16. Ions and Equilibrium;
       Acids and Bases
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       The difference between strong and weak acids

Moreover, the large excess of water molecules present in an aqueous solution will push the equilibrium

far to the right, in the direction of dissociated acid.

For strong acids such as HClO4, HNO3, HCI, and H2SO4 (or weak B-L bases such as ClO4-, NO3-, Cl- and HSO4-), the donation of the proton by the acid to H2O, as shown in the reaction to the left above, is effectively complete.

We refer to these as the strong acids, in contrast to the weak acids that show only partial dissociation in aqueous solution. Although these strong acids do have different degrees of attraction for their protons, they all appear equally strong when water is the solvent. This is called the leveling effect of the solvent. If we want more information about the relative strengths of these acids, we must turn to a different solvent, which exerts less of a pulling power on the acid protons.

Methanol, CH3OH, has a smaller attraction for protons than water does. HCl behaves as a weak acid in methanol solution, because it is only partially dissociated (see diagram).

One can measure dissociation constants in methanol or other nonaqueous solvents for compounds that are completely dissociated strong acids in water solution, and thus can make the relative strength ranking shown in the table on Page 8.

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