CFR 14

Chemical Equilibrium

  • Definition: Chemical equilibrium exists when the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant.
  • Concepts:
  • Reactants convert to Products and vice versa.
  • Dynamic process where both reactions occur simultaneously.

Equilibrium Constant (K)

  • General Form:
  • For a reaction: "aA + bB ⇌ cC + dD"
  • K = [C]^c [D]^d / [A]^a [B]^b (where concentrations are raised to the power of their respective coefficients)
  • Characteristics:
  • K is unit-less and dependent on temperature.

Equilibrium Constant Expressions

  • Solids in Reactions:

  • The concentration of solids does not change and is therefore not included in the equilibrium expression.

  • Example: For the reaction: Fe(s) + 5CO(g) ⇌ Fe(CO)5(g), K = [Fe(CO)5]/[CO]^5

  • Liquids in Reactions:

  • Similar to solids, pure liquids are also excluded from equilibrium expressions due to constant density.

  • Example: CH3CO2H + H2O ⇌ CH3COO- + H3O+, therefore K = [CH3COO-][H3O+]

Forward and Reverse Reactions Relationship

  • Equation: Kfor = 1/Krev
  • Example: For the reaction SO2 + O2 ⇌ SO3, Kfor = [SO3]^2 / ([SO2]^2[O2]) and Krev = 1 / K_for.

Interpreting Equilibrium Constants

  • K >> 1: Indicates a reaction that favors products.
  • K << 1: Indicates a reaction that favors reactants.
  • Example at 25°C: CH3CO2H + H2O (K = 1.8 x 10^-5) vs. 2H2 + O2 ⇌ 2H2O (K = 3.5 x 10^81).

Le Chatelier’s Principle

  • Concept: If a system at equilibrium experiences a change (stress), the system will adjust to counteract the change and restore equilibrium.
  • Types of Stress:
  1. Change in concentration of reactants/products.
  2. Change in pressure/volume for gaseous reactions.
  3. Change in temperature.

Solubility Product (Ksp)

  • Definition: Ksp is the equilibrium constant for the dissolution of a solid substance.
  • General Form: e.g., for CaF2 ⇌ Ca^2+ + 2F^-, Ksp = [Ca^2+][F^-]^2.
  • Greater surface area of the solid can lead to greater solubility, hence influencing Ksp.

Factors Influencing Solubility

Common Ion Effect
  • Adding a common ion decreases solubility.
  • Example: Adding AgNO3 reduces the solubility of Ag2CrO4 in a solution with Ag+ ions already present.
pH Influence
  • Changes in pH influence solubility for certain compounds:
  • Increasing [OH-] can shift equilibrium leftward, reducing solubility (e.g., Mg(OH)2).
  • Decreasing [OH-] increases solubility by shifting equilibrium rightward.
Applications in Biological and Pharmaceutical Systems
  • Understanding Ksp is crucial for drug solubility in biological systems versus formulations requiring specific solubility characteristics (e.g., suspensions with barium sulfate).
  • Fluoridation prevents tooth decay by forming a more soluble fluorapatite compared to hydroxyapatite under acidic conditions.

Calculating Molar Solubility

  • Example with PbI2:
  • Ksp: 7.1 x 10^-9, derived from dissolution: PbI2 ⇌ Pb^2+ + 2I^-
  • Let "s" be molar solubility: Ksp = (s)(2s)^2 = 7.1 x 10^-9 generates results leading to s = 1.2 x 10^-3.

Summary of Learning Outcomes

  • Understand and define the solubility product, calculate it from molar solubility, and recognize the common ion effect and pH influence on solubility.