LON-CAPA CHM 2200C Spring 2005--Predicting the Direction of Acid-Base Reactions

Predicting the Direction of Acid-Base Reactions from Approximate pK_a'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, K_a. For the reaction:

HA H⁺ + A^-

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

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

pK_a = -logK_a

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

pH = -log[H⁺]

The larger the pK_a, 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 pK_a.

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
pK_a ~5						pK_a~9
Equilibrium lies to the right: →


C₂H₅NH₂	+	H₂O		C₂H₅NH₃⁺		OH^-
base		acid		acid		base
		pK_a ~16		pK_a ~11
← Equilibrium lies to the left ^*


NH₃	+	HCN		NH₄⁺	+	CN^-
base		acid		acid		base
		pK_a ~9		pK_a ~9
pK's the same, equilibrium has approximately equal amounts of product and reactant.

Some Generalizations

The larger the difference in pK_a's, the further the equilibrium lies toward the higher pK_a (weaker) acid.
If the pK_a's are equal, the equilibrium constant of the reaction will be ~1,and the ratio of products and reactants will be about 1.^*
If the pK_a difference is about 2, there will be about a 10 to 1 ratio of products and reactants in the favored direction.^*
If the pK_a difference is about 4, there will be about a 100 to 1 ratio of products and reactants in the favored direction.^*

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