How do I use pKa values to tell if an acid base reaction is favorable or unfavorable?

In an acid-base reaction, the direction of the reaction favors the formation of the compound with a higher pKa from one with a lower pKa. The compound with the lower pKa is considered the stronger acid, while the one with the higher pKa is the weaker acid. Equilibrium favors the side with the weaker acid. So, if the acid has a lower pKa value, the reaction will favor the side where the acid donates a proton. If the base has a lower pKa value, the reaction will favor the side where the base accepts a proton.