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what is the difference between commion ion effect and buffer solution

Common ion effect

In a solution of a weak electrolyte, the moleculer of the electrolyte are in equilibrium with its ions. For example a weak electrolyte AB is in equilibrium with A⁺ and B^- in solution.
that is AB ?  A⁺ + B^-

The equilibrium is characterized by equilibrium constant,

If the concentration of A⁺ or B^- is increased, the equation would shift towards ldft, that is more AB is formed due to the combination of A⁺ and B^-. The concentration of A⁺ and B^- can be chaneged by adding a strong electrolyte having one ion is common with the electrolyte(AB)

Definition

A shift in the position of the equilibrium involving molecular form (AB) and ionic form (A⁺, B^-) of an electrolyte, by adding a strong electrolyte having one ion common to it is called common-ion effect.

Buffer solutions:

A buffer is a solution that can maintain a nearly constant pH when diluted or when strong acids/ bases are added. A buffer solution consists of a mixture of a weak acid and its confugate base/ a weak base and its confugate acid.

Example : CH₃COOH and CH₃COONa

Any strong base that is added to the solution is neutralized by acetic acid.

CH₃COOH   +    OH^-   ?  CH₃COO-   +   H₂O
         (aq)              (aq)                    (aq)          (aq)

Any strong acid that is added to the solution is neutralized by acetate.

CH₃COO^-       +   H⁺   ?       CH₃COOH
            (aq)            (aq)                      (aq)

The amount of strong acid or base that a buffer can neutralise is called buffer capacity. After neutralization, the equilibrium will be reestablished.

In general, a buffer can be prepared by,

1. Mixing a weak acid and a salt of its confugate base in solution.
2. Mixing of a weak acid and anough strong base to neutralize a portion of the weak acid.
3. Mixing a weak base and enough strong acid to neutralise a portion of the weak base.