Consider HA- the intermediate form of a diprotic acid. Ka for this species is 10-4 and Kb is 10-8. Nonetheless, the Ka and Kb reactions proceed to nearly the same extent when NaHA is dissolved in water. Can anyone explain that?
To understand why the reactions of HA's Ka and Kb proceed to nearly the same extent when NaHA is dissolved in water, we need to examine the nature of diprotic acids and their conjugate bases.
First, let's define the terms Ka and Kb:
- Ka is the acid dissociation constant, which measures the degree of dissociation of an acid in water. It indicates the strength of an acid.
- Kb is the base dissociation constant, which measures the degree of dissociation of a base in water. It indicates the strength of a base.
Now, HA is a diprotic acid, meaning it can donate two protons (H+) in two separate steps. It can be represented as H2A, where the first proton can be donated to form HA- (the conjugate base). The second proton can be donated to form A2- (the second conjugate base).
Given that the Ka of HA is 10^-4 and the Kb is 10^-8, we can conclude that HA is a weaker acid (because its Ka is smaller) and that A2- is a weaker base (because its Kb is smaller).
When NaHA is dissolved in water, it dissociates into Na+ and HA-. Since HA- is a weak acid, it doesn't dissociate significantly. Therefore, the reaction proceeds to only a limited extent.
However, when HA- reacts with water, it can accept a proton (H+) from water since it is a weak base (due to its conjugate acid H2A being a weak acid). This forms H2A and OH- ions. This reaction occurs to a significant extent because HA- is a weak base, and water (H2O) acts as the stronger acid.
The reactions of HA's Ka and Kb proceed to nearly the same extent because Ka measures the level of dissociation of HA as an acid, while Kb measures the level of reaction of the conjugate base HA- as a base. Despite the large difference between the magnitude of Ka and Kb, the extent of dissociation for HA and the extent of reaction for HA- are similar.
In summary, when NaHA is dissolved in water, the weak acid HA dissociates to a limited extent, but its conjugate base HA- reacts with water to a significant extent due to its weaker acidity. This leads to the nearly equal extent of reaction observed for both Ka and Kb.