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Activity.ppt

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					                  Equilibrium
• Equilibrium Constant, K (or Keq) describes
  conditions AT equilibrium

                       [ products]n
                K     i

                      [reactants]n
                       i




         CaCO3(calcite) + H+  Ca2+ + HCO3-
                                      
                       [Ca 2 ]1[ HCO3 ]1
                K eq 
                             [H  ]1
                 Activity
• Sometimes called ‘effective concentration’,
  which is misleading and reflects a poor
  understanding of the property…

• Think of more of the effect the rest of a
  solution has on how easily two ions come
  together..
                    Activity
• For solids or liquid solutions:
    ai=Xigi

                      Xi=mole fraction of component i
• For gases:          Pi = partial pressure of component i
                      mi = molal concentration of component i
     ai=Pigi = fi

• For aqueous solutions:
     ai=migi
          Activity Coefficients
• Where do they come from??
• The standard state for dissolved ions is
  actually an infinitely dilute solution…
• Activity of phases - gases, minerals, and bulk
  liquids (H2O) are usually pretty close to 1 in
  waters
• Dissolved molecules/ ions have activity
  coefficients that change with concentration
  (ions are curved lines relating concentration
  and activity coefficients, molecules usually
  more linear relation)
  Application to ions in solution
• Ions in solutions are obviously nonideal
  mixtures!
                 ai = gimi
• The activity coefficient, gi, is found via
  some empirical foundations

• Dependent on the other ions in water…
          Dissolved species gi
• First must define the ionic strength (I) of the
  solution the ion is in:

               I   mi z i   2

                      i

Where mi is the molar concentration of species i
 and zi is the charge of species I
         Activity Coefficients
• Debye-Huckel approximation (valid for I:
                             1
                        2    2
                      Az I
           log g               1
                   I  aBI       2

• Where A and B are constants (depending
  on T, see table 10.3 in your book), and a is
  a measure of the effective diameter of the
  ion (table 10.4)
    Different ways to calculate gi

• Limiting law
• Debye-Huckel
• Davies
• TJ, SIT
  models
• Pitzer, HKW
  models
            Neutral species
• Setchnow equation:
• Logan=ksI
  For activity coefficient (see table 4-2 for
  selected coefficients)

				
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posted:2/15/2013
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