EquilibriumEquilibrium Constant, K (or Keq) describes conditions AT equilibrium

CaCO3(calcite) + H+ Ca2+ + HCO3-

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..

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

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)

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

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

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)

Different ways to calculate gi Limiting lawDebye-HuckelDaviesTJ, SIT modelsPitzer, HKW models

Neutral speciesSetchnow equation:Logan=ksIFor activity coefficient (see table 4-2 for selected coefficients)