In aqueous solution all very strong bases like Na+H–, Na+NH2–, Na+OC2H5– are levelled to the strength of OH– ion, for they react completely with H2O to produce OH–ions. In other words, they are stronger bases than the conjugate base OH-.
Na+H- +HOH = OH- + NaH2+
So bases stronger than the conjugate base of the solution are levelled (behave of equal strength ) and this effect is called leveling effect or solvent effect.
Similarly
Since HF and HCl both are ~ 100% ionised in liquid NH3 to give ~100% NH4+ ions, these appear to be of equal strength and liquid NH3 acts as a levelling solvent for HF and HCl. In H2O, HF is only partially ionised, whereas HCl and HBr are ~ 100% ionised. So H2O is a differentiating solvent for HF and HBr/HCl. In other words, HF and H3O+ are of about equal strength acids. And HCl/HBr are stronger acids than H3O+.
HF+NH3=NH4+ +F-
0 100
HBr/HCl+NH3=NH4+ +Cl-
0 100
HF+H2O =H3O+ +F-
45 55
HBr/HCl+H2O =H3O+ +Cl-
0 100
The solvents in which complete proton-transfer occurs are called levelling solvents?
In other words, the solvent in which the solute is ~100% ionised, are called levelling solvents.
Thus H2O is a differentiating solvent for HF and HCl, but for HCl and HBr it is a leveling solvent. Several mineral acids are partially ionised in glacial CH3COOH medium because CH3COOH is a poor proton-acceptor but rather a better proton donor. CH3COOH, therefore, acts as a differentiating solvent towards the mineral acids. But, for bases, CH3COOH act as a leveling solvent.
CH3COOH+H+=CH3COOH2+ Poor
(weaker base)
CH3COOH=CH3COO- +H Good
(stronger acid)
HCl+CH3COOH = CH3COOH2+ +Cl-
10 90
HBr
15 85 (suposed figures)
HF
20 80
NaOH+CH3COOH = CH3COONa+ H2O
0 100
KOH
0 100
This phenomenon viz the strength of all acids becomes equal to that of H3O+ion is called leveling effect of the solvent, and here water is called a leveling solvent for all these acids.
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