16. Ions and Equilibrium;
       Acids and Bases
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       The Meaning of Acids and Bases

In contrast, the cyanide ion, CN-, is a strong B-L base. It has a great attraction for protons, and the reaction

is heavily shifted to the right. As a result, HCN is a very weak acid.

In the examples discussed so far, the acidic species has been electrically neutral and the basic species charged. This need not always be so. Ammonia is a B-L base, and the ammonium ion is a B-L acid, capable of releasing a proton:

We can write an acid-dissociation equilibrium espression in the usual way:

This is related to the base-dissociation constant seen previously, Kb, by


This is the value of Ka for the ammonium ion that is given in the table on Page 8. You can either think of NH3 as a moderately strong base with a Kb of 1.76 x 10-5, or NH4+ as a very weak acid with a Ka of 5.68 x 10-10. The results of any calculation involving this acid-base equilibrium always will be the same whether you use Ka or Kb, as long as you know what you are doing.

Water can act as both a B-L acid and a B-L base.

We can regard the hydrating water molecules around a proton in solution as a weak B-L base

The Bronsted-Lowry theory has so influenced chemistry that two generations of chemists have written H3O+ for the hydrated form of H+ and called it the "hydronium ion", even though the true hydration state of the proton is not known, but it is probably more like H+(H2O)4 or H9O4+.

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