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Section 4: Solvent Leveling

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  • The strong hydrohalic acids, HI, HBr, and HCl, at the same concentration, cannot be differentiated in water.

    \[\ce{HI (g) + H2O (l) -> H3O+ (aq) + I- (aq)}\]

    \[\ce{ HBr (g) + H2O (l) -> H3O+ (aq) + Br- (aq)}\]

    \[\ce{ HCl (g) + H2O (l) -> H3O+ (aq) + Cl- (aq)}\]

    The three acids, at 1 M concentrations, would all produce 1 M H3O+ and would have a pH of 0. This observation would apply to the other monoprotic strong acids as well. This observation is called solvent – leveling which notes that the strongest acid that can exist in water is hydronium. All strong acids are levels to hydronium. The pKa values listed in the table are estimates made in other solvents--dimethyl sulfoxide is a common solvent for these measurements.

    The same observation can be made for bases when added to water. They automatically dissociate to produce hydroxide. Acids that are weaker than water (pKa > 14) do exist, and they do form conjugate bases stronger than hydroxide, but once we add these to water, they revert to hydroxide. When solvent leveling is occurring, thermodynamics favors product formation (\(K > 1\), \(\Delta G < 0\)).

    Given the solvent leveling effect of water, this chemistry does readily occur, but we may need to alter our solvent to prevent leveling to hydroxide or hydronium. For example, if we shift to ammonia as the solvent, bases stronger than hydroxide can be observe. Recall auto ionization occurs for all solvents, the strength of the acid and base resulting from auto ionization will determine the maximum strength of acids and bases that can be observed in those solutions.

    In ammonia, the strongest acids that can ever be observed is ammonium (NH4+) and the strong base is amide (NH2-). Acids stronger than ammonium will be leveled to ammonium. Bases stronger than amide will be leveled to amide.