Acid-base reactions involve a proton (H⁺) transfer.
This proton transfer is reversible as the products of the reaction are also an acid and a base:

HA	+	B		BH+	+	A–
stronger acid		stronger base		weaker acid		weaker base

The reaction is product-favoured (products predominate in the reaction mixture) only if the reactant acid is stronger than the product acid.
Thus pK_a of the two conjugate pairs can be used to predict the extent of proton transfer.
 

To decide whether a proton transfer between two species is product-favoured:
 

1. Check that one species can act as an acid and the other can act as a base.
2. Write the equation for the proton transfer.
3. Products predominate at equilibrium only if pK_a(reactant acid) < pK_a(product acid).