Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.8.4Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Hydrolysis of Halogenoalkanes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 49 of 53 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5The Silver Nitrate Method

Halide ions released by hydrolysis are detected as a precipitate.

1
Place equal volumes of ethanol and acidified aqueous silver nitrate in three test tubes, held in a water bath at 50 °C.
2
Add the same small number of drops of a chloroalkane, a bromoalkane and an iodoalkane to each tube, then start a stopwatch.
3
The halogenoalkane is hydrolysed by the water present, releasing halide ions: RX + H₂O → ROH + H⁺ + X⁻.
4
Silver ions combine with the halide ions as soon as they form, giving a precipitate: Ag⁺(aq) + X⁻(aq) → AgX(s).
5
Record the time taken for each precipitate to appear; a shorter time means a faster rate of hydrolysis.

Part 2 of 5Observations and Explanation

Observations and Explanation
Iodoalkane
  • Iodoalkane A yellow precipitate of silver iodide forms first, because the C–I bond is the weakest and breaks most easily.
Bromoalkane
  • Bromoalkane A cream precipitate of silver bromide forms at an intermediate speed, with a C–Br bond of intermediate strength.
Chloroalkane
  • Chloroalkane A white precipitate of silver chloride forms most slowly of the three, because the C–Cl bond is stronger.
Fluoroalkane
  • Fluoroalkane No precipitate forms because the C–F bond is so strong, and silver fluoride is also soluble in water.
Halogenoalkane typePrecipitate colourRelative rate
IodoalkaneYellowFastest
BromoalkaneCreamIntermediate
ChloroalkaneWhiteSlowest
FluoroalkaneNoneNo visible reaction

Part 3 of 5Primary, Secondary and Tertiary

The same test can compare the carbon skeleton if the halogen is kept constant.

Fair test
  • Fair test Use isomers with the same halogen, for example 1-chlorobutane, 2-chlorobutane and 2-chloro-2-methylpropane.
Tertiary
  • Tertiary The tertiary halogenoalkane gives its precipitate fastest, so it hydrolyses most quickly.
Primary
  • Primary The primary halogenoalkane gives its precipitate slowest of the three isomers.
Secondary
  • Secondary The secondary halogenoalkane falls between the primary and tertiary isomers in rate.

Part 4 of 5Worked Example

Question

  • Hydrolysis of 1-bromobutane releases bromide ions that precipitate with silver ions.
  • Calculate the mass of silver bromide formed when 0.0200 mol of 1-bromobutane is completely hydrolysed (Ar: Ag = 107.9, Br = 79.9).

  1. Write the overall changeC₄H₉Br + H₂O → C₄H₉OH + H⁺ + Br⁻, then Ag⁺ + Br⁻ → AgBr.
  2. One mole of halogenoalkane releases one mole of Br⁻, so n(AgBr) = 0.0200 mol.
  3. Mr of AgBr = 107.9 + 79.9 = 187.8.
  4. Mass = 0.0200 × 187.8 = 3.756 g.

Answer Mass of AgBr = 3.76 g (3 significant figures)

Part 5 of 5Exam Checklist & Tips

2.8.4 – Key Points for the Exam

  • Hydrolysis
    • Breaking a bond with water or hydroxide ions;
      • for halogenoalkanes it replaces X with OH.
  • Precipitate An insoluble solid formed when two solutions react.
  • Bond enthalpy The energy needed to break one mole of a covalent bond in gaseous molecules.
  • Isomers Compounds with the same molecular formula but different arrangements of atoms.

Common Mistakes

  • Explaining the trend by polarity;
    • the rate order is controlled by the bond strength, not by how polar the bond is.
  • Saying that the precipitate forms from the halogenoalkane itself;
    • it forms from the halide ions released by hydrolysis.
  • Changing the halogen when comparing primary, secondary and tertiary isomers, so the comparison is no longer fair.
  • Mixing up precipitate coloursAgCl is white, AgBr is cream and AgI is yellow.

Last updated:

\n