Equilibrium

Solubility Product Constant (Ksp) - AP Chemistry Rundown

Introduction

The solubility product constant (Ksp) is a crucial concept in AP Chemistry that describes the equilibrium between a solid ionic compound and its dissolved ions in a saturated solution. A saturated solution is one where the maximum amount of solute has been dissolved at a given temperature. It’s important to remember that Ksp applies only to slightly soluble or insoluble ionic compounds, not to readily soluble ones.

Definition

The Ksp is the equilibrium constant for the dissolution of a solid ionic compound in water. Consider a generic ionic compound AxBy(s):

$ A_xB_y(s) \rightleftharpoons xA^{y+}(aq) + yB^{x-}(aq) $

The solubility product constant expression is given by:

$ K_{sp} = [A^{y+}]^x[B^{x-}]^y $

Where:

• $ [A^{y+}] $ and $ [B^{x-}] $ represent the molar concentrations of the respective ions at equilibrium.
• x and y are the stoichiometric coefficients of the balanced dissolution equation.

Important Note: The solid reactant, AxBy(s), is not included in the Ksp expression because its concentration is effectively constant.

Factors Affecting Ksp

• Temperature: Ksp is temperature-dependent. Generally, the solubility of solids increases with increasing temperature, thus increasing Ksp. Temperature and Solubility
• Common Ion Effect: The presence of a common ion in the solution decreases the solubility of the slightly soluble salt, thus shifting the equilibrium to the left. Common Ion Effect
• pH: The solubility of some salts, particularly those containing basic anions (e.g., hydroxides, sulfides, carbonates), is affected by the pH of the solution. pH and Solubility
• Complex Ion Formation: The formation of complex ions can significantly increase the solubility of a metal salt. Complex Ion Equilibria

Calculating Ksp

Ksp can be calculated from the molar solubility (s) of the compound, which is the number of moles of the compound that dissolve per liter of saturated solution. The relationship between Ksp and molar solubility depends on the stoichiometry of the dissolution reaction.

Example:

For the compound MX(s) dissolving in water:

$ MX(s) \rightleftharpoons M^+(aq) + X^-(aq) $

$ K_{sp} = [M^+][X^-] = (s)(s) = s^2 $

For the compound MX2(s) dissolving in water:

$ MX_2(s) \rightleftharpoons M^{2+}(aq) + 2X^-(aq) $

$ K_{sp} = [M^{2+}][X^-]^2 = (s)(2s)^2 = 4s^3 $

Relationship between Ksp and Solubility

A larger Ksp value indicates higher solubility. However, direct comparison of Ksp values to determine relative solubilities is only valid for compounds with the same stoichiometry (e.g., MX and MY, but not MX and MY2).

Predicting Precipitation

The ion product (Q) can be used to predict whether precipitation will occur when two solutions containing ions are mixed. Q is calculated in the same way as Ksp but using the initial ion concentrations.

• Q < Ksp: The solution is unsaturated, and no precipitation will occur.
• Q = Ksp: The solution is saturated, and the system is at equilibrium.
• Q > Ksp: The solution is supersaturated, and precipitation will occur until Q = Ksp. Precipitation Reactions

This rundown provides a comprehensive overview of the solubility product constant for AP Chemistry. Remember to consult the linked notes for more in-depth explanations of specific topics.