4.6 Carboxylic AcidsEdexcel IGCSE Chemistry: Revision notes

Carboxylic acids are a homologous series containing the functional group -COOH. This group combines a carbonyl (C=O) and a hydroxyl (-OH) on the same carbon atom, which together make the -H acidic enough to ionise slightly in water, giving these compounds their acidic properties.

Vinegar is an aqueous solution containing ethanoic acid — the everyday example of a carboxylic acid.

The -COOH group is always at the end of the carbon chain (carbon 1). The unbranched-chain carboxylic acids up to four carbons are:

In the displayed formula, the -COOH group is drawn showing the C=O double bond and the separate -O-H single bond.

Like other acids, aqueous carboxylic acids react with reactive metals and with metal carbonates.

With a reactive metal (e.g. magnesium): carboxylic acid + metal → salt + hydrogen. Bubbles of hydrogen gas are seen, and the metal gradually dissolves.

ethanoic acid + magnesium → magnesium ethanoate + hydrogen

With a metal carbonate: carboxylic acid + metal carbonate → salt + water + carbon dioxide. Effervescence (fizzing) is seen as carbon dioxide gas is released.

ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide

The salts formed from carboxylic acids are named by replacing '-oic acid' with '-oate' (e.g. ethanoic acid gives ethanoate salts).

Carboxylic acids are described as weak acids: they only partially ionise in water, so a carboxylic acid solution of the same concentration as hydrochloric acid has a higher pH (is less acidic) and reacts more slowly, even though it undergoes the same types of reaction (with metals, carbonates, and bases).