Predicting Acid-Base Reactions from pKa

While pKa's directly reflects an acid's strength in water, its power extends beyond. By understanding pKa, we can indirectly predict the equilibrium constant (how far a reaction goes) for any acid-base pair, not just reactions involving water.

Let's consider a simple acid base reaction as follows.  Let's predict the equilibrium constant (Keq) for this reaction and determine if equiliobrium favors one side over the other.

1. Identify the species behaving as the acid on each side of the equilibrium equation. On the left side, the acid is benzoic acid since it gives up a proton (H+) to the base trimethylamine.  If the reaction were going in reverse the trimethylammonnium ions gives up a proton to the benzoate ion.  The Pka for these two acids are 4.2 and about 11 respectively.
   
   
2. The reaction will favor the side of the weakest acid and base.  The trimethylammonium ion has a pKa of 11 so it is the weakest acid, therefore this reaction will favor the product side.  We can calculate the equilibrium constant as follows.
   
          K_eq = 10^ΔpKa = 10^(11-4.2) = 6.3 x 10⁶
   
   Thus Keq${\mathrm{<br>}}_{}$ is a very large number (much greater than 1) and the equilibrium lies far to the right-hand side of the equation, just as we had predicted.