Chemistry 112

Acid/Base Reactions

Consider an aqueous solution of a weak acid, HA, and a weak base, B:

HA(aq) + B(aq) ?

Brønsted-Lowry theory tells us that a hydrogen ion should transfer from the acid to the base, but what is the extent of reaction?

HA(aq) + B(aq) ? HB⁺(aq) + A^–(aq)

Does the reaction go to completion or is it an equilibrium?

To answer the question, consider the individual reactivities:

The acid reacts with water:

HA(aq) + H₂O(l) → ← H₃O⁺(aq) + A^–(aq) K_a

The base reacts with water:

B(aq) + H₂O(l) → ← HB⁺(aq) + OH^–(aq) K_b

A high concentration of both hydronium ion and hydroxide ion is incompatible with the water autoionization equilibrium, so:

H₃O⁺(aq) + OH^–(aq) → ← 2 H₂O(l) 1/K_w

If we add the reactions:

HA(aq) + B(aq) → ← HB⁺(aq) + A^–(aq) K_c

From the Law of Multiple Equilibria:

K_c = K_aK_b/K_w

If K_c is a large value (>100), we can justifiably say that the reaction essentially goes to completion (> 90 % reaction).

If K_c is a small value (<10^–14), we can say that the reaction does not occur.

For values of K_c in between, the reaction is an equilibrium.

If both the acid and the base are weak, the net reaction may go to completion or may be an equilibrium, depending upon the sizes of K_a and K_b.

Note: K_b = K_w/K_a(conjugate acid)

This means that K_c = K_a(acid)/K_a(conjugate acid)

Examples

Complete and balance:

HCN(aq) + NH₃(aq) ?

HF(aq) + CH₃NH₂(aq) ?

Titrations:

A titration is an experiment where one reagent is added to another, causing a reaction to occur, and some property is monitored at each addition.

For acid/base reactions, the quantity monitored is usually the pH.

Titration of an acid with a strong base:

Equivalence point: the volume of added base required to exactly satisfy the reaction stoichiometry.

The pH at the equivalence point = 7 for a strong acid/strong base titration. At equivalence, the only species present in the solution are neutral ions (the cation from the strong base and the anion from the strong acid) and water.

For a weak acid/strong base titration, the pH at the equivalence point is > 7. This is because the solution is composed of water, the cation of a strong base (neutral), and the anion of a weak acid (a base).

The pH at half the equivalence point volume (the half-equivalence point) is the pK_a. This is another method for finding the acid ionization constant.

The region around the half-equivalence point is flat and is called the buffering region. This provides another method for preparing a buffer.

Titration of a base with a strong acid.

The plot looks pretty much the same, except that it is mirrored about pH = 7.