What is the rate of reaction?
The rate of reaction tells us how fast or slow a chemical change happens. In science we define it as the change in the quantity of a reactant used up, or a product formed, in one unit of time. A reaction that finishes in a few seconds, such as an explosion, has a very high rate, while the rusting of an iron gate in humid Malaysian air has a very low rate that takes weeks.
Key idea
Average rate of reaction = total change in quantity ÷ total time taken. Common units are cm³ s⁻¹ (gas volume), g s⁻¹ (mass), or mol s⁻¹.
Ways to measure the rate
To find a rate we must measure a change that we can observe. The method chosen depends on the reaction:
- Volume of gas released — collected in a gas syringe or over water, e.g. hydrogen from zinc and acid.
- Loss of mass — the flask sits on an electronic balance and the mass falls as gas escapes, e.g. carbon dioxide from marble and acid.
- Time for a fixed change — such as the time for a solid mark under a beaker to disappear when a precipitate of sulphur forms between sodium thiosulphate and acid.
Example
If 48 cm³ of gas is collected in 24 seconds, the average rate of reaction is 48 ÷ 24 = 2 cm³ s⁻¹.
Reading the graph
When the volume of gas is plotted against time, the curve is steepest at the start because the concentration of reactant is highest, so reaction is fastest. As reactants are used up the curve becomes less steep, and it finally becomes flat (horizontal) when the reaction stops. The average rate is found from the overall change, while the instantaneous rate at any moment is the gradient of the tangent to the curve at that point.
Remember
A flat graph does not mean the reaction is slow — it means the reaction has already finished because a reactant has run out.