Validation of debye huckel law onsager equation presented by ameer Hamza

2. Ghazi University dgk (city campus)

3. Presentation
Validation of debye huckel onsager equation
Presented by Ameer hamza
Ghazi university Dear ghazi Khan

4. Experiment Verification to check the validity of Debye
Huckle limiting law
Several experimental methods for calculating activity
coefficients have been tried to check the validity of
Debye–Hückel theory:
The main problem is that we need extremely high dilutions.
Main problem

5. Some other problems
1. Measurements of the freezing point,
2.
4. Electric potential
Measurement of vapour pressure
3. Osmotic pressure (which is an indirect method)

6. Method to check validity
Using liquid membrane cells, it was possible to examine the
aqueous media 104 M at high dilutions and obtain good results
and it also has been found that for the ratio 1:1 electrolytes (as
either KCl or NaCl) the Debye–Hückel equation is completely
correct, but for the ratio 2:2 or 3:2 electrolytes it can be possible
to find the negative deviation from Debye–Hückel limit law: this
strange behavior may be noticed only in the very dilute area, and
in many concentrate regions the deviation becomes positive.

7. •
It is also possible that Debye–Hückel equation
is unable to foresee this particular behavior
because of the linearization of
Poisson–Boltzmann equation, or may be not:
about this, studies have been started only
during the last years of the 20th-century
because prior to it, it was not possible to
investigate the region of 10−4 M, so it can be
possible that during the next years new theories
will be come up.

8. • The equivalent conductance is usually a linear function of
the square root ofthe concentration. The validity of the
Onsager equation is proved by showing thatthe slope of
the line is numerically equal to A+Bλ 0 where the values
of A and B areknown. The Onsager equation is regarded
as a limiting expression applicable to

9. very dilute solutions only since the ionic atmosphere is
related with 1/k. Theconductances for solutions of low
concentration have to be obtained in order totest the
accuracy of the Onsager equation. The observed equivalent
conductanceof aqueous solutions of a few uni-univalent
electrolytes are plotted against thesquare roots of the
corresponding concentrations. The theoretical slopes of
thestraight lines to be expected from the Onsager equation
calculated from thevalues of A and B

10. in connection with an estimated equivalent conductance
atinfinite dilution are shown by the dotted lines in the figure.
It can be seen fromthe graph that for aqueous solutions of
the uni-univalent electrolytes, theOnsager equation is very
closely obeyed at concentrations up to about 2x10 -
3equivalents per liter.

11. The validity for Onsager theory is also provided by
conductancemeasurements of a number of electrolytes
made at 0 0 C and 100 0 C. At bothtemperatures, the
observed slope of the plot of agrees with the calculated
resultwithin the limits of experimental error.Thus the
Onsager equation represents the dependence of the
concentrationof the equivalent conductances of uni-
univalent and uni-bivalent (or bi-uni-)electrolytes. But major
discrepancies are observed for bi-bivalent solutes since

12. • the plot of λ against square root of the concentration is
not a straight line but isconcave to the axis of (C) ½ . The
shapes of the curves indicate that in sufficientlydilute
solutions the slopes would be very close to the theoretical
Onsager values.

13. For methyl alcohol solutions, chlorides and thiocyanates of
the alkali metals,the experimental values closely agree with
the theoretical values. But nitratesolutions, tetra alkyl
ammonium salts and salts of higher valence types
showappreciable deviations. The discrepancies are more
prominent when the dielectricconstant of the medium is low.
The conductance of KI in a number of solvents
wasdetermined and the slopes of the plots of λ against (C)
½ found out. At higher

14. dielectric constants, there is close agreement between the
experimental andcalculated slopes. But at lower dielectric
constants, greater discrepancies areobserved. The
deviation from theoretical Onsager behavior for non-
aqueoussolutions occurs because many strong electrolytes
which are completelydissociated in water behave as weak,
incompletely dissociated electrolytes insolvents of low
dielectric constant.