Predicting the Direction of Acid-Base Reactions from Approximate pKa's

An acid-base reaction is simply the transfer of a proton from an acid to a base, forming the conjugate base of the acid and the conjugate acid of the base.

For Example:

Acid   Base   Conjugate Base   Conjugate Acid
CH3COOH
+
HCO3-

CH3COO-
+
H2CO3


There is actually an equilibrium between reactants and products, and the equilibrium position can be determined quantitatively from the acid dissociation constants of each acid/conjugate base pair. However one can qualitatively tell whether a reaction goes predominantly to the right or to the left by comparing the relative strength of the acids and bases involved.

The following statements are equivalent:

The reaction will proceed in the direction to form the weaker base!
The reaction will proceed in the direction to form the weaker acid!

Strength of Acids

The strength of an acid can be given quantitatively from its acid dissociation constant, Ka. For the reaction:

HA            H+      +        A-

In aqueous solution, the Ka is the [H+] at which you have equal amounts of the acid and its conjugate base in equilibrium.

Because Ka can vary over many orders of magnitude, it is often expressed in logarithmic form:

pKa = -logKa

Just as [H+] is expressed in logarithmic form:

pH = -log[H+]

The larger the pKa, the weaker the acid (and the stronger its conjugate base).

Therefore, the reaction will proceed in the direction to form the acid with the larger pKa.

Click here for a table of approximate pKa values. (Values below 1 and above 16 are extrapolated from reactions measured in non-aqueous solvents.)


Examples
             
CH3COOH
+
NH3

CH3COO-
+
NH4+
acid
 
base
 
base
 
acid
pKa ~5
         
pKa~9
Equilibrium lies to the right: →
             
C2H5NH2
+
H2O

C2H5NH3+
OH-
base
acid
acid
base
pKa ~16
pKa ~11
←   Equilibrium lies to the left *
             
             
NH3
+
HCN
NH4+
+
CN-
base
acid
acid
base
pKa ~9
pKa ~9
pK's the same, equilibrium has approximately equal amounts of product and reactant.

Some Generalizations


*Generalizations about ratios in aqueous solution hold only hold for pKa values between 0 and 14. Below 0 and above 14 they apply to reactions in non-aqueous, non-protic solvents only.