4.1.4Unit 4. Rates, Equilibria and Further Organic Chemistry
Obtaining Rate Data — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 13 of 56 Unit quiz
Part 1 of 6Physical Methods
A property that changes steadily with concentration can be measured to follow a reaction.
Gas volume
- Collect a gas in a gas syringe and record the volume at set times, which works when the reaction produces a gas.
Mass loss
- Weigh the flask on a balance as gas escapes, but this suits only the release of dense gases like carbon dioxide.
Colorimetry
- Measure the absorbance of light by a coloured species, which changes in proportion to its concentration.
Conductivity
- Follow the change in conductivity when the number or type of ions in solution changes.
Part 2 of 6Sampling and Titration
Sampling and Titration
Sampling
- Withdraw small portions of the reaction mixture at known times.
Quenching
- Stop the reaction in the sample immediately by cooling, dilution or adding a reagent that removes a reactant.
Titration
- Titrate the quenched sample to find the concentration of a reactant or product at that time.
Limitation
- The method is slower than a continuous measurement and the process of sampling takes time and so adds uncertainty.
| Method | Suitable when |
|---|---|
| Gas syringe | A gas is produced |
| Colorimeter | A coloured species changes |
| Titration | A sample can be quenched |
| Conductivity | The ions change in number |
Part 3 of 6Planning the Measurement
1
Choose a property that changes measurably and consistently as the reaction proceeds.
2
Prepare the reactants separately at the same temperature, such as in a thermostatic water bath.
3
Mix them and start the timer at the moment of mixing, then record readings at regular intervals.
4
Continue until the reaction is nearly complete or enough points are collected for a graph.
5
Plot concentration, or a property proportional to it, against time.
Part 4 of 6Two Approaches to the Data
Continuous monitoring
- One experiment is followed over time, and the curve gives the rate at different times.
Initial rates
- Many experiments with different starting concentrations are run, and only the first rate is found from each.
Choice
- Use continuous monitoring if the order of one reactant is needed from a single run, and initial rates for several reactants.
Accuracy
- Instrumental methods with data loggers reduce human error in timings.
Part 5 of 6Worked Example
Question
- 48 cm³ of hydrogen is collected in 60 s at room temperature and pressure, where 1 mol of gas occupies 24 000 cm³.
- Find the average rate in mol s⁻¹.
- Amount of hydrogen = 48 ÷ 24 000 = 2.0 × 10⁻³ mol.
- Time = 60 s.
- Average rate = 2.0 × 10⁻³ ÷ 60 = 3.3 × 10⁻⁵ mol s⁻¹.
Answer Average rate = 3.3 × 10⁻⁵ mol s⁻¹
Part 6 of 6Exam Checklist & Tips
4.1.4 – Key Points for the Exam
- Quenching Stopping a reaction in a sample so that its composition can be analysed.
- Colorimeter An instrument that measures the absorbance of coloured solutions.
- Continuous monitoring Following one reaction mixture over time.
- Gas syringe A syringe that collects and measures the volume of gas given off.
- Sampling Taking small portions of a reaction mixture at known times.
Common Mistakes
- Choosing a gas-volume method for a reaction that does not release a gas.
- Forgetting to quench a sample before titrating, so the reaction continues during the analysis.
- Starting the timer before the reactants are fully mixed.
- Using a colorimeter with a filter that the solution does not absorb.
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