Periodic Table

Definition of Equilibrium constant

The equilibrium between reactants and products is described by an equilibrium constant . For the balanced reaction: aA + bB <--> cC + dD The equilibrium constant, Keq is defined as: [C]c [D]d Keq = --------- [A]a [B]b where the [] brackets indicate the concentration of the chemical species. Rules for Writing K Expressions

  • Products are always in the numerator.
  • Reactants are always in the denominator.
  • Express gas concentrations as partial pressure, P, and dissolved species in molar concentration, [].
  • The partial pressures or concentrations are raised to the power of the stoichiometric coefficient for the balanced reaction.
  • Leave out pure solids or liquids and any solvent. Only variables will be in a K expression: partial pressure of gases and concentrations of solutes in solution.

    Example:

    Zn (s) + 2 H+(aq) <--> Zn2+(aq) + H2 (g) PH2 [Zn2+] K = ----------- [H+]2 Specific Equilibrium Constants

    The equilbrium constant has specific names for several classes of reactions:

  • Gas-phase reactions that use units of partial pressure: Kp
  • Dissociation of water: dissociation constant of water, Kw
  • Dissociation of acids: acid dissociation constant, Ka
  • Reaction of bases with water: base hydrolysis constant, Kb
  • Solubility of precipitates: solubility product, Ksp
  • Formation of complexes: formation constant, Kf

    Each of these classifications of reactions will have a convention for how to write the direction of the reaction. The different conventions are illustrated in the introduction to reactions document.




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