Common ion effect
Solubility of CH3COOH Explained by Common Ion Effect
The solubility of acetic acid (CH3COOH) decreases when sodium acetate (CH3COONa) is added to its solution. This phenomenon is primarily explained by the common ion effect.
Understanding Acetic Acid and Sodium Acetate
- Acetic acid, CH3COOH, is a weak acid. In water, it undergoes partial dissociation according to the equilibrium:
CH3COOH(aq) $\rightleftharpoons$ CH3COO-(aq) + H+(aq)
- Sodium acetate, CH3COONa, is a salt formed from the weak acid CH3COOH and a strong base (NaOH). Being a soluble salt, it dissociates completely in water:
CH3COONa(aq) $\rightarrow$ CH3COO-(aq) + Na+(aq)
The Common Ion Effect Explained
The common ion effect describes the decrease in the solubility of a substance when another substance containing a common ion is added to the solution. In this case:
- Both acetic acid (CH3COOH) and sodium acetate (CH3COONa) produce the acetate ion (CH3COO-).
- When CH3COONa is added to a solution containing CH3COOH, the concentration of the acetate ion (CH3COO-) increases significantly because CH3COONa dissociates completely.
- According to Le Chatelier's principle, the system will counteract this increase in product concentration (CH3COO-) by shifting the equilibrium of the acetic acid dissociation reaction to the left.
- This shift favors the undissociated form of acetic acid (CH3COOH), effectively reducing the concentration of dissolved CH3COOH molecules in the solution. Consequently, the solubility of CH3COOH decreases.
The equilibrium expression for acetic acid is:
$K_a = \frac{[\text{CH}_3\text{COO}^-][\text{H}^+]}{[\text{CH}_3\text{COOH}]}$
An increase in $[\text{CH}_3\text{COO}^-]$ (from the added CH3COONa) requires a decrease in $[\text{CH}_3\text{COOH}]$ or an increase in $[\text{H}^+]$ to maintain the constant $K_a$ value. Since CH3COOH is already a weak acid, its dissociation is limited. The added acetate ions suppress further dissociation and favor the molecular form, thus reducing solubility.
Evaluating Other Options
- Increased temperature: While temperature can affect solubility, it's not the direct cause for the decrease observed specifically due to the addition of CH3COONa. The effect is chemical, not primarily thermal.
- Ion pairing: Ion pairing involves association between ions, but it doesn't explain the fundamental decrease in solubility caused by the presence of a shared ion in the equilibrium system.
- Oxidation: Oxidation is a redox process and is unrelated to the solubility changes of acetic acid in the presence of sodium acetate.
Therefore, the decrease in the solubility of CH3COOH in the presence of CH3COONa is due to the common ion effect.
Important Questions from Equilibrium
What is the pH of a solution prepared by dissolving $0.0025$ moles of $HNO_3$ in $250\,mL$ of water?
Which species acts as acid like behaviour in the following reaction?
\(HPO_4^{2-}+NH_4^{1+}\rightarrow H_2PO_4^{1-}+NH_3\)
Aqueous solution of CH 3COONa is:
pH of a neutral solution is ___________.
For the reaction N 2(g) + 3H 2(g) \(\rightleftharpoons\) 2NH 3(g) ΔH = -ve :
