Leveling effect (chemistry)

The term leveling effect refers to a solvent's ability to level the effect of a strong acid or base dissolved in it.

Process

When a strong acid is dissolved in water, it reacts with it to form hydronium ion (H₃O⁺) in the following reaction (where "HA" is a generic strong acid such as hydrochloric acid (HCl)):

HA + H2O -> A- + H3O+

Any acid that is stronger than H₃O⁺ reacts with H₂O to form H₃O⁺; therefore, no acid can be stronger than H₃O⁺ in H₂O.

This is true with any solvent, although usually to a lesser extent than with water. Bases may be also leveled in solvents.

Example

Solomons & Fryhle ^[1] describe the leveling effect, using the example of the reaction between sodium amide (NaNH₂) and water:

 H-O-H         + NH2-           -->   H-O-        + NH3
(Stronger Acid) (Stronger Base) --> (Weaker Base) (Weaker Acid)

The amide ion reacts with water to produce a solution containing hydroxide ions (a much weaker base) and ammonia.

Leveling and differentiating solvents

A weakly basic solvent has less tendency than a strongly basic one to accept a proton. Similarly a weak acid has less tendency to donate protons than a strong acid. As a result a strong acid such as perchloric acid exhibits more strongly acidic properties than a weak acid such as acetic acid when dissolved in a weakly basic solvent. On the other hand, all acids tend to become indistinguishable in strength when dissolved in strongly basic solvents owing to the greater affinity of strong bases for protons. This is called the leveling effect. Strong bases are leveling solvents for acids, weak bases are differentiating solvents for acids.

Solution

To use bases stronger than the hydroxide ion, use a solvent that is a weaker acid than water.

To use acids stronger than the hydroxonium ion, use a solvent that is a stronger acid than water.

See also

• Nonaqueous titration

References

1. ^ Solomons & Fryhle's Organic Chemistry (8th Ed.)