Natural fats and oils are esters formed from long-chain carboxylic acids and glycerol. (i) Name the type of reaction used to convert a fat or oil into soap, and state the reagent required. (ii) Identify the other organic product formed alongside soap in this reaction. (iii) The reverse reaction — forming an ester from an acid and an alcohol — is called esterification. State the reagent and condition used and explain why this reaction is described as reversible.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
(i) The reaction is called saponification. The reagent required is sodium hydroxide (NaOH) — a strong alkali (or potassium hydroxide, KOH).
(ii) The other organic product formed alongside soap is glycerol (propane-1,2,3-triol).
(iii) Reagent: concentrated sulphuric acid (H₂SO₄) as catalyst; Condition: heating. The reaction is reversible because the ester and water formed can react with each other to regenerate the original acid and alcohol, so neither reaction goes to completion — both forward and backward reactions occur simultaneously.
Source: Chapter 4, Section 4.5 Soaps and Detergents
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Explanation
- Saponification is the key term examiners expect for (i); simply writing "hydrolysis" may lose the mark.
- For (ii), glycerol is the specific organic by-product — do not write water (it is inorganic).
- For (iii), the two marks cover: naming the reagent/condition and explaining reversibility. The reversibility point must mention that the backward reaction (hydrolysis of ester) also occurs, making it an equilibrium — this is what earns the mark.