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3.2Physical Chemistry

Rates of Reaction — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

10 min read 13 parts Lesson 5 of 9 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Rates of Reaction: Measuring Rate and Effect of Concentration, Surface Area and Temperature — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Plan and carry out experiments to measure rate and explain the effect of concentration, surface area and temperature using collision theory.The rat…

  • rate of a reaction
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Rate of reaction

Part 1 of 6Defining and measuring rate

The rate of a reaction tells us how fast reactants are used up or products are formed. Rate = change in amount ÷ time. Reactions range from very slow (rusting, weathering of rocks) to very fast (explosions, precipitation of silver chloride). To measure a rate, choose a property that changes steadily during the reaction.

TechniqueMeasuredExampleLimitation
Gas collection (syringe or inverted cylinder over water)Volume of gas at timed intervalsMagnesium + hydrochloric acidGas must not be very soluble
Mass lossDecrease in mass as gas escapesMarble chips + acidLight gases such as hydrogen give small changes
Disappearing crossTime for cross to vanishSodium thiosulfate + acidSubjective endpoint; only for precipitates
Colour change / colorimeterAbsorbance of coloured solutionIodine reactionsNeeds a coloured species

Part 2 of 6Method: concentration and surface area

1
Measure 50 cm3 of sodium thiosulfate solution into a conical flask standing on a black cross on paper.
  • Use different concentrations made by diluting with water (for example 50, 40, 30, 20 and 10 cm3 of thiosulfate topped up to 50 cm3).
2
Add 10 cm3 of dilute hydrochloric acid, start the stopwatch and swirl.
3
Stop the clock when the cross can no longer be seen through the sulfur precipitate.
4
Record the time, and calculate rate as 1 ÷ time (s-1). Repeat the experiment and use a mean.
5
Plot rate against concentration; a straight line through the origin shows rate is proportional to concentration.

For surface area, react a fixed mass of calcium carbonate as large chips, small chips and powder with the same volume and concentration of acid, and measure gas volume every 10 s. Smaller pieces give a steeper curve because more particles are exposed at once. Equal masses are used so that the final volume of gas is the same.

Part 3 of 6Explaining the effects with collision theory

Particles can only react if they collide with at least the activation energy.

Explaining the effects with collision theory
Concentration or pressure: more particles per unit volume, so collisions are more frequent and the rate increases.
Surface area: more exposed particles, so more frequent collisions.
Temperature: particles gain kinetic energy, move faster, collide more often and a greater proportion of collisions have the activation energy,
  • so the rate rises steeply.

Temperature experimentwarm the thiosulfate solution to 20, 30, 40, 50 and 60 °C in a water bath, add acid and time the cross.

Explaining the effects with collision theory
A graph of 1 ÷ time against temperature curves upward: the rate roughly doubles for each 10 °C rise.
Control the volume, concentration and the same cross-and-flask setup, and note that the fastest reaction has the largest rate and the shortest time.

Part 4 of 6Fair testing and safety

Variables

  • the independent variable is the one you change;
    • the dependent variable is what you measure;
    • control variables are kept the same.

Fair testing and safety
Safety: thiosulfate with acid releases sulfur dioxide, a toxic gas,
  • so work in a well-ventilated area, use small volumes, and dispose of waste promptly.
Wear eye protection and avoid having flames when collecting hydrogen.
Industrial relevance: coal dust and flour dust explosions arise from very large surface areas,
  • and food is stored in refrigerators to slow the rate of reactions caused by bacteria.

Part 5 of 6Evaluating the experiments

Evaluation questions ask how reliable and valid the results are.

1
Reliability improves by repeating each run and ignoring anomalies when calculating the mean.
2
Validity improves by changing only one variable.
3
In the disappearing cross method, different observers may judge the end point differently,
  • so the same person should do all runs, in the same lighting, looking from above.
4
In gas collection experiments, gas may leak around a bung or dissolve in water; a gas syringe is more accurate than a measuring cylinder over water.
5
When temperature is the variable, a thermostatically controlled water bath keeps the temperature constant during the run; the reaction itself may warm the solution slightly,
  • so the starting temperature of each solution should be measured immediately before mixing.

Plotting data

  • place the independent variable on the x-axis, label axes with units, choose a scale that uses more than half the grid, plot points accurately and draw a best-fit straight line or smooth curve.
  • State the pattern in words: "as concentration doubles, rate doubles", or "as temperature increases, rate increases at an increasing gradient".

Part 6 of 6Worked examples

  1. A student times the thiosulfate cross experiment at 20 °C (80 s) and 40 °C (21 s). Calculate rate (1/time) for each and state the effect. [3]3 marks

  2. Explain why powdered zinc reacts faster with acid than zinc granules. [2]2 marks

  3. State three variables to control in a concentration experiment. [3]3 marks

  4. Describe one advantage of collecting gas in a syringe instead of measuring mass loss. [1]1 mark

Key terms

TermMeaning
Rate of reactionHow quickly reactants are used or products formed
Collision frequencyNumber of collisions per second
Surface areaExposed area of a solid
ConcentrationAmount of solute per unit volume
Control variableA factor kept constant in an experiment
PrecipitateInsoluble solid formed in a solution

Avoid these mistakes

1
Saying a higher concentration means particles move faster — it means more particles in the same volume.
2
Forgetting to keep the total volume constant when diluting.
3
Reading the time as the rate (a longer time is a slower rate).
4
Describing the cross experiment for reactions that do not form a precipitate.

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