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AS Level · Chemistry 3 (IAL) WCH13

Inorganic & Organic Chemistry Core Practicals

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Edexcel IAL Chemistry · Core Practicals 5–8

Inorganic & Organic Chemistry Core Practicals

The big idea: Every core practical here is really the same story told four ways — you take a molecule or ion, force a change on it (break a bond, add an oxidiser, swap a functional group), and then use a simple, observable signal (a precipitate colour, a gas turning litmus blue, a flame colour) as your evidence for what actually happened at the molecular level.

Summary — What This Chapter Covers

Core Practical 5 — Hydrolysis of Halogenoalkanes: reacting halogenoalkanes with warm aqueous silver nitrate to measure how fast the C–X bond breaks, based on bond enthalpy.
Core Practical 6 — Chlorination of 2-Methylpropan-2-ol: converting an alcohol into a haloalkane using concentrated HCl, then purifying the product using a separating funnel, drying agent, and distillation.
Core Practical 7 — Oxidation of Propan-1-ol: using acidified potassium dichromate(VI) under reflux (to make the acid) or distillation (to stop at the aldehyde), and understanding why the apparatus choice matters.
Core Practical 8 — AS Qualitative Analysis: a toolkit of test-tube reactions to identify ammonium, carbonate/hydrogencarbonate, and sulfate ions, plus flame tests for metal ions.

Core Practical 5: Hydrolysis of Halogenoalkanes

Why halogenoalkanes hydrolyse

A halogenoalkane has a polar C–X bond (X = F, Cl, Br, or I), because the halogen is more electronegative than carbon. This makes the carbon slightly positive (δ+) — a perfect target for an electron-rich species (a nucleophile) to attack. When that nucleophile is hydroxide, OH⁻, the halogen atom is kicked out and replaced by an –OH group. This is called hydrolysis because water (or its more reactive cousin, hydroxide) is doing the breaking-apart.

CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻
(bromoethane) (ethanol) (bromide ion)

Notice this is very slow at room temperature — that's why the reaction mixture always needs warming. Warming gives the molecules enough kinetic energy to collide successfully and overcome the activation energy of breaking the C–X bond.

Why hydroxide beats water as a nucleophile Water's oxygen only carries a negative charge (δ−) because it's still bonded to two hydrogens pulling electron density away. Hydroxide's oxygen carries a formal negative charge, because one of those O–H bonds has been replaced by a lone pair. A fuller negative charge means a stronger pull towards the δ+ carbon, so OH⁻ attacks faster than H₂O — that's why we use aqueous NaOH or KOH rather than just water to hydrolyse halogenoalkanes at a reasonable rate.

Bond enthalpy controls the rate

This is the part students most often get backwards, so let's slow down. The rate of hydrolysis depends on how strong the C–X bond is — not on how electronegative the halogen is, and not on the size of the halogen atom directly (though those two things are related to bond strength).

The Rule Bond enthalpy order: C–F > C–Cl > C–Br > C–I
Rate of hydrolysis order: C–I > C–Br > C–Cl (C–F essentially doesn't react)

Think of it like this:

Common mistake bond enthalpy

The experiment: measuring the rate

Practice Question 1

Explain, in terms of bond enthalpy, why 1-chlorobutane hydrolyses more slowly than 1-iodobutane. Your answer should reference the strength of the relevant bonds.

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