Alcohols and Carboxylic Acids — Part 2: Ethanoic (acetic) acid: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

The sour taste of vinegar

Part 2 — Vinegar is a dilute solution (about 5–8 per cent) of ethanoic acid, also called acetic acid, CH₃COOH. It is the best-known carboxylic acid. Carboxylic acids contain the carboxyl group, –COOH.

In this lesson you will

Draw the structure of acetic acid and identify the carboxyl group
Describe the physical properties of acetic acid and explain what glacial acetic acid is
Write equations for acetic acid with sodium hydroxide and with ethanol, and describe its effect on litmus
Use the fruity smell of the ester as evidence of esterification and list uses of acetic acid

Table of three carboxylic acids with formulae and where they are found
Carboxylic acid	Formula	Found in
Methanoic acid (formic acid)	HCOOH	Ant and bee stings
Ethanoic acid (acetic acid)	CH₃COOH	Vinegar
Propanoic acid	CH₃CH₂COOH	Used to preserve bread

Concept

Structure: the carboxyl group

In ethanoic acid a methyl group is joined to a carboxyl group, –COOH. The carboxyl carbon is joined to one oxygen by a double bond (C=O) and to an –OH group by a single bond. Molecular formula C₂H₄O₂; molecular mass 60. The hydrogen of the –OH can be released as H⁺, which makes the compound an acid.

Concept

Physical properties of ethanoic acid

A colourless liquid with a sharp, pungent smell of vinegar and a sour taste.
Boiling point 118 °C; melting point about 17 °C (16.6 °C).
Miscible with water in all proportions; density a little more than water (1.05 g cm⁻³).
Pure ethanoic acid is corrosive and blisters the skin.

Table comparing the smell, boiling point, melting point and effect on litmus of ethanol and ethanoic acid
Property	Ethanol	Ethanoic acid
Smell	Pleasant, spirit-like	Pungent, of vinegar
Boiling point	78 °C	118 °C
Melting point	about −114 °C	about 17 °C
Effect on blue litmus	No change	Turns red

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