2.12
Solubility Equilibria: Overview
When a substance such as sodium chloride is added to water, it dissolves, forming an aqueous solution. The extent of dissolution is called solubility.…
Consider a saturated solution of sparingly soluble salt calcium fluoride with excess solid calcium fluoride. The already dissolved calcium fluoride has dissociated into Ca2+ and fluoride ions.
At equilibrium, the dissociation rate of calcium fluoride is equal to its formation rate.
The extent of dissociation is calculated quantitatively from the equilibrium constant called the solubility product constant or the solubility product.
Ksp is defined as the product of all the concentrations of ions raised to the power of its coefficient in the balanced equation at equilibrium.
According to Le Châtelier's principle, the solubility of a sparingly soluble salt increases with an increase in temperature and decreases in the presence of a common ion because of the common ion effect.
If the sparingly soluble salt is the conjugate base of a weak acid, the solubility increases by increasing the acidity.
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Q1: What is the solubility product constant and how is it calculated?
The solubility product constant, or Ksp, is the equilibrium constant for a sparingly soluble salt. It is defined as the product of all ion concentrations raised to the power of their coefficients in the balanced equation at equilibrium. Ksp is independent of the solid salt concentration because the solid concentration remains constant in a saturated solution.
Q2: How does temperature affect the solubility of sparingly soluble salts?
According to Le Châtelier's principle, the solubility of a sparingly soluble salt increases with an increase in temperature. When temperature rises, the system shifts to counteract the stress, promoting the dissociation of the solid salt into dissolved ions and increasing overall solubility.
Q3: What is the common ion effect and how does it reduce solubility?
The common ion effect occurs when a sparingly soluble salt's solubility decreases in the presence of a common ion already in solution. This happens because the added ion shifts the equilibrium backward, reducing dissociation of the salt and decreasing its solubility according to Le Châtelier's principle.
Q4: How does pH influence the solubility of salts formed from weak acids?
When a sparingly soluble salt is the conjugate base of a weak acid, increasing acidity raises its solubility. Lower pH provides more hydrogen ions, which react with the conjugate base ions, shifting the equilibrium forward and promoting dissolution of the solid salt.
Q5: What is the difference between solubility equilibria and precipitation equilibria?
Solubility equilibria and precipitation equilibria describe the same reversible process from opposite directions. Solubility is the forward process where a solid dissolves into ions, while precipitation is the reverse process where dissolved ions form a solid. Both exist in dynamic equilibrium in a saturated solution.
Q6: Why is solubility equilibrium important in biological systems?
Solubility equilibria affect critical biological processes. For example, acidic foods dissolve hydroxyapatite in tooth enamel, causing decay. Additionally, calcium oxalate, a sparingly soluble salt, precipitates in kidneys if not flushed out, forming kidney stones. Understanding these equilibria helps prevent health complications.
Q7: What factors besides temperature and common ions affect solubility equilibria?
Solubility equilibria are influenced by solvent type, pH of the solution, and the extent of complex ion formation. These factors shift the equilibrium position by changing ion concentrations or availability, altering the overall solubility of the sparingly soluble salt in the system.