CFR 13

Chemical Equilibrium

  • 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} ]
  • Solving for s gives the molar solubility.