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Equilibrium

Equilibrium. Equilibrium Constant, K (or K eq ) describes conditions AT equilibrium. CaCO 3(calcite) + H +  Ca 2+ + HCO 3 -. Activity. Sometimes called ‘effective concentration’, which is misleading and reflects a poor understanding of the property…

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Equilibrium

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  1. Equilibrium • Equilibrium Constant, K (or Keq) describes conditions AT equilibrium CaCO3(calcite) + H+ Ca2+ + HCO3-

  2. 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..

  3. Activity • For solids or liquid solutions: ai=Xigi • For gases: ai=Pigi = fi • For aqueous solutions: ai=migi Xi=mole fraction of component i Pi = partial pressure of component i mi = molal concentration of component i

  4. 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)

  5. 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…

  6. Dissolved species gi • First must define the ionic strength (I) of the solution the ion is in: Where mi is the molar concentration of species i and zi is the charge of species I

  7. Activity Coefficients • Debye-Huckel approximation (valid for I: • 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)

  8. Different ways to calculate gi • Limiting law • Debye-Huckel • Davies • TJ, SIT models • Pitzer, HKW models

  9. Neutral species • Setchnow equation: • Logan=ksI For activity coefficient (see table 4-2 for selected coefficients)

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