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5.2.2Additional lessons

Measuring Rates of Reaction — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 97 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Choosing a Method

The best method depends on which property of the mixture changes measurably as the reaction proceeds.

Gas volume
  • Gas volume Collect gas in a gas syringe or inverted measuring cylinder, and record volume at regular time intervals.
Mass loss
  • Mass loss Place the reaction flask on a balance and record the mass falling as a gas escapes.
Colour change
  • Colour change A colorimeter measures absorbance of a coloured species, which is proportional to its concentration.
Conductivity or pH
  • Conductivity or pH Use a probe if the reaction changes the number or type of ions or the acidity of the solution.

Section 2 of 5Collecting Reliable Data

1
Start the timer at the moment the final reactant is added and mix the solutions immediately.
2
Record the property at regular time intervals, with more frequent readings at the start when change is fastest.
3
Control other variables such as temperature, volume and concentration unless they are being varied.
4
Repeat the experiment and calculate a mean to improve reliability.
5
Plot the data and draw a smooth curve, then use tangents to find the rate at selected times.

Section 3 of 5Alternative Approaches

Alternative Approaches
Initial rate method
  • Initial rate method Measure the time for a fixed small change and compare rates at several starting concentrations.
Clock reaction
  • Clock reaction Time how long a sudden colour change takes to appear; a shorter time indicates a faster rate.
Sampling
  • Sampling Remove samples at intervals and quench the reaction before titrating to find the concentration.
Limitations
  • Limitations Gas can dissolve or leak, and mass loss is hard to detect for light gases like hydrogen.

Section 4 of 5Worked Example

Question

  • A gas syringe collects 24.0 cm³ of carbon dioxide in 40.0 s.
  • Calculate the average rate of gas production and state one improvement to the method.

  1. Average rate = volume ÷ time = 24.0 ÷ 40.0 = 0.600 cm³ s⁻¹.
  2. Improvementtake readings at shorter intervals and repeat the experiment to find a mean.
  3. Another improvementkeep the temperature constant using a water bath.

Answer 0.600 cm³ s⁻¹

Section 5 of 5Exam Checklist & Tips

5.2.2 – Key Points for the Exam

  • Colorimeter An instrument that measures absorbance of light by a coloured solution.
  • Quenching Stopping a reaction suddenly so that a sample can be analysed.
  • Clock reaction A reaction in which a sudden visible change marks a set amount of reaction.
  • Reliability How closely repeated measurements agree with each other.

Common Mistakes

  • Starting the timer before all reactants have been mixed.
  • Using too few readings in the early stage when the rate changes quickly.
  • Using a mass-loss method for a gas of very low molar mass.
  • Failing to keep temperature constant when comparing different concentrations.

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