Question

The common ion effect for weak acids is to significantly decrease the dissociation of the acid in water. Explain the common ion effect.

Below is the answer that I created. I want to be sure if the answer correct or does it need any correction? It was supposed to be 250 words minimum. But I could only reach 130 words so far. Can anybody please help elaborate it so I can submit the assignment? Also, is the chemical equation complete?

The common ion effect is the phenomenon in which a solution with dissociated ions or of a weak acid or base is suppressed by adding strong electrolyte that contains common ions. The effect of common ion is seen when there is a decrease of solubility of solids because when common ions are added in a compound, its solubility generally decreases because of the shift in the equilibrium (causes the system to shift towards the left direction resulting in the formation of weaker acids). Let us consider weak acid as acetic acid.
The dissociation takes place as follows:

CH3COOH + H2O = H3O = CH3COO

With the addition of strong acids like HCL or H2SO4, the concentration of H3O + tend to increase shifting the reaction to the left side that causes the dissociation of acetic acid to decrease.

Answer 1

The common ion effect is the phenomenon which is used to describe effect on a equilibrium system of weak acid or base or sparingly soluble substances when concentration of an ion is changed significantly that is already involved in equilibrium. This effect in equilibrium occurs due to the Le Châtelier's Principle which states that if equilibrium becomes unbalanced, the reaction will shift to restore the balance. If a common ion is added or removed from a system of weak acid or weak base equilibrium, then the equilibrium will shift towards the reactants or products respectively. Let us consider weak acid as acetic acid.
The dissociation takes place as follows:

                             CH3COOH + H2O = H3O + CH3COO

With the addition of strong acids like HCL or H2SO4, the concentration of H3O + tend to increase shifting the reaction to the left side that causes the dissociation of acetic acid to decrease. This happens because every system wants to attain equilibrium for stability or if we talk mathematically the Ka value of the acid should remain constant at certain temperature. So if the concentration of H3O+ is increased by adding strong acids then concentration of acetate ion should be reduced for constant ka value because

                                        Ka = [CH₃COO^-] [H₃O⁺]

This reduction in concentration of acetate ion occurs when the dissociation of acid is reduced and this is how common ion effect reduces the dissociation weak acid or base or sparingly soluble substances like AgCl , AgBr , PbCl2 etc.