Advanced Electrochemistry

1. 1
In the name of
GOD
Advanced
Electrochemistry
2
ADELK

2. For electrolytic solution:
Dimension: -1 m-1 OR S m-1
electric conductance (G) : G = 1/R
G=   l
Dimension: -1, mho, Siemens, S
conductivity () or spedific conductance:
Definition:  = 1/ 

3. Solution Electric Conductivity (Sm-1)
Seawater 5
Drinking water 0.0005 to 0.05
Deionized water 5.5 x 10-6

4. Influential factors for
conductivity
Concentration
Temperature
Type of electrolyte

5. Influential factors for
conductivity
Concentration
Temperature
Type of electrolyte

6. Influential factors for
conductivity
Concentration
Temperature
Type of electrolyte

7. Strong and Weak Electrolytes
strong electrolyte
substance that dissociates or ionizes completely when dissolved in water

HCl  H O 
weak electrolyte
substance that ionizes only partially when dissolved in water
 
CH3 COOH  H2 O  CH3 COO  H3 O
nonelectrolyte
substance that does not produce ions when dissolved in water
C12 H22O11  H2O  C12 H22O11

9. Influential factors for
conductivity
• Concentration.
• Type of electrolyte
• Temperature
1. Acids and bases have higher
conductance
2. C < 5 mol dm-3,  increases with C
3. For CH3COOH conductance does not
depend on C

10. m


C

1) Definition
Molar conductivity
The conductivity of a solution is approximately
proportional to the concentration
m is the conductivity contributed by 1 mole of
electrolyte between electrodes of 1 m apart

11. Example:
The specific conductivity of a KCl solution with a concentration of 1
mol/dm3 at 25 degrees and a pressure of 1 atmosphere is equal to
0.112 -1cm-1. What is the molar conductivity of KCl in this
solution?
m,KCl

 
0.1121
cm1
103
cm3
C
Molar conductivity

12. Dependence of molar conductivity on concentration
m decreases with
concentration.
Due to the interaction
between ions:
interionic attraction

13. Plotted
m against C
Kohlrausch replotted
m against C1/2

14. Kohlrausch empirical formula
m m
  
 A c
To extrapolate the linear part of m ~ C1/2 at low
concentration to C = 0, m
 can be obtained.
  the limiting value of 
m m at infinite
dilution: limiting molar conductivity. It is the
conductivity of 1 mol of solution at infinite
dilution.

15. Electrolyte m° (S cm2 mol-1)
HCl 426.16
HBr 428.10
NaOH 248.10
NaCl 126.45
KBr 151.80
KCl 149.86
NaNO3 121.55
KNO3 144.96
NH4Cl 149.70
KHCO3 118.00
m
m  A c
  
Kohlrausch law

17. Kohlrausch law
m
m
  
 A c

18. Kohlrausch’s law of independent ionic
mobilities

m
 
 
m m
At infinite dilution, m
 should be the
sum of the separate contributions of the
ions

19. Kohlrausch law was also able to establish experimentally that
°m
can be expressed as the sum of contributions from its
individual ions.
 and − are the limiting molar conductivity of cations and anions
 and − are the numbers of cations and anions per formula unit of electrolyte
    −  1 for HCl, NaCl
    −  1 for CuSO4
   1,  −  2 for MgCl2

20. Cation °+ / Scm2mol-1 Anion °- / Scm2mol-1
H+ 349.6 OH- 197.8
Li+ 38.7 Cl- 76.4
Na+ 50.1 Br- 78.2
K+ 73.5 I- 76.8
Fe2+ 108.0 CH3COO- 40.9
Fe3+ 204 CO2-
3 138.6
NH4
+ 73.4 NO-
3 71.5
Ba2+ 127.3 SO2-
4 160.0

21. values for
For acetic acid CH3COOH ("HAc"), we combine the λ0
H3O+ and CH3COO– given in the table:

(CH COOH)  v 
 v 
3    
1(349.6)1(40.9)
 390.5Scm2
mol1

22. limiting molar conductivity of weak
electrolyte

(HAc)  
(H 
)  
(Ac
)
m m m
 
(H 
)
(Cl
)
(Na
)
m m m

(Ac
)
(Na
)
(Cl
)
m m m
 
(HCl)  
(NaAc)  
(NaCl)
m m m

23. Kohlrausch law:
 °m sum of contributions from its individual ions
Calculate m° for a weak electrolyte NH4OH from the ° values for
these strong electrolytes:
NH4Cl: 149.7 Scm2mol-1
NaCl: 126.5 Scm2mol-1
NaOH:248.1 Scm2mol-1
(NH4OH)  (NH4Cl)  (NaOH )(NaCl)
149.7  248.10126.5
 271.3

24. Uses of Kohlrausch’s law
 Calculation of Degree of dissociation and Dissociation Constant
for weak electrolytes
 Calculation of solubility of sparingly soluble salt
 Calculation of Molar Conductivity for weak electrolytes at infinite
dilution