9.30C - The carboxyl group and acidity
Connect the carboxyl group to hydrogen ions in water, then use familiar acid reaction patterns to predict what carboxylic acids do.
The carboxyl group and hydrogen ions
Carboxylic acids contain the carboxyl group, –COOH. The whole group matters: it contains a carbon double bonded to one oxygen and single bonded to an O–H group. Ethanoic acid is CH3COOH. Its O–H belongs to the carboxyl group; it does not make ethanoic acid an alcohol.
In water, the hydrogen on this oxygen can leave as a hydrogen ion:
CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq)
Hydrogen ions give the solution acidic properties. CH3COO⁻ is the ethanoate ion. More generally, RCOOH ⇌ H⁺ + RCOO⁻, where R represents the rest of the molecule.
Why a weak acid is still acidic
These carboxylic acids partially dissociate in water: some molecules form ions while many remain as molecules. This makes them weak acids. The reversible arrow shows that the process can occur in both directions.
Weak and dilute mean different things. Weak describes incomplete dissociation; dilute means little acid per unit volume of solution. A weak acid still supplies H⁺ and has a pH below 7. At the same acid concentration, it has a lower hydrogen-ion concentration than a strong acid.
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