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Concentrations vs. Activities. Activities are not identical to concentrations Result from interactions of charged species Interactions Electrostatic: between charged ions Not complexes – that is dealt with differently Hydration shells: water as a polar molecule

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concentrations vs activities
Concentrations vs. Activities
  • Activities are not identical to concentrations
    • Result from interactions of charged species
  • Interactions
    • Electrostatic: between charged ions
    • Not complexes – that is dealt with differently
    • Hydration shells: water as a polar molecule
  • Interactions influence Gibbs free energy
practical applications
Practical applications
  • Most water less concentrated than seawater:
    • Salinity 3.5%
    • aseawater ≈ 0.98 mseawater
  • Fresh water
    • awater ≈ mwater
  • Recall
    • Activity coefficient, g = a/m
uncharged species
Uncharged Species
  • Uncharged solutes in water close to ideal
    • E.g., a ~ m
    • There is no electrical interaction
    • Only change from hydration of ions
slide4

For uncharged species:

    • g ~ 1 for dilute solution
    • Typically, g > 1 for concentrated solution
    • E.g., a > m
  • Why?
    • Results from hydration of charged species
    • “Removes” water from solution
slide5

Activity coefficient of uncharged species:

g = 100.1I

Where I is ionic strength:

I = ½ Smizi2

ionic strengths
Ionic Strengths
  • Complicated to determine in natural waters:
    • Need a total analyses of all dissolved solids, approximate with major elements
    • Possible to estimate with TDS or SpC
slide7

Values depend on type of solution:

    • I ≈ 2X10-5(TDS); NaCl waters
    • I ≈ 2.5x10-5(TDS); “average” waters
    • I ≈ 2.8x10-5(TDS); CaHCO3 waters
slide8

Or:

    • I ≈ 0.8X10-5(SpC); NaCl waters
    • I ≈ 1.7x10-5(SpC); “average” waters
    • I ≈ 1.9x10-5(SpC); CaHCO3 waters
  • SpC typically closer to ionic strength because it measures charge of solution
charged species
Charged Species
  • Problem
    • To determine gi = ai/mi, need to know how much G varies when m changes
    • Impossible to change just one ion (i), violates electrical neutrality
    • If one ion changes, then another oppositely charged specie changes
    • Generally calculated in terms of uncharge components, e.g., NaClo
slide11

For dilute solution, assume change in single ion concentration

  • Can consider single ion activity coefficient
debye huckel fomulation
Debye-Huckelfomulation
  • Reduction in “effective” concentrations results from:
    • Charged species surrouned by “cloud” of oppositely charge species – adds structure to solution
    • Charged species surrounded by “hydration sheath” – water as polar molecule
slide13

Changes state variables:

    • Less random
    • Increases entropy
    • Reduces the G of the ion
  • Assumptions:
    • Charged species are point charges
    • All interaction are electrostatic
    • Boltzmann distribution around ions