Equilibrium• Equilibrium Constant, K (or Keq) describes

conditions AT equilibrium

i

ni

n

reactants

productsK

][

][

1

13

12

]H[][][

HCOCaKeq

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

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..

Activity

• For solids or liquid solutions:ai=Xii

• For gases:ai=Pii = fi

• For aqueous solutions:ai=mii

Xi=mole fraction of component iPi = partial pressure of component imi = molal concentration of component i

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)

Application to ions in solution

• Ions in solutions are obviously nonideal mixtures!

ai = imi

• The activity coefficient, i, is found via some empirical foundations

• Dependent on the other ions in water…

Dissolved species i

• 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

2izmI

ii

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)

21

21

2

logaBII

IAz

Different ways to calculate i

• Limiting law• Debye-Huckel• Davies• TJ, SIT

models• Pitzer, HKW

models

Neutral species

• Setchnow equation:• Logn=ksI

For activity coefficient (see table 4-2 for selected coefficients)