Why do we use a double arrow (
) to show the dissociation of a weak acid or weak base in aqueous solution?
The Correct Answer and Explanation is:
We use a double arrow (⇌) to show the dissociation of a weak acid or weak base in aqueous solution because these substances do not fully dissociate into ions, unlike strong acids or bases. A single arrow (→) would suggest that the dissociation goes to completion, which is not the case with weak acids and bases. Instead, the double arrow indicates that the dissociation is reversible, meaning that the system can reach an equilibrium where both the dissociated ions and the undissociated molecules coexist in solution.
In the case of weak acids, such as acetic acid (CH₃COOH), only a small fraction of the molecules ionize to produce hydrogen ions (H⁺) and acetate ions (CH₃COO⁻). For example:
CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO⁻ (aq)
This means that at equilibrium, some of the acetic acid molecules will remain in their undissociated form (CH₃COOH), and some will be dissociated into ions (H⁺ and CH₃COO⁻). The concentrations of these species do not change unless the conditions of the solution, such as temperature or the concentration of reactants, are altered.
The same principle applies to weak bases. For example, ammonia (NH₃) partially dissociates in water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻):
NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
Because weak acids and bases only partially dissociate, their dissociation reactions do not go to completion. This partial dissociation leads to an equilibrium that can be quantified by the acid dissociation constant (Ka) for acids or the base dissociation constant (Kb) for bases. The use of the double arrow is a shorthand way to indicate that both the dissociation and recombination processes are occurring simultaneously and in balance.
