Solubility and Le Chatelier's Principle
- A solution is formed when a solute dissolves in a solvent.
- Solubility refers to the maximum amount of solute that can be dissolved in the solvent at a specified temperature and pressure.
- A saturated solution is one in which no more solute can be dissolved in a given amount of solvent at a specified temperature and pressure.
- If excess solute is present, then the undissolved solute is in equilibrium with dissolved solute in the saturated solution.
- When a solute dissolves in a solvent heat is either :
(i) absorbed (endothermic, ΔH is a positive value)
(ii) released (endothermic, ΔH is a negative value)
- The amount of heat absorbed or released when the solute dissolves in the solvent is known as its heat of solution (or enthalpy of solution), symbol is ΔHsoln
If water is the solvent, the term heat of hydration, or enthalpy of hydration, is used and the symbol is ΔHhyd
- Le Chatelier's Principle can be used to decide the effect of changing the temperature or the pressure of a saturated solution at equilibrium.
- Temperature changes:
(i) Solubility of a solute decreases with increasing temperature if ΔHsoln is negative (exothermic)
(ii) Solubility of a solute increases with increasing temperature if ΔHsoln is positive (endothermic)
- Pressure changes:
(i) Solubility of a gaseous solute increases if its partial pressure increases.
(ii) Solubility of a gaseous solute decreases if its partial pressure decreases.