Ionic equilibrium I - Application of Dissociation Constants.

In this entry, we will be discussing about dissociation constants. But before that, let's further justify the importance of these constants in determining the strength of the acid (or base).

Importance of dissociation constants

HA + H₂O <-> H₃O⁺ + A^-

where, < - > refers to the reversible arrow, thus showing that HA is a weak acid.

With the above equilibrium, and the application of Le Chatelier's principle, you can observe that by diluting HA (creating a diluted HA), we have the equilibrium position to shift to the right and resulting in HA to be fully dissociated. Thus, percentage dissociation seems to be in 100%.

In addition, if the concentration of acid (or base) is increased, by Le Chatelier's principle the equilibrium position will shift to the left. So if we measure percentage dissociation of the acid, it appears to have decrease.

Thus from the above paragraph, it appears that when the acid is diluted it becomes stronger (since it fully dissociates) and when it is concentrated, it becomes weaker.

Isn't that an anomaly? Isn't that strange that for the same acid, it's strength differs?

Thus pH and concentrations are poor means to gauge acid strength and therefore, K_a is the best (and only) gauge of deciding the acid strength as its value remains constants despite changing concentration.

Application of Dissociation Constants

By understanding how an equilibrium work and the property of a dissociation constant, you will be able to appreciate that despite adding HCl (a strong acid) to HA, the K_a remains the same. But, the percentage of dissociation of HA decreases. The decrease can be explained using Le Chatelier's principle.

Lastly, making use of the K_a in caluations will help you determine the pH of buffers, since pH of a solution changes when different (maybe slightly) acids/bases are added. To do such calculation, you need to understand and appreciate how the ICE table is used. (Please refer to tutorial questions for this part).

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Article written by Kwok YL 2007. (edited in Apr 2009)

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