The extent of reaction of a weak acid or weak base with water depends on
- the acid or base strength
When comparing solutions of the same concentration prepared by dissolving a series of
- weak acids, the extent of reaction is highest in the solution containing the strongest acid (largest Ka or lowest pKa), and the pH of that solution is lowest.
- weak bases, the extent of reaction is highest in the solution containing the strongest base. The strongest base is the base of the conjugate pair having the highest pKa, and the pH of that solution is highest.
- the concentration
The extent of reaction is higher in more dilute solutions.
This is because the power of concentrations in the numerator of the expression (squared) is higher than in the denominator.
Diluting the solution reduces all concentrations by the same amount, BUT because of the squared concentration in the numerator, the value of the expression becomes smaller than Ka; therefore reaction occurs to produce more H3O+ and more A– until equilibrium is restored.
- whether one or both members of the pair are major species
If both members of the pair are major species, their extent of reaction of each is much smaller than if only one member of the pair is a major species. This is because for both solutes, one product of their reaction with water is already a major species.