What is rate of reaction?
The rate of reaction tells us how fast reactants are used up or how fast products are formed in a given time. A reaction that finishes in 10 s is faster than one that takes 100 s. We measure rate by choosing a quantity that changes as the reaction proceeds, then recording how that quantity changes with time.
Common measurable changes include the volume of gas released (using a gas syringe or an inverted burette), the loss in mass of the flask (using an electronic balance), a change in colour or turbidity (a cross under a beaker disappearing), or the pressure of gas produced. The general definition is:
Key idea
Average rate = change in quantity ÷ time taken. When we time how long a reaction takes to reach a fixed stage (for example a fixed volume of gas), a useful shortcut is rate ∝ 1 ÷ time, so a shorter time means a higher rate.
Reading a graph
Plotting the measured quantity (y-axis) against time (x-axis) gives a curve that is steep at the start and flattens as reactants run out. The gradient of the tangent at any point gives the instantaneous rate at that moment. The curve becomes horizontal when the reaction stops.
A classic experiment reacts marble chips with dilute hydrochloric acid: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g). The CO2 gas can be collected to measure volume, or the flask can be weighed to follow the loss in mass.
Worked example
In a reaction, 48 cm³ of gas is collected in 24 s. The average rate of reaction = 48 cm³ ÷ 24 s = 2 cm³ s⁻¹. If a second experiment reaches the same volume in 12 s, its rate (48 ÷ 12 = 4 cm³ s⁻¹) is twice as fast.
Remember
- Rate units are amount per time, e.g. cm³ s⁻¹, g s⁻¹ or mol dm⁻³ s⁻¹.
- The steepest part of the curve is at the start when reactant concentration is highest.
- A shorter reaction time means a higher rate (rate ∝ 1 ÷ time).