Molar solubility decreases primarily due to the common ion effect.
Understanding the Common Ion Effect
The common ion effect describes the decrease in solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution. Le Chatelier's principle explains this phenomenon; adding a common ion shifts the equilibrium of the dissolution reaction to the left, thus reducing the concentration of the sparingly soluble salt in solution, i.e., decreasing its molar solubility.
Example:
Consider the sparingly soluble salt silver chloride (AgCl). Its dissolution equilibrium is:
AgCl(s) ⇌ Ag+(aq) + Cl-(aq)
If we add a soluble chloride salt, such as sodium chloride (NaCl), to this solution, the concentration of Cl-(aq) increases. According to Le Chatelier's principle, this increase in [Cl-] will shift the equilibrium to the left, causing more AgCl to precipitate out of solution and thereby decreasing the concentration of Ag+(aq). Consequently, the molar solubility of AgCl decreases.
Factors Influencing Molar Solubility
Besides the common ion effect, several other factors can influence molar solubility:
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Temperature: The solubility of many salts changes with temperature. For most ionic compounds, solubility increases with increasing temperature. However, there are exceptions.
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pH: The solubility of salts containing basic anions (e.g., hydroxides, carbonates, fluorides) is affected by pH. Lowering the pH (increasing acidity) generally increases the solubility of these salts by reacting with the basic anion.
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Complex Ion Formation: The formation of complex ions can increase the solubility of sparingly soluble salts. For instance, silver chloride (AgCl) is practically insoluble in water. However, in the presence of ammonia (NH3), Ag+ ions can form a soluble complex ion, [Ag(NH3)2]+, thereby increasing the solubility of AgCl.
| Factor | Effect on Molar Solubility |
|---|---|
| Common Ion Effect | Decreases solubility |
| Temperature | Usually increases, but exceptions exist |
| pH | Affects salts with basic anions; lower pH often increases solubility |
| Complex Ion Formation | Increases solubility |
Conclusion
In summary, the most significant factor directly decreasing molar solubility is the common ion effect, where the presence of a common ion from another soluble salt reduces the solubility of the sparingly soluble salt. Other factors like temperature, pH, and complex ion formation can also influence molar solubility, though their effects may vary depending on the specific salt and solution conditions.
