Question

What does it mean to say that an acid is weak in aqueous solution? What does this reveal about the ability of the acid's anion to attract protons?

Short answer

A weak acid in an aqueous solution is one that does not completely dissociate into its ions, remaining partially in its molecular form. This establishes an equilibrium reaction, represented as \( HA + H_2O \rightleftharpoons H_3O^+ + A^- \), with a small acid dissociation constant (Ka), indicating the majority of the acid molecules remain undissociated. The weak acid's anion has a low affinity for protons, thus does not readily attract protons, contributing to the acid's low degree of ionization in the solution.

Step-by-step solution

Defining weak acids

A weak acid is an acid that does not completely dissociate into its ions when dissolved in water. In other words, in an aqueous solution, a weak acid only partially ionizes, with a significant amount of the acid remaining in its molecular form.

Behavior of weak acids in aqueous solutions

In an aqueous solution, a weak acid partially ionizes into its constituent ions: protons (H+) and the conjugate base (anion). This process establishes an equilibrium reaction, where the acid (HA) donates a proton to a water molecule (H2O), forming hydronium ions (H3O+) and the conjugate base (A-). The chemical reaction can be represented as follows: \( HA + H_2O \rightleftharpoons H_3O^+ + A^- \) The equilibrium constant of this reaction, known as the acid dissociation constant (Ka), is relatively small, indicating that the equilibrium lies to the left side. This means that only a small fraction of the weak acid molecules dissociate into protons and conjugate base anions, while the majority of the molecules remain undissociated.

Anions and proton attraction

The ability of an acid's anion to attract protons is associated with its basicity, or its affinity for protons. In a weak acid, the conjugate base (anion) has a low affinity for protons, reflecting the weak acid's low degree of ionization. Because the anion does not readily attract protons, the weak acid remains largely as undissociated molecules in the aqueous solution.