# Activity.ppt by dffhrtcv3

VIEWS: 5 PAGES: 10

• pg 1
```									                  Equilibrium
• Equilibrium Constant, K (or Keq) describes
conditions AT equilibrium

 [ products]n
K     i

 [reactants]n
i

CaCO3(calcite) + H+  Ca2+ + HCO3-

[Ca 2 ]1[ HCO3 ]1
K eq 
[H  ]1
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=Xigi

Xi=mole fraction of component i
• For gases:          Pi = partial pressure of component i
mi = molal concentration of component i
ai=Pigi = fi

• For aqueous solutions:
ai=migi
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 = gimi
• The activity coefficient, gi, is found via
some empirical foundations

• Dependent on the other ions in water…
Dissolved species gi
• First must define the ionic strength (I) of the
solution the ion is in:

I   mi z i   2

i

Where mi is the molar concentration of species i
and zi is the charge of species I
Activity Coefficients
• Debye-Huckel approximation (valid for I:
1
2    2
Az I
 log g               1
I  aBI       2

• 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 law
• Debye-Huckel
• Davies
• TJ, SIT
models
• Pitzer, HKW
models
Neutral species
• Setchnow equation:
• Logan=ksI
For activity coefficient (see table 4-2 for
selected coefficients)

```
To top