Common Ion Effect
  • Shift in equilibrium position due to the addition of an 
   ion already involved in the equilibrium process.
  • Involves Le Châtelier’s principle.
                 A Common Ion Effect
    Consider the following equilibrium:
    •                              +            -
       CHCOH(aq)  +  H O(l) ⇌ H O (aq) + CH CO (aq)
          3   2         2       3           3  2
    •  Addition of NaCH CO  shifts equilibrium to the left 
                        3  2
       due to increase of [CH CO -].
                           3   2
    •                  +
       Shift causes [H3O ] to decrease, and pH increases.
    •  Solutions containing a mixture of HC H O  and 
                                         2 3 2
       NaCHO have pH between those of pure HC H O  
           2 3  2                               2  3 2
       and NaC H O .
               2 3 2
      pH of Acetic Acid & Sodium Acetate Solutions
    •
    • Consider the following solutions: 
        
    •  (1) A solution of 1.0 M CH CO H (K  = 1.8 x 10-5) has: 
                              3  2    a
            +
        [H O ] =   = 0.0042 M
          3
           pH = -log(0.0042) = 2.37
     
    •  (2) A solution of 1.0 M NaCH CO  (K  = 5.6 x 10-10) has:
                                3   2  b
           -            -5
        [OH] =  = 2.4 x 10  M;
        pOH = -log(2.4 x 10-5) = 4.62;  pH = 14.00 – 4.62 = 9.38
           pH of Acetic Acid + Sodium Acetate Solution  
        •    Now Consider a solution composed of a mixture of 1.0 M 
        •  
             CHCOH and 1.0 M NaCH CO . What is the pH of this 
                   3      2                                  3      2
             solution?
        •                                                        +                           -
             Equilibrium:  CH CO H(aq)  H (aq)  +  CH CO (aq)
                                           3      2                                  3      2
        
         [Initial]       1.0 M                     0.00        1.0 M
         D[  ]                        -x                 +x                 +x 
         [Equil’m]        (1.0 – x)                        x                (1.0 + x)
        
        (next slide)
          pH of Acetic Acid + Sodium Acetate Solution
       •   (from previous slide)
       •  
       •   K =  =  = 1.8 x 10-5  
             a
       •   By approximation, x << 1.0; (1.0 + x) ~ (1.0 – x) ~ 1.0;
       •                                 -5          +
           Then,   ~ x = 1.8 x 10  M = [H ]
       •                            -5
           pH = -log(1.8 x 10 ) = 4.74
       •   Thus, solution containing a mixture of CH3CO2H and NaCH3CO2 has a 
           pH value between that of the acid solution and the conjugate base 
           solution.