Aqueous Environmental Geochemistry

Document Sample
Aqueous Environmental Geochemistry Powered By Docstoc
					                                Fall 2007




Aqueous Environmental
    Geochemistry

          Brooklyn College
   The City University of New York
Natural Systems
    Open and Close Natural Systems

•   Natural systems can be large or small
•   Different phases
•   In relative sense
•   Reaction rates vs. Flux rates
         Gibbs Phase Rule
• F=C–P+2
  What do C and P mean?

• Example:
    H2O (ice) = H2O (liquid) = H2O (vapor)

• F = C’-R – P + 2:
     What are C’ and R? Give one example.
                Ehthalpy (H)
•   Heat content at constant pressure
•   Heat of formation from elements
•   ΔHf0 for (most stable form) element is zero
•   ΔHr0 : Heat exchange
       ΔHr0 : H>0, endothermic
       ΔHr0 H<0, exothermic
• How T & P change affect the equilibrium of
  a reaction?
                Entropy (S)
• Degree of randomness or disorder
                      T   Cp
       ST  S 0            dT
                     0    T
• Cp: heat capacity. Energy needed to raise the
  temperature of 1 mole by 1 K
• At 0 K, most substances S=0
• S increases as temperature increases
      Gibbs Free Energy (G)
• At equilibrium condition:
           ΔGr0 = - RT ln Keq
           R: gas constant, 1.9872 cal/mol K
           T: 25 ºC, or 298.15 K
• Can be transformed into:
           ΔGr0 = -0.59248 ln Keq
                = -1.3642 log Keq
     Direction of Reaction

        ΔGr = ΔGr0 + RT ln Q
• Q: reaction quotient
• Sign of ΔGr indicate the direction of reaction
         Chemical Potential (µ)
• Similar to Gibbs Free Energy
• µi = µi0 + RT ln(αi)
  - Chemical potential is for individual component

  - αi is activity, which is different than concentration!

• At equilibrium, is the same for the same
  component in different phases!
                         Solutions
• In solutions
               (αi) = λi · Ni
  -λ   is the rational activity coefficient
  - for ideal solutions, λ ≈ 1 for major components (Raoult’s Law);
  - for ideal solutions, λ ≠ 1 for minor components or dilute solutions
  (Henry’s Law)
  - See Fig. 1.3 for differences
     Solid Solutions (binary mixing)
• Only equations to remember:
     ln λA = α0 NB2
     ln λB = α0 NA2

 Can be treated as exchange reaction:

               KAX   AmA    BNB 
         Kex                    
               KEX   BmB    ANA 
              Keq Dependence on T
•   Empirical constants
•   Determined in experiments
•   Table 1.1 (P39-45)
•   Equation:

                                        F
    log Keq  A  BT   D log T  ET 
                       C
                       T
                                      2
                                            GT 1/ 2

                                        T2
• Calculate ΔGr0 , ΔHr0 ΔCpr0 ΔSr0
• ΔHr0 controls the dependence.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:5
posted:9/14/2012
language:English
pages:12