chapter 7 activity
TRANSCRIPT
Activity and Activity Coefficients
Experimental Observation:
A precipitate is more soluble in a solution containing an inert salt than it is in water
Ex. BaSO4
2-fold increase in solubility when KNO3 concentration is increased from 0 to 0.02M
Activity and Activity Coefficients
Experimental Observation:
Degree of dissociation increases with the addition of an inert salt
Ex. Acetic Acid
At 0.00 M NaCl, Ka’ = 1.75 x 10-5
At 0.02 M NaCl, Ka’ = 2.29 x 10-5
At 1.01 M NaCl, Ka’ = 3.16 x 10-5
More solubleas conc. of inert salt increases
Activity and Activity Coefficients
• Effect due to electrostatic attractions between ions
• Effect is essentially independent of the kind of electrolyte but depends on the ionic strength ()
memorize = ½ Ci zi2
C = concentration of the ion “i”
z = charge on that ion
Activity and Activity Coefficients
aA + bB = cC + dD
K = [C]c[D]d / [A]a[B]b
Does not predict any effect on the ionic strength
Why?
Activity and Activity Coefficients
activity Activity coefficient
Ac = [C] C
K = ACcAD
d / ABbAA
a
K = Cc[C]c D
d[D]d / Bb[B]b A
a[A]a
Activity and Activity Coefficients
Examples:
Sparingly soluble salt
BaSO4(s) = Ba2+ + SO42-
Ksp = ABaASO4 = [Ba][SO4] BaSO4
pH
pH = - log AH+ = -log [H+] H+
Activity and Activity Coefficients
Weak Acid
HA + H2O = A- + H30+
Ka = ([A-][H3O+] / [HA]) (A-H3O / HA)
To calculate the activity coefficient, use theDebye-Huckel Equation,
(be aware of limitations)
Equation
3051
51.0log
2
Z
Depends on:
Z: charge: diameter of hydrated ion (pm): ionic strength
Ion size (will be given on a test)
Use D-H equation to calculate:
Do not use these values.
Why?
Activity and Activity Coefficients
Properties of Activity Coefficients:
• Measure of the “effectiveness” with which that species influences an equilibrium
0, x 1, ax [X], K’ K
• Dilute solutions, x is independent of the nature of
electrolyte and dependent on
• For a given , x departs farther from unity as the charge increases
Activity coefficient gets smalleras ionic strength increases
