Weak Acid Base Equilibrium

Weak bases only partially ionize in aqueous solutions, so they form a system of equilibrium with this general formula right here. Now when we apply the equilibrium expression to this reaction, we’re going to get this expression right here.

Notice here that I took the right side of this reaction and put it in the numerator of this fraction, and I took the left side of this reaction and put it in the denominator of this fraction. Now you may be thinking we left water out, but we didn’t because that’s just excess water, so we don’t need to take it into account in this reaction.

Now we have this equation right here and this equation is to solve for the base dissociation constant, which looks like this with the capital \(K\) and then a lower-case subscript \(b\), so the value of \(K_b\) indicates the strength of the base. High \(K_b\) values indicate strong basis which dissociate readily in water, and low \(K_b\) values indicate weak bases which dissociate only a little in water.

By looking just at the \(K_b\) values, we can determine which base is stronger. Right here we’re taking a look at ammonia and pyridine, so the \(K_b\) value of ammonia is 1 point 8 times 10 to the negative 5th. Then the \(K_b\) value of pyridine is 1 point 7 times 10 to the negative 9. Obviously here, ammonia has a higher \(K_b\) value, so we know that ammonia is a stronger base than pyridine, which means that ammonia is going to dissociate more readily in water.