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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 ExpressionsProducts 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 ConstantsThe equilbrium constant has specific names for several classes of reactions: Gas-phase reactions that use units of partial pressure: KpDissociation of water: dissociation constant of water, KwDissociation of acids: acid dissociation constant, KaReaction of bases with water: base hydrolysis constant, KbSolubility of precipitates: solubility product, KspFormation 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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