Chemistry 112

Weak Bases

The general Brønsted–Lowry reaction for a weak base is:

B(aq) + H2O(l)HB+(aq) + OH(aq)

The equilibrium constant for this reaction is

Kb is the base ionization constant. As with Ka, Kb defines the reaction.

We can also define a log scale:

pKb = –logKb

Better bases give a higher concentration of hydroxide ion: this is indicated by a larger Kb or a smaller pKb.


Three categories of commonly encountered weak bases:

  1. Metal hydroxides. The strong bases all fit this category. Most metal hydroxides are only sparingly soluble, so are weak bases. We will look at the equilibrium chemistry of salts later in the semester.

  2. Most anions. The negative charge on the anion usually can act as a hydrogen ion acceptor in the presence of water. Exceptions: anions of strong acids (Cl, Br, I, ClO4, NO3, HSO4) and anions with ionizable hydrogen ions (HSO4, H2PO4, and others).

  3. Organic amines. These have the general structure of R3N: where R is H, C, O, or other main group atoms. The lone pair on the nitrogen is the underlying reason for the ability to accept a hydrogen ion. The conjugate acid of an amine is called an ammonium ion.

  4. R3N:(aq) + H2O(l) R3NH+(aq) + OH(aq)

    RNH2(aq) + H2O(l) RNH3+(aq) + OH(aq)

    Common amines:

A problem: outside of general chemistry textbooks, Kb (or pKb) is rarely reported. Rather, we find the pKa of the conjugate acid.

What is the relationship between Ka and Kb of a conjugate acid/base pair?



We treat equilibrium problems involving weak bases in exactly the same fashion as weak acids.


Example

Find the pH and percent ionization of a 0.10 M solution of ammonia at 25 oC.

Strategy: Write the reaction, write the mass action expression, find Kb, set up a table of concentrations, plug into the mass action expression, solve for the variable, and then use this information to answer the questions asked.

What are the limits of pH?

If ammonia were a strong base (100% ionized), then pOH = –log(0.10) = 1.00 so the pH = 14.00 – 1.00 = 13.00. If ammonia were completely unreactive, then the pH = 7.00 (pure water).