A carboxyl group is –COOH. Name its carbon as part of the parent chain and use oic acid; compounds with two terminal groups use dioic acid. Carboxylic acids can form by oxidation of primary alcohols or aldehydes. In water they are weak acids: RCOOH +H2O ⇌ RCOO−+H3O+. Weak means partial ionisation, not necessarily low concentration.
Hydroxide neutralises a carboxylic acid to a carboxylate and water. Carbonates and hydrogencarbonates also produce CO2. The ionic sodium or potassium salt often dissolves more readily than its parent acid, particularly for longer hydrocarbon chains, because ions interact strongly with water. Match the number of acidic groups to the base amount: a molecule with two carboxyl groups can consume two moles of hydroxide per mole.
Example 1
Write the reaction of ethanoic acid with NaOH.
CH3COOH + NaOH → CH3COONa +H2O. The net ionic form is CH3COOH +OH−→ CH3COO−+H2O.
Example 2
Find hydroxide needed for 0.0200 mol HOOCCH2COOH, assuming full neutralisation.
There are two carboxyl groups per molecule. Required OH−=2×0.0200=0.0400 mol.
Watch out:Weak describes a small degree of ionisation; concentration describes amount per volume. A weak acid can be concentrated.
Carboxylic acids, salts and reactions practice | Year 12 Chemistry | Ascent Arcade