Definition of Solubility Product Constant (Ksp)
The solubility product constant (Ksp) is a specific type of equilibrium constant used to describe the extent to which an ionic compound dissolves in a solvent, typically water. It represents the product of the concentrations of the dissolved ions in a saturated solution, each raised to the power of their stoichiometric coefficients in the balanced dissolution equation.
Calculating Ksp
For a generic sparingly soluble ionic compound A_x_B_y_ that dissolves into xA^(y+)(aq) and yB^(x-)(aq) ions, the Ksp expression is written as [A^(y+)]^x * [B^(x-)]^y. This calculation is only applicable to saturated solutions where the solid compound is in equilibrium with its dissolved ions at a specific temperature.
Example with Silver Chloride (AgCl)
Consider the dissolution of silver chloride: AgCl(s) ⇌ Ag+(aq) + Cl-(aq). In a saturated solution of AgCl, the Ksp expression is [Ag+] * [Cl-]. If the molar solubility of AgCl is 's' (moles/L), then [Ag+] = s and [Cl-] = s, resulting in Ksp = s^2. A common Ksp value for AgCl at 25°C is approximately 1.8 × 10^-10.
Importance and Applications
Ksp values are vital for predicting whether an ionic compound will precipitate from a solution when concentrations are known. By comparing the ion product (Qsp) to Ksp, chemists can determine if a solution is unsaturated (Qsp < Ksp), saturated (Qsp = Ksp), or supersaturated (Qsp > Ksp), which indicates that precipitation will occur. It also allows for the comparison of relative solubilities among different ionic compounds.