The impact of a solvent on the characteristics of acids and bases is known as the solvent leveling effect or solvent effect. According to this effect, the basicity of the solvents limits or levels the strength of a strong acid. Similarly, the acidity of the solvent balances the strength of a strong base.
The solvent in which the complete proton transfer occurs or the solute is completely ionized is called leveling solvent. One of the most common examples of leveling solvents is water(H2O).
In such a solvent, several acids are completely dissociated and are thus of the same strength. Strong bases are leveling solvents for acids.
Liquid ammonia(NH3) acts as leveling solvent for HCl(Strong acid) and CH3COOH(Weak acid) since these acids ionize completely in Liq.NH3 to give NH4+ cations.
Although HCl is a strong acid and CH3COOH is a weak acid, due to their complete ionization in Liq.NH3 appears to be equally strong in NH3 solvent which acts as a leveling solvent for these acids.
The solvent in which various acids dissociates to different degrees and thus have different strength is called differentiating solvent. Weak bases are differentiating solvents for acids.
CH3COOH is a differentiating solvent for strong acids like HClO4, HBr, H2SO4, HCl, and HNO3, since these acids ionize to different extents in CH3COOH solvent.
Solvent leveling effect
When a strong acid is dissolved in water, it reacts with water to form a hydronium ion(H3O+).
Any acid that is stronger than H3O+, reacts with water to form H3O+. Therefore, no acid stronger than H3O+ exists in H2O. A similar effect arises in the case of the base also. In water, OH– is the strongest base, thus any base stronger than OH– when reacts with water, generates OH–. Therefore, no base stronger than hydroxide(OH–) exists in H2O. The Levelling effect of water is due to its high dielectric constant and strong proton accepting tendency.
Hence, any acid stronger than H3O+ in water donates a proton to H2O. Any base stronger than OH– in water accepts a proton from H2O. Therefore, no acid stronger than H3O+ and no base stronger than OH– can exist in H2O. This is the effect called the solvent effect.
In conclusion, the solvent leveling effect can be defined as the phenomenon due to which the acids or bases appear to be equally strong.