The average rate of reaction is the change in a measured quantity (gas volume, mass or concentration) divided by the time taken for that change.
This page walks through average rate of reaction for SPM Chemistry, step by step: the formula you need, the units to watch, and a worked example.
When you use this
Rate of reaction is a whole Form 4 chapter, and the first thing it asks you to calculate is the average rate over a period. Whether the experiment collects gas, watches a mass fall, or records a concentration, the calculation is the same shape.
The formula and units
average rate = change in quantity / time taken
The quantity can be the volume of gas released, the loss in mass, or the change in concentration. So the unit of the rate depends on what you measured:
- gas collected: cm3 s−1 (or cm3 min-1)
- mass lost: g s−1
- concentration change: mol dm−3 s−1
Units. Always divide the amount of change by the time for that change, and quote the matching unit. Rate is a positive quantity describing how fast the reaction goes.
Worked example 1 (easy)
A reaction releases 48 cm3 of gas in 24 s. Find the average rate.
- average rate = 48 / 24 = 2 cm3 s−1.
Worked example 2 (medium)
In a reaction that gives off gas, the flask’s mass falls from 50.00 g to 48.20 g in 60 s. Find the average rate of the mass loss.
- change in mass = 50.00 - 48.20 = 1.80 g.
- average rate = 1.80 / 60 = 0.03 g s−1.
Worked example 3 (SPM level)
Marble chips and acid produce 120 cm3 of carbon dioxide in total over 60 s, but the first 30 cm3 is collected in just the first 10 s. Compare the average rate over the first 10 s with the overall average rate.
- first 10 s: average rate = 30 / 10 = 3 cm3 s−1.
- whole reaction: average rate = 120 / 60 = 2 cm3 s−1.
The rate is higher at the start (3 cm3 s−1) than over the whole reaction (2 cm3 s−1), which tells you the reaction slows down as it proceeds, a favourite SPM point.
Why the rate falls with time
The average over the first few seconds is always larger than the average over the whole reaction. At the start the reactant concentration is highest, so there are the most frequent effective collisions and gas comes off fastest. As reactant is used up, collisions become less frequent and the curve flattens until the reaction stops. This is the difference between an average rate over an interval and the instantaneous rate at a moment, which you read from a graph gradient.
Choosing the right measure
Which quantity you track depends on the reaction. If a gas is given off, collect it in a syringe or over water and measure its volume. If a gas escapes an open flask, follow the falling mass on a balance. If a colour or a precipitate forms, follow the concentration or the time to reach a fixed mark. The method changes but the formula does not: it is always change in quantity divided by time. That is why you must first be clear about what each experiment measures before choosing the unit of the rate, because the unit has to match the quantity observed.
Common traps
- Forgetting the unit. A rate is meaningless without cm3 s−1, g s−1 or mol dm−3 s−1.
- Wrong time interval. Use the time for the exact change you are measuring, not the total time of the experiment, unless the question asks for the overall average.
- Not finding the change. Subtract the initial value from the final value first; do not divide the final reading by time.
- Confusing average with instantaneous. The average spans an interval; the instantaneous rate is a single instant.
Our teachers link every rate figure back to the collision theory picture, so you can both calculate the number and explain in words why it changes, the two things Paper 2 rate questions ask together.
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