Reactions reach equilibrium when the concentrations of products and reactants remain constant.
Forward and reverse reactions occur simultaneously, meaning the reaction hasn't stopped.
Equilibrium can be represented by an equilibrium expression.
Equilibrium Expression: [ K = \frac{[Products]^{n}}{[Reactants]^{m}} ] where n and m are the stoichiometric coefficients.
Units are unitless.
The equilibrium constant (K) is specific to a reaction and depends on temperature.
Construction of Equilibrium Expressions
Solids and liquids are not included in equilibrium expressions because their concentrations do not change appreciably.
Example: In a weak acid dissociation expression, water is not included.
The equilibrium expression for a reverse reaction is the inverse of the forward reaction.
K > 1 indicates products are favored.
K < 1 indicates reactants are favored.
Solubility Equilibrium
Solubility Product (Ksp): The equilibrium constant for a dissolution reaction.
Constructing the solubility product expression allows calculation from molar solubility and vice versa.
Ksp expressions exclude solids and are based solely on dissolved ions.
Example for CaF₂: [ K_{sp} = [Ca^{2+}][F^{-}]^{2} ]
Saturated Solutions
A saturated solution contains the maximum amount of solute that can dissolve, reaching dynamic equilibrium.
Rate of dissolution equals the rate of precipitation.
Example: Dissolving salt until undissolved salt remains at the bottom indicates saturation.
Effects on Solubility
Common Ion Effect
Introduction of a common ion shifts equilibrium to reduce solubility.
Example: Dissolving a salt (like silver chromate) in a solution already containing a common ion (like Ag⁺) will reduce the amount of salt that dissolves.
pH Effects on Solubility
Solubility can change based on pH.
Increasing pH (adding OH⁻) may decrease dissolution (shifts equilibrium left).
Decreasing pH (adding H⁺) may enhance dissolution (shifts equilibrium right).
Practical Applications of Solubility
Drug Formulations: Knowledge of solubility can help design drugs that are effective in specific environments.
Barium sulfate is used in X-ray imaging since it has low solubility (Ksp) and is not absorbed by the body.
Tooth Decay: Hydroxyapatite in teeth dissolving creates OH⁻. Acidic environments lead to more cavities through reduced solubility and slower remineralization.
Fluoride enhances dental health by incorporating into enamel reducing solubility.
Drug Example: Erythromycin
Erythromycin is less stable in acidic conditions.
Its solubility can be adjusted with formulations (like covered tablets) or altered salts for different routes of administration (IV vs pills).
Molar Solubility Calculations
If Ksp is known, molar solubility (s) can be calculated by rearranging the Ksp expression.
Example for lead iodide: [ K{sp} = [Pb^{2+}][I^{-}]^{2} \Rightarrow K{sp} = s (2s)^{2} = K_{sp} = 4 s^{3} ]