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1.11.4Physical Chemistry

Measuring the Rate of a Reaction — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 61 of 63

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
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Part 1 of 6Choosing a Method

1.11.4 Measuring the Rate of a Reaction

The best method depends on which property changes measurably during the reaction.

Gas volume
  • Collect the gas in a syringe and record volume at fixed times; suitable when a gas is a product.
Mass loss
  • Weigh the flask on a balance as gas escapes; best when the gas is dense, such as carbon dioxide.
Colorimetry
  • Measure absorbance as a coloured reactant or product changes, using a calibration curve to convert to concentration.
Sampling and titration
  • Remove samples at intervals, quench the reaction, then titrate to find the concentration remaining.
Other properties
  • Electrical conductivity or pH can be followed when ions or hydrogen ions change during the reaction.

Part 2 of 6Quenching and Sampling

1
Withdraw a measured sample from the reaction mixture at a recorded time.
2
Add it immediately to a quenching solution, such as ice-cold water or a reagent that removes a reactant.
3
Titrate the quenched sample against a standard solution to find the amount of reactant or product present.
4
Repeat at regular time intervals to build up a concentration–time data set.
5
Plot the graph of concentration against time to analyse the rate.

Part 3 of 6Rates from Graphs

Rates from Graphs
Tangent
  • Draw a tangent at the chosen time; its gradient is the rate of reaction at that moment.
Initial rate
  • Draw the tangent at time zero, where concentrations are known exactly from the mixture.
Gradient
  • The rate equals change in concentration divided by change in time, taken from two well-separated points on the tangent.
Fall in rate
  • The tangent becomes shallower with time as reactant concentration falls.

Part 4 of 6Clock Reactions

Principle
  • A sudden visible change, such as colour appearing, occurs after a fixed small amount of product has formed.
Rate estimate
  • Because the same amount forms each time, rate is proportional to 1/time for the same extent of reaction.
Limitation
  • The result approximates the initial rate only if concentrations hardly change before the end point.
Control
  • Temperature and total volume must be kept constant, and only the concentration being studied is varied.

Part 5 of 6Worked Example

Question

  • The concentration of a reactant at t = 0 is 0.50 mol dm⁻³.
  • A tangent drawn at t = 0 meets the time axis at t = 125 s.
  • Calculate the initial rate.

  1. The tangent falls from 0.50 mol dm⁻³ at 0 s to 0 mol dm⁻³ at 125 s.
  2. Gradient = change in concentration / change in time = −0.50 / 125.
  3. Rate is the magnitude of the gradient: 0.50 / 125 = 4.0 × 10⁻³.

Answer Initial rate = 4.0 × 10⁻³ mol dm⁻³ s⁻¹.

Part 6 of 6Exam Checklist & Tips

1.11.4 – Key Points for the Exam

  • Continuous monitoring Measuring a property of the reaction mixture repeatedly as the reaction proceeds.
  • Quenching Stopping a reaction in a sample, for example by rapid cooling or dilution.
  • Tangent A straight line touching a curve at one point, whose gradient gives the rate at that point.
  • Clock reaction A reaction in which a sudden change shows when a fixed amount of product has formed.

Common Mistakes

  • Drawing a line joining two points on the curve instead of a tangent at the point.
  • Stating a rate without units; rates have units such as mol dm⁻³ s⁻¹.
  • Forgetting that the mass-loss method fails for gases with very low density, such as hydrogen.
  • Not quenching titration samples, so the reaction continues while titrating.

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