Identify proton donors, acceptors and conjugate acid–base pairs.
Method and worked examples
A Brønsted–Lowry acid donates a proton, H+, and a base accepts one. When an acid loses H+ it forms its conjugate base; when a base gains H+ it forms its conjugate acid. Conjugate partners differ by one proton and one unit of charge. In HCl+H2O→H3O++Cl−, HCl is the acid and water acts as the base.
Water can act as either an acid or a base depending on its reaction partner. Monoprotic acids can donate one proton per molecule; diprotic and triprotic acids can donate two or three in successive steps. Count ionisable protons, not every hydrogen atom: ethanoic acid CH3COOH is monoprotic. Indicators have acidic and basic forms with different colours, so a supplied colour table can identify a pH interval.
Example 1
Identify conjugate pairs in NH3+H2O ⇌ NH4++OH−.
NH4+NH3 differ by H+, as do OH−H2O. NH3 accepts the proton and is the base; water donates it and is the acid.
Example 2
Classify H3PO4, using the structure O=P(OH)₃.
Three O–H groups can donate protons successively, so phosphoric acid is triprotic.
Watch out:Loss of H+ makes the charge one unit more negative.
Proton transfer and conjugate pairs | Year 11 Chemistry | Ascent Arcade