Chapter 8 Flashcards
Thermodynamic equilibrium constant expression
wW+xX->yY+zZ
K=([Y]^y[Z]^z)/([W]^w[X]^x)
The experimentally determined K values depend upon the electrolyte concentration of the medium. This happens even when the added electrolyte contains no ion in common with those involved in equilibria.
Concentration-based equilibrium constants are indicated by
adding a prime mark, ex: Kw’,Ksp’,Ka’
As the electrolyte concentration
becomes very small,
concentration based equilibrium
constants approach their
thermodynamic values: Kw, Ksp ,
Ka .
effect of charge of reactants of products
- The magnitude of the electrolyte effect is highly dependent upon the charges of the participants in an equilibrium.
- For neutral species, no effect on the added electrolyte.
- With ionic participants, the magnitude of the electrolyte effect increases with
charge
Source of the salt effect
An ionic atmosphere, shown as a spherical cloud of charge +
and -, surrounds ions in solution. The charge of the atmosphere
is less than the charge of the central ion. The greater the ionic
strength of the solution, the greater the charge in each ionic
atmosphere.
As a result of this effect a decrease in overall attraction between
cation and anions and an increase in solubility.
Activity
To quantitatively describe the effective concentration of participants in
an equilibrium at any given ionic strength, activity, a is used.
The activity, a of a species X is defined as
ax=vx[X], vx- activity coefficient, ,varies with ionic strength, u
the Ksp of a precipitate, XmYn is defined by
Ksp=(vx^m)(vy^n)[X]^m[Y]^n=vx^mvy^nKsp’
Where Ksp’ is th econcentration solubility product and Ksp is thermodynamic solubility product
The activity of a species
is a measure of its effective concentration as determined by colligative properties by electrical conductivity and by the mass action effect
Debye huckel equation
the theoretical expression that permits the calculation of activity
coefficients of ions from their charge and their average size
-logvx=(0.51(Zx^2 sqrt(U)))/ (1+3.3aX(sqrt(u))) at 25 c
where vX= activity coefficient of the species X
Zx is the charge on the species X
u= ionic strength of the solution
aX=effective diameter of the hydrated ion X is nanometers
Effect of ionic strength
ionic strength= u=1/2([A}ZA^2+[B}ZB^2+[C]ZC^2….
where [A],[B],[C]… represent the species molar concentration of ions A,B,C,… and ZA,ZB,ZC are their charges
Properties of activity coefficients,vX
- The activity coefficient of a species is a measure of the effectiveness with which that species influences an equilibrium in which it is a participant. In dilute solutions, where the ionic strength is minimal, this effectiveness becomes constant, and the activity coefficient is unity.
- In solutions that are not too concentrated, the activity coefficient for a given species is independent of the nature of the electrolyte and
dependent only upon the ionic strength. - For a given ionic strength, the activity coefficient of an ion departs
farther from unity as the charge carried by the species increases. - At any given ionic strength, the activity coefficient of ions of the
same charge are approximately equal. - The activity coefficient of a given ion describes its effective behavior
in all equilibria in which it participates
interpolation
(unknown y interval)/(triangle y)= (known x interval)/ (triangle x)
pH definition
pH=-log a H+=-log[H+]vH+
equilibrium calculations employing activity coefficients
dealt with examples
