Average rate is the change in the amount divided by the time taken. Pick the two rows that bound the interval, subtract, divide, and write the unit.
This lesson is part of rates of reaction. It puts numbers on the ideas in explaining collision ideas at Science depth.
How do I pick the numbers?
Decide the interval first, then read both the amounts and the times at its two ends. The change is the later amount minus the earlier amount, or the other way round when the mass falls.
Write the working as a fraction so the unit follows automatically: grams over seconds gives g per s.
Worked example: mass lost from an open flask
The table is original. A flask of acid and marble chips sits on a balance, and carbon dioxide escapes.
| Time (s) | 0 | 30 | 60 | 90 | 120 | 150 |
|---|---|---|---|---|---|---|
| Mass (g) | 85.00 | 84.46 | 84.10 | 83.90 | 83.80 | 83.80 |
Mass lost in each 30 s. Each interval uses the earlier mass minus the later mass.
- 0 to 30 s: 85.00 − 84.46 = 0.54 g
- 30 to 60 s: 84.46 − 84.10 = 0.36 g
- 60 to 90 s: 0.20 g
- 90 to 120 s: 0.10 g
- 120 to 150 s: 0 g
Rates. Divide each mass lost by 30 s.
- 0 to 30 s: 0.54 ÷ 30 = 0.018 g per s
- 30 to 60 s: 0.36 ÷ 30 = 0.012 g per s
- 60 to 90 s: 0.20 ÷ 30 = 0.0067 g per s
- 90 to 120 s: 0.10 ÷ 30 = 0.0033 g per s
- 120 to 150 s: 0 g per s
Reading. The rate falls each interval, and it is 0 from 120 s, so the reaction finished at about 120 s. The total mass lost is 1.20 g.
The mistake that costs marks
The common slip is to divide the total mass lost by the last time in the table. Here that gives 1.20 ÷ 150 = 0.008 g per s, which understates the rate while the reaction was running.
| Step | Wrong | Right |
|---|---|---|
| Interval | 0 to 150 s | 0 to 120 s, where the reaction happened |
| Working | 1.20 ÷ 150 | 1.20 ÷ 120 |
| Answer | 0.008 g per s | 0.010 g per s |
| Unit | (missing) | g per s |
If the question asks for the rate over 0 to 150 s, 0.008 is correct. Always check which interval the question names.
Checking your answer
Check two things. The rate should fall as time passes, and the units should match the table.
Also check the total. Adding the mass lost in each interval must give the final change: 0.54 + 0.36 + 0.20 + 0.10 + 0 = 1.20 g, which matches 85.00 − 83.80.
Check yourself
A gas syringe records 0 cm³ at 0 s, 14 cm³ at 20 s and 32 cm³ at 50 s. Find the average rate between 20 s and 50 s in cm³ per second, then in cm³ per minute.
Answer
Change in volume = 32 − 14 = 18 cm³. Change in time = 50 − 20 = 30 s.
Average rate = 18 ÷ 30 = 0.6 cm³ per second. To convert to minutes, multiply by 60: 0.6 × 60 = 36 cm³ per minute.
What to study next
Test all four lessons together with the rates of reaction practice set. If the subtraction step slips, return to reading rate graphs.
If you want a teacher to go through your working with you, see online one-to-one Science tuition.