Aqueous Environmental Geochemistry

Document Sample
Aqueous Environmental Geochemistry Powered By Docstoc
					                                Lecture 3




Aqueous Environmental
    Geochemistry

          Brooklyn College
   The City University of New York
         Activity vs. Concentration

              ai  i  Ni
              Here
              ai is activity,
              i is activity coefficient,
              N i is mole fraction of a conponent

Case 1: when Ni  1 (major components), i  1, obeys Raoults Law
Case 2: when Ni  0 (minor components), i  1, obeys Henry's Law
       Henry’s Law Constant
• For Gases

     [Gas (aq )]  K H  Pgas


• For Solutions

           ai   i  mi
   Elementary and Overall Reactions

• Examples
   CO2 (aq)  OH   HCO3
   CaCO3 ( s)  CO2 ( g )  H 2O  Ca 2  2HCO3

• Rate laws more dependent on elementary reactions.
• Write the rate law for reaction A+2B=C

         d ( A)    d ( B)    d (C )
                2                  K  ( A)( B ) 2
           dt        dt        dt
                K
         K eq   for reaction A  B
                K
      Pseudo first-order reaction
• One of the reactants can be assumed constant.

        Fe3  SO4 2  FeSO4 
             d [ Fe3 ] d [ SO4 2 ]
        R                           K  [ Fe3 ][ SO4 2 ]
                 dt          dt
• In seawater, [SO42-] is very large at 0.028 M, assuming constant


                                         3
                 R  K  [ Fe ]*
        Activation Energy (Ea)
• Arrhenius expression
                        Ea
           K  AF exp(    )
                        RT
                               Ea
           log K  log AF 
                            2.303RT
• Plot logK and 1/T will yield a straight line, with
  slope of –Ea/(2.303R), interception of log AF.
       Activation Energy (Ea)
• Assuming at two temperatures T1 & T2
                                  Ea
           log KT 1  log AF 
                               2.303RT1
                                    Ea
           log KT 2    log AF 
                                 2.303RT2


         KT 1   Ea   1 1
     log           (  )
         KT 2 2.303R T2 T1
Activation Energy (Ea)


   Activation energy ranges for different
    types of reactions, see P62 Table.
   Rule of Thumb: rates of reactions
    roughly double for every 10 degree
    (celcius) increase in temperature.
              Oxidation of Ferrous Iron
 Below pH=2.2
          1           1
         2                        3
     Fe  O2  H  Fe  H 2O
         4             2

 pH=2.2-3.5
                  1        1
        Fe 2  O2  H 2O  FeOH 2
                  4        2
         d [ Fe( II )]
                         K  [ Fe( II )]  PO 2
              dt
 pH>4
                  1       5
              2
         Fe  O2  H 2O  Fe(OH )3  2 H 
                  4       2
         d [ Fe( II )]       [ Fe2 ]
                        K          P
                                  2 O2
              dt             [H ]

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:7
posted:9/14/2012
language:English
pages:9