3.2.1Unit 3. Practical Skills in Chemistry I
Hydrolysis of Halogenoalkanes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 7 of 10 Unit quiz
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.
Part 1 of 5Aim and Principle
This core practical links bond enthalpy to reactivity.
Aim
- Aim Compare how quickly different halogenoalkanes are hydrolysed by water, using the appearance of a silver halide precipitate as the signal.
Hydrolysis
- Hydrolysis Water acts as a weak nucleophile and replaces the halogen with –OH, releasing a halide ion into solution.
Detection
- Detection Silver ions in the mixture react with each halide ion as it forms, giving an insoluble silver halide.
Why water, not alkali
- Why water, not alkali Hydroxide ions would react far faster and hide differences, while water slows the reaction enough to time it.
Part 2 of 5Method
Reaction conditions must be identical for every halogenoalkane.
1
Add 1 cm³ of ethanol and 1 cm³ of acidified silver nitrate solution to each of three clean test tubes.
2
Stand the three tubes in a water bath at 50 °C and allow them to reach the same temperature.
3
Add the same number of drops (for example three) of 1-chlorobutane, 1-bromobutane and 1-iodobutane to separate tubes, starting the stopwatch.
4
Observe each tube and record the time at which a precipitate first becomes visible.
5
Repeat the experiment to check reliability and calculate a mean time for each halogenoalkane.
Part 3 of 5Results and Interpretation
Results and Interpretation
Iodoalkane
- Iodoalkane A yellow precipitate of AgI appears fastest, showing that the C–I bond is the weakest and breaks most readily.
Bromoalkane
- Bromoalkane A cream precipitate of AgBr appears after a longer delay than for the iodoalkane.
Chloroalkane
- Chloroalkane A white precipitate of AgCl appears last and slowest because the C–Cl bond has the highest enthalpy of the three.
Conclusion
- Conclusion The rate of hydrolysis depends on bond strength: RI > RBr > RCl, and polarity does not control the order.
| Halogenoalkane | Precipitate | Equation |
|---|---|---|
| 1-chlorobutane | White AgCl | Ag⁺(aq) + Cl⁻(aq) → AgCl(s) |
| 1-bromobutane | Cream AgBr | Ag⁺(aq) + Br⁻(aq) → AgBr(s) |
| 1-iodobutane | Yellow AgI | Ag⁺(aq) + I⁻(aq) → AgI(s) |
Part 4 of 5Safety and Evaluation
Hazards
- Hazards Halogenoalkanes are harmful and volatile, ethanol is flammable and silver nitrate stains skin black, so wear eye protection and gloves.
No naked flames
- No naked flames Use a water bath rather than a Bunsen burner because ethanol vapour is flammable.
Precision
- Precision Deciding when the precipitate first appears is subjective, so a mean of repeated readings or a light sensor improves reliability.
Control variables
- Control variables Equal volumes, equal drops and equal temperature make sure that only the halogen differs between tubes.
Part 5 of 5Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Hydrolysis A reaction with water in which a bond is broken and the halogen is replaced by –OH.
- Nucleophilic substitution A reaction in which an electron-pair donor replaces an atom or group on a carbon atom.
- Bond enthalpy The energy required to break one mole of a given bond in the gas phase.
- Control variable A condition kept constant so that it does not affect the outcome being compared.
Common Mistakes
- Using different volumes or numbers of drops of each halogenoalkane, so the test is no longer fair.
- Claiming that the precipitate is the halogenoalkane itself rather than a silver halide formed from released halide ions.
- Using a Bunsen burner flame beneath tubes containing flammable ethanol.
- Saying the order of reactivity follows polarity, when it follows bond strength.
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