How to Predict the Equilibrium Direction of an Acid-Base Reaction
pKa values allow you to predict the equilibrium direction of acid-base reactions for organic molecules. The pKa value of an acid is a quantitative measurement of a molecule’s acidity. The pKa is derived from the equilibrium constant for the acid’s dissociation reaction, Ka, and uses a logarithmic scale to allow the pKa values to span wide ranges.
pKa = –log Ka
The lower the pKa value of an acid, the stronger the acid. The higher the pKa value, the weaker the acid. Very strong acids have pKa values of less than zero, while weak acids generally have pKa values of between 0 and 9.
A brief pKa table of acids is shown here.
With pKa table in hand (or in memory, if your professor insists upon it), you can predict the equilibrium direction of acid-base reactions. Weak acids and bases are lower in energy than strong acids and bases, and because equilibria favor the reaction side with the lowest-energy species, acid-base reactions will go to the side with the weakest acids and bases.
As a rule, the equilibrium of a reaction will favor the side with weaker acids and bases.
For example, you can predict the direction of the acid-base reaction between hydrogen cyanide (HCN) and acetate (C2H3O2–) shown here.
Because hydrogen cyanide (pKa = 9) has a higher pKa value than acetic acid (pKa = 5), the equilibrium will lie to the left, in the direction of the weaker acid and base. That’s really all there is to predicting the direction of an acid-base reaction. If you know the pKa values of the two acids on both sides of the equation, then you know in which direction the equilibrium lies, because equilibrium will favor the side with the acid that has the highest pKa.