IGCSE Chemistry Rates of Reaction Notes
Rate of reaction is how quickly reactants become products. Measure it by tracking a change in mass, volume of gas, or colour over time.
Key concepts
- Rate
- Rate = change in amount ÷ time; units include g/s, cm³/s and mol/dm³/s.
- Measuring rate
- Gas syringe (volume of gas), mass loss on a balance (CO₂ escaping), disappearing cross (Na₂S₂O₃ + HCl).
- Rate graphs
- Steepest gradient at t = 0; flattens as reactants are used up; horizontal when reaction ends.
- Doubling concentration typically doubles the initial rate.
- A 10 °C rise roughly doubles the rate for many reactions.
- Catalysts remain chemically unchanged at the end of a reaction.
- The end of a reaction is when a reactant is used up (curve flattens).
Worked example
In an experiment, 48 cm³ of gas was collected in 40 s. Calculate the average rate of reaction.
- 1Rate = volume ÷ time
- 2= 48 ÷ 40
- 3= 1.2 cm³/s
Answer: 1.2 cm³/s
Common mistakes
✗ Reading the initial rate from the flat part of the graph.
✓ Initial rate is the gradient at t = 0, not later on.
✗ Forgetting units in rate calculations.
✓ Always match units of amount and time (e.g. cm³/s, g/min).
✗ Assuming a catalyst is used up because concentration falls.
✓ The reactant concentration falls; the catalyst mass stays the same.
Quick check
Try these, then reveal the answers.
How can you measure the rate of reaction between HCl and marble chips?Reveal answer
Loss of mass on a balance (CO₂ escapes) or volume of gas in a syringe over time.
Why does the rate slow down as the reaction proceeds?Reveal answer
Reactant concentration falls, so fewer collisions per second.
What is measured in the disappearing cross experiment?Reveal answer
Time for the cross to disappear as sulfur clouds the solution.
Exam tips
- Practise drawing a tangent to find rate at a specific time on a curve.
- State the variable measured and how it changes with time in every 'describe the method' answer.
- Explain trends using collision theory — 'more collisions per second with sufficient energy'.
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Frequently asked questions
What variables affect the rate of a reaction?+
Concentration, temperature, surface area, pressure (for gases) and the presence of a catalyst.
How do you find the average rate from a graph?+
Total change in the measured quantity divided by the total time taken.
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