Properties of Ethanoic Acid

Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles. The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gave rise to its name glacial acetic acid.

The group of organic compounds called carboxylic acids are obviously characterised by a special acidity. However, unlike mineral acids like HCl, which are completely ionised, carboxylic acids are weak acids.

Activity 4.7

1. Compare the pH of dilute acetic acid and dilute hydrochloric acid using both litmus paper and universal indicator.
2. Are both acids indicated by the litmus test?
3. Does the universal indicator show them as equally strong acids?

Activity 4.8

1. Take 1 mL ethanol (absolute alcohol) and 1 mL glacial acetic acid along with a few drops of concentrated sulphuric acid in a test tube.
2. Warm in a water-bath for at least five minutes as shown in Fig. 4.11.
3. Pour into a beaker containing 20-50 mL of water and smell the resulting mixture.

Reactions of ethanoic acid:

(i) Esterification reaction: Esters are most commonly formed by reaction of an acid and an alcohol. Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester –

Esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents. Esters react in the presence of an acid or a base to give back the alcohol and carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.

ii) Reaction with a base: Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:

NaOH + CH₃COOH → CH₃COONa + H₂O

How does ethanoic acid react with carbonates and hydrogencarbonates?

Let us perform an activity to find out.

Activity 4.9

1. Set up the apparatus as shown in Chapter 2, Activity 2.5.
2. Take a spatula full of sodium carbonate in a test tube and add 2 mL of dilute ethanoic acid.
3. What do you observe?
4. Pass the gas produced through freshly prepared lime-water. What do you observe?
5. Can the gas produced by the reaction between ethanoic acid and sodium carbonate be identified by this test?
6. Repeat this Activity with sodium hydrogencarbonate instead of sodium carbonate.

iii) Reaction with carbonates and hydrogencarbonates: Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to a salt, carbon dioxide and water. The salt produced is commonly called

sodium acetate.

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂