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: For gases: ai=Pigi = fi
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

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