equilibrium
DESCRIPTION
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… - PowerPoint PPT PresentationTRANSCRIPT
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EquilibriumEquilibrium Constant, K (or Keq) describes conditions AT equilibrium
CaCO3(calcite) + H+ Ca2+ + HCO3-
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ActivitySometimes 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..
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ActivityFor solids or liquid solutions:ai=Xigi
For gases:ai=Pigi = fi
For aqueous solutions:ai=migiXi=mole fraction of component iPi = partial pressure of component imi = molal concentration of component i
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Activity CoefficientsWhere do they come from??The standard state for dissolved ions is actually an infinitely dilute solutionActivity of phases - gases, minerals, and bulk liquids (H2O) are usually pretty close to 1 in watersDissolved molecules/ ions have activity coefficients that change with concentration (ions are curved lines relating concentration and activity coefficients, molecules usually more linear relation)
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Application to ions in solutionIons in solutions are obviously nonideal mixtures! ai = gimiThe activity coefficient, gi, is found via some empirical foundations
Dependent on the other ions in water
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Dissolved species giFirst 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
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Activity CoefficientsDebye-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)
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Different ways to calculate gi Limiting lawDebye-HuckelDaviesTJ, SIT modelsPitzer, HKW models
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Neutral speciesSetchnow equation:Logan=ksIFor activity coefficient (see table 4-2 for selected coefficients)