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Chemistry · Glossary

What is Oxonium ion, ampholyte?

Also known as: oxonium ion · ampholyte

Definition 44.2 School Chemistry — Grades 1 to 12 · Chapter 44 — Acids and Bases: Ka and pKa

In water a hydrogen ion never stays alone: it is taken by a water molecule, giving the oxonium ion HX3OX+\ce{H3O+}. A species that is the acid of one couple and the base of another is an ampholyte; water is one, acid of the couple HX2O\ce{H2O}/OHX−\ce{OH-} and base of the couple HX3OX+\ce{H3O+}/HX2O\ce{H2O}.

A hydrogen ion passes from the acid to water: a lone pair of water forms the new O-H bond (orange), and the old O-H pair stays on the oxygen of the ethanoate ion (blue).
A hydrogen ion passes from the acid to water: a lone pair of water forms the new O−H\ce{O-H} bond (orange), and the old O−H\ce{O-H} pair stays on the oxygen of the ethanoate ion (blue).

Examples

Example 44.3 (An acid in water)

When ethanoic acid dissolves, a lone pair of a water molecule takes the hydrogen of its O−H\ce{O-H}, and the O−H\ce{O-H} pair stays on the acid’s oxygen:

CHX3COOH+HX2O⇌CHX3COOX−+HX3OX+.\ce{CH3COOH + H2O <=> CH3COO- + H3O+} .

Two couples exchange a hydrogen ion: CHX3COOH\ce{CH3COOH}/CHX3COOX−\ce{CH3COO-} gives it, HX3OX+\ce{H3O+}/HX2O\ce{H2O} takes it. From now on, the hydrogen ions of earlier chapters are written HX3OX+\ce{H3O+} whenever water is the solvent.

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