Calculating Rates of Reaction — AQA GCSE Chemistry

The rate of reaction measures how quickly reactants are used up or products are formed.

Measuring rate

The rate of reaction can be found by measuring:

• the amount of product formed over time (e.g. volume of gas with a gas syringe), or
• the amount of reactant used up over time (e.g. loss of mass on a balance).

$$\text{mean rate} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}$$

Units are often cm³/s (gas) or g/s (mass).

Interpreting rate graphs

On a graph of amount of product against time:

• The steeper the line, the faster the rate.
• The line levels off (becomes horizontal) when the reaction has finished (a reactant has run out).
• Reactions usually start fastest and slow down as reactants are used up.

Finding the rate at a specific time (Higher Tier)

The rate at a particular moment is the gradient of the tangent to the curve at that time. Draw a tangent, then calculate its gradient (change in y ÷ change in x).

Exam tips

• Learn the two ways to measure rate (gas volume; mass loss).
• Calculate mean rate = quantity ÷ time, with correct units.
• Steeper graph = faster rate; level line = reaction finished.
• For Higher Tier, find an instantaneous rate using the gradient of a tangent.