5.2.2Additional lessons
Measuring Rates of Reaction — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 67 of 98
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.
Section 1 of 6Choosing a Property
The best method depends on the reaction being studied.
Principle
- Principle Any property that changes steadily as reactants are converted into products can be monitored over time.
Gas evolved
- Gas evolved Collect the gas in a syringe or over water and record the volume at regular time intervals.
Mass loss
- Mass loss Where a gas escapes, a balance shows the mass falling, though light gases give small changes that are hard to measure.
Colour change
- Colour change A colorimeter measures absorbance of a coloured reactant or product, which is linked to its concentration.
Section 2 of 6Methods
Methods
Gas syringe
- Gas syringe Measures the volume of gas given off, as in the reaction of magnesium or calcium carbonate with acid.
Balance
- Balance Measures decreasing mass of the flask and contents when a gas leaves, such as CO₂ from carbonates and acid.
Colorimetry
- Colorimetry Measures how much light of a chosen wavelength is absorbed by a coloured solution as its concentration changes.
Conductivity and pH
- Conductivity and pH Track changes in ion concentration or acidity during reactions that produce or use ions or H⁺.
| Reaction | Method |
|---|---|
| Metal + acid | Gas syringe |
| Carbonate + acid | Mass loss or gas volume |
| Iodine formed or used | Colorimetry |
| Ionic reaction | Conductivity |
Section 3 of 6Collecting and Using Data
1
Set up the apparatus, and start timing at the moment the reactants are mixed.
2
Record the measurement at fixed intervals, such as every 10 s, until the readings stop changing.
3
Plot the property against time and draw a smooth curve.
4
Calculate the initial rate from the gradient of the tangent at time zero and compare it with rates under other conditions.
5
Repeat the experiment and calculate a mean to improve reliability.
Section 4 of 6Practical Considerations
Practical Considerations
Fair test
- Fair test Change only one variable at a time and keep the temperature, volumes and concentrations of other factors the same.
Initial rates
- Initial rates Comparing initial rates avoids the effects of changing concentration during the reaction.
Disappearing cross
- Disappearing cross Timing how long a cross takes to vanish gives a simple measure, though it is subjective, and rate is taken as 1 ÷ time.
Sources of error
- Sources of error Delay in starting the timer, gas escaping before the syringe is fitted and poor mixing all affect the data.
Section 5 of 6Worked Example
Question
- In an experiment 24.0 cm³ of gas is collected in 40.0 s at a roughly constant rate.
- Calculate the average rate in cm³ s⁻¹, and the rate if the volume after 80.0 s is 36.0 cm³ in total.
- Rate over first 40.0 s = 24.0 ÷ 40.0 = 0.600 cm³ s⁻¹.
- Rate over 80.0 s = 36.0 ÷ 80.0 = 0.450 cm³ s⁻¹.
- The average rate is lower over the longer period because the reaction slows as reactants are used.
Answer 0.600 cm³ s⁻¹ over 40 s and 0.450 cm³ s⁻¹ over 80 s
Section 6 of 6Exam Checklist & Tips
5.2.2 – Key Points for the Exam
- Colorimeter An instrument measuring the absorbance of coloured solutions.
- Gas syringe A syringe used to measure the volume of gas produced.
- Tangent A line drawn touching a curve at one point, whose gradient gives the instantaneous rate.
- Reliable data Results that are consistent when the experiment is repeated.
Common Mistakes
- Starting the timer after mixing so that the first readings are missed.
- Using the same volume and concentration for all runs and changing nothing, so the test shows no trend.
- Choosing a method unsuitable for the reaction, such as mass loss for a reaction producing no gas.
- Reading the rate from a graph without drawing a tangent.
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