Weak Acid Equilibrium

Weak acids only partially ionize in aqueous solutions. They form an equilibrium system with this general formula. Now, when we apply the equilibrium expression to this reaction, we get the acid dissociation constant, so the equilibrium expression is going to look like this.

This is the equilibrium expression, and notice that I took the right side of this reaction and put it in the numerator of this fraction, and then I took the left side of this reaction and put it in the denominator of this fraction. Now you may be thinking that we left water out, but this is just excess water, so we don’t need to take it into account.

\(K_a\) right here is the acid dissociation constant, so the value of \(K_a\) indicates the strength of the acid. High \(K_a\) values indicate strong acids which dissociate readily in water, and then low \(K_a\) values indicate weak acids which dissociate only a little in water.

Now if we were to take a look at the \(K_a\) of hydrofluoric acid and the \(K_a\) of acetic acid, we could determine which was the stronger acid. The \(K_a\) of hydrofluoric acid is 6 point 8 times 10 to the negative 4th. The \(K_a\) of acetic acid is 1 point 8 times 10 to the negative 5th. Hydrofluoric acid has a higher \(K_a\) value which means it’s a stronger acid, which means it dissociates more readily in water than acetic acid does.