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

In the view formalized for acids and bases at the turn of the century, an acid was any substance that dissociated to give H+ ions in aqueous solution, and a base was any substance that dissociated to give OH- ions.This definition (proposed by Arrhenius) was fine for potassium hydroxide, but where was the hydroxide ion in ammonia, NH4+?

Ammonia releases hydroxide ions when it dissolves, but only because the ammonia steals a proton from a water molecule:

One could talk about "ammonium hydroxide," NH4OH, but there is no evidence that the compound exists.

A better definition of acids and bases in aqueous solutions is: An acid is any substance that, when added to an aqueous solution, increases the hydrogen ion concentration; a base is any substance that, when added to an aqueous solution, decreases the hydrogen ion concentration and increases the hydroxide ion concentration.

Since [H+] and [OH-] are linked by the water dissociation equilibrium, when one is increased, the other must decrease. By this definition, both NaOH and NH3 are bases because they increase the hydroxide ion concentration, [OH-]. In the case of NaOH, the hydroxide ions come from the crystal lattice of the solid, and in the case of NH3 they come from dissociated water molecules whose protons have combined with NH3 to form, NH4+. The overall effect is the same.

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