Question

What is common ion effect? Explain it through H2PO4 - (aq) + H2O(l) ⇋ H3O + (aq) + HPO4 2- (aq

Answer 1

The Common Ion Effect

If a salt of a weak acid is added to a solution of the acid itself, the dissociation of the acid is diminished further. For example, the addition of sodium acetate to a solution of acetic acid suppresses the dissociation of acetic acid which is already very small.

Consider the equilibrium,

CH₃COOH ⇔

H⁺ + CH₃COO^-

The addition of one of the products of dissociation (example:acetate ion), supplied by the largely dissociated salt (example:sodium acetate) pushes the equilibrium to the left. In other words, the dissociation of acetic acid is suppressed. Similarly, the addition of hydrogen ions furnished by the addition of a largely dissociated acid such as hydrochloric acid , also suppresses the dissociation of acetic acid.

Thus, the common ion effect is the "suppression of the dissociation of a weak acid or a weak base on the addition of its own ions.

H2PO4 - (aq) + H2O(l) ⇋ H3O + (aq) + HPO4 2- (aq

conjugate     conjugate      conjugate     conjugate

acid          base    acid base ..............

In this case if we add more H2O   then reaction will shift towards right side

if we add acid (H)+) , equilibrium will shift towards left to undo effect of addition of acid