Weak Acids and Bases

Learning Goals

Here you will learn how to rank weak acids and bases in terms of their equilibrium constants.

Synopsis

Most acids and bases are weak, meaning that their water dissociation is not complete. So, rather than writing the reaction as complete, we will have to write the reaction as an equilibrium as in the case of acetic acid (contained in vinegar)

CH₃COOH(aq) + H₂O(l) --> CH₃COO^-(aq) + H3₃⁺(aq)

We then can write an equilibrium constant for this reaction where we will use the symbol Ka for the equilibrium constant of an acid:

Ka = [CH₃COO^-][H₃O⁺]/[CH₃COOH]

We would write the equilibrium constant for a weak base in water in exactly the same way. Table 17.3 lists equilibrium constants for some common acids and bases. Acids are listed on the left and their conjugate bases are listed on the right. The best way to remember which acids and bases are weak is to memorize the ones that are strong and assume all others are weak. Strong acids are HCl, HBr, HI, HNO₃, H₂SO₄, and HClO₄. Strong bases are LiOH, NaOH, KOH, Sr(OH)₂, and Ba(OH)₂.

Review Questions

Caffeine is a weak base. Look up its K_b and compare its strength with some of the other bases in Table 17.3.
Notice in the list of strong acids that we skipped the anion F^- (the halogens are F, Cl, Br, and I). In Table 17.3 you see that its K_a is 7.2 x 10^-4. Speculate as to why HF is not given as a strong acid.