|---|
| Volume of gas produced (cm3) | 42 |
| Time (mins) | 3 |
|---|---|
| Volume of gas produced (cm3) | 48 |
| Time (mins) | 4 |
|---|---|
| Volume of gas produced (cm3) | 50 |
| Time (mins) | 5 |
|---|---|
| Volume of gas produced (cm3) | 50 |
The results recorded here show that the reaction had finished by four minutes, as no more gas was produced after that.
The mean rate of reaction = 50 ÷ 4 = 12.5 cm3/min
However, the rate decreased during the reaction. The table shows how this happened.
| Minute | Volume of gas (cm3) | Rate of reaction (cm3/min) |
| First (0 to 1) | 34 – 0 = 34 | 34 ÷ 1 = 34 |
| Second (1 to 2) | 42 – 34 = 8 | 8 ÷ 1 = 8 |
| Third (2 to 3) | 48 – 42 = 6 | 6 ÷ 1 = 6 |
| Fourth (3 to 4) | 50 – 48 = 2 | 2 ÷ 1 = 2 |
| Fifth (4 to 5) | 50 – 50 = 0 | 0 ÷ 1 = 0 |
| Minute | First (0 to 1) |
|---|---|
| Volume of gas (cm3) | 34 – 0 = 34 |
| Rate of reaction (cm3/min) | 34 ÷ 1 = 34 |
| Minute | Second (1 to 2) |
|---|---|
| Volume of gas (cm3) | 42 – 34 = 8 |
| Rate of reaction (cm3/min) | 8 ÷ 1 = 8 |
| Minute | Third (2 to 3) |
|---|---|
| Volume of gas (cm3) | 48 – 42 = 6 |
| Rate of reaction (cm3/min) | 6 ÷ 1 = 6 |
| Minute | Fourth (3 to 4) |
|---|---|
| Volume of gas (cm3) | 50 – 48 = 2 |
| Rate of reaction (cm3/min) | 2 ÷ 1 = 2 |
| Minute | Fifth (4 to 5) |
|---|---|
| Volume of gas (cm3) | 50 – 50 = 0 |
| Rate of reaction (cm3/min) | 0 ÷ 1 = 0 |
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 (showing that the rate of reaction is greater)
[Higher tier only]
You are expected to be able to calculate the rate of reaction at any time during a reaction by drawing a tangent to the curve at that time and then calculating the gradient of the tangent. An example is shown below.
