What is the Difference Between pKa and pKb?

The difference between pKa and pKb lies in the type of dissociation they represent. pKa is the negative logarithm of the acid dissociation constant (Ka), while pKb is the negative logarithm of the base dissociation constant (Kb). Here are the key differences:

• pKa: This value is related to the acid dissociation constant. The lower the pKa value, the stronger the acid. For example, nitrous acid (HNO2) with a pKa of 3.25 is about 1000 times stronger than hydrocyanic acid (HCN) with a pKa of 9.21.
• pKb: This value is related to the base dissociation constant. The lower the pKb value, the weaker the base. Smaller values of pKb correspond to larger base ionization constants and hence stronger bases.

In summary:

• pKa is associated with acid dissociation and indicates the strength of an acid.
• pKb is associated with base dissociation and indicates the strength of a base.

For any conjugate acid-base pair, the relationship between pKa and pKb is given by the equation: $$pKa + pKb = pK_w$$ where pKw is a constant equal to 14.00 at 25°C.

Comparative Table: pKa vs pKb

Here is a table comparing the key differences between pKa and pKb:

Both pKa and pKb are used to determine and measure the strength of acids and bases. They are calculated using negative logarithms, which makes it easier to compare and work with these values.