Chemical Equilibrium

Chem-Eng.21

Chemical Equilibrium

       When a chemical reaction takes place in a container which prevents the entry or escape of any of the substances involved in the reaction, the quantities of these components change as some are consumed and others are formed. Eventually this change will come to an end, after which the composition will remain unchanged as long as the system remains undisturbed. The system is then said to be in its equilibrium state, or more simply, "at equilibrium".

A chemical reaction is in equilibrium when there is no tendency for the quantities of reactants and products to change.

• There is never actually a complete conversion of reactants to product in a chemical reaction, there is only a chemical equilibrium. 
• A chemical equilibrium state occurs when the ratio of concentration of reactants and products is constant.  An equilibrium-constant expression is an algebraic equation that describes the concentration relationships that exist among reactants and products at equilibrium.
  
  

  aA + bB <=> cC + dD

  
  

  K = [C]c [D]d / [A]a [B]b

  
  

  
  

  
  

  Typical Equilibrium Constant Expressions

   

  • Dissociation of water

  2H2O « H3O+ + OH-      Kw = [H3O+ ][OH-] 

  • Acid base

  NH3 + H2O « NH4+ + OH-       Kb  = [NH4+][OH-] / [NH3]

  • Solubility

  PbI2(s) « Pb2+  +  2I-              Ksp  = [Pb2+ ][I-]2

  • Oxidation-Reduction 

  IO3- + 5I- + 6H+ « 3I2(aq) + 3H20      Keq = [I2]3 / [IO3-][I-]5[H+]6
  Cl2(g) + 2AgI(s) « 2AgCl(s) + I2 (g)    Keq = pI2/ pCl2