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Acid-Base Reaction

The acidity or basicity of a protonic solvent has great influence on the usefulness of a protonic solvent, because of what is known as leveling effect of the solvent. In any protonic solvent, the strongest acidic species or the strongest basic species which can exist are the cation or the anion, respectively, which arise form the self-ionization of the solvent. For example,

2H2O  <==>  H3O+ + OH-
2NH<==>  NH4+ + NH2-

The strongest acid which can exist in aqueous solutions is H3O+ ion and the strongest base is the hydroxide ion, OH- . All protonic acids which are fully ionized in water will generate the same acidic species. H3O+ to the same extent and irrespective of their intrinsic strength will all behave as strong acids in aqueous solutions. This is known as leveling effect. Therefore, water acts as a leveling solvent for all acids and they (HCIO4, H2SO4, HNO3, HCI) are all leveled to the strength of H3O+ in aqueous solutions.

Similarly, the very strong bases such as hydride ion (H- ) amide ion (NH2- ) and ethoxide ion (OC2H5-), all react completely with water to yield the anion OH- . It means that all these bases are leveled to the strength of OH- ion in aqueous solutions. It would thus be a waste of effort to use strong bases such as NaH, NaNH2 or  NaOC2H5 in aqueous solutions, because they all ionize in water to give hydroxide ions.

         NaH + H2O    →      Na+ + OH- + H2
    NaNH2 + H2O    →      Na+ +OH- + NH3
NaOC2H2 + H2O   →      Na+ + OH- + C2H5OH

In case we have to differentiate between he strengths of different acids such as HCIO4, H2SO4 and HCI, instead of water (which is a strong proton acceptor), we may employ a weak proton acceptor like acetic acid. Perchloric acid and sulphuric acid fully protonate acetic acid while hydrochloric acid behaves as a weak electrolyte

   HCIO4 + CH3COOH   →     CH3COOH2+ + CIO4-
H2SO4 + CH3COOH     →    CH3COOH2+ + HSO4-
        HCI + CH3COOH  <==>    CH3COOH2+ + CI -
Aprotic solvents (like liquid SO2), which have neither proton accepting nor proton donor properties, may be used as differentiating solvents for both acids and bases. 

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