1. 每日一句
凡是有大成功的人，都是有绝顶聪明而肯作笨功夫的人。
胡适致白薇

2. 7 4 3 2 1
30-34
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>85
61.2

3. §7.1 Electrolyte and electrolytic solution
Out-class reading: pp
Section 10.6 solutions of electrolytes
Section 10.9 ionic association pp
Section 16.6 electrical conductivity of electrolyte solutions.

4. Main contents:
Electrolyte: origin of the concept
Existence of ions in solution
Ion-dipole interaction--Hydration theory
Interionic interaction
Motion under electric field
Conducting mechanism
Faraday’s law and its application

5. 1) Definition of electrolyte
An electrolyte is a substance that, when dissolved in solvent, produces a solution that will conduct electricity.
Progress of the definition:
(1) molten salt;
(2) solid-state electrolyte
(3) room-temperature ionic liquids (RTIL).

6. 2) Dissociation of substance
In 1886, van’t Hoff published his quantitative research on the colligative properties of solution.
For sucrose, the osmotic pressure () can be expressed as:
 = c R T
But for some other kind of solvates such as NaCl, the osmotic pressure had to be modified as:
 = i c R T
i , van’t Hoff factor, is larger than 1.
In the paper written in Achieves Neerlandaises (1885) and Transactions of the Swedish. Academy (1886), van't Hoff showed analogy between gases and dilute solutions.

7.  3) Dissociation theory for weak electrolytes  +
In 1887, Svant A. Arrhenius postulated that, when dissolved in adequate solvent, some substances can split into smaller particles, the process was termed as dissociation. +   +
AB  A B –
molecule cation anion
positive ion negative ion
The charged chemical species are named as ions and the process is termed as ionization.

8. New definitions: Ion, cation, anion; Dissociation, ionization
Weak / strong electrolyte?
True / potential electrolyte?
Theory of Electrolytic Dissociation
Acid-base theory
Cf. Levine p.295

9. 2. Ions in solution Solvation shells
In what state do ions exist in solution?
Solvated (hydrated) ion
ion
Primary hydration shell
secondary hydration shell
Disordered layer
Bulk solution
Solvation shells

10. Hydration of ion Coordination number: Li+: 4, K+: 6
The water molecules in the hydration sphere and bulk water have different properties which can be distinguished by spectroscopic techniques such as nuclear magnetic resonance (NMR), infrared spectroscopy (IR), and XRD etc.
Coordination number:
Li+: 4, K+: 6
Primary solvation shell:
4-9, 6 is the most common number
Secondary slovation shell:
6-8, for Al3+ and Cr3+: 10-20

11. 3. Hydration Theory / Solvation Theory
Why does NaCl only melt at higher temperature, but dissolve in water at room temperature?
H / kJ mol-1 4
NaCl(s)
Na+(aq) + Cl(aq)
Na+(g) + Cl(g)
788
784
hydration energy:
784 kJ mol-1
1948, Robinson and Storks

12. 4. Interionic interaction
Long-range forces
The interionic distance for NaCl crystal is 200 pm, while for 0.1 moldm-3 solution is 2000 pm.
To draw Na+ and Cl apart from 200 nm to 2000 nm, the work is:
W (/kJ) = 625 / r
for melting: r =1, W = 625 kJ, m.p. = 801 oC。
for dissolution in water: r = 78.5, W = 8 kJ.
Therefore, NaCl is difficult to melt by easy to dissolve in water at room temperature.

13. At medium concentration + +  + 
At high concentration
At low concentration
At medium concentration
In equilibrium -- Bjerrum
Cf. Levine, p. 304

14. Owing to the strong interaction, ionic pair forms in concentrated solution.
ionic pair vs free ion
In an ionic pair, the cation and anion are close to each other, and few or no solvent molecules are between them. Therefore, HCl does not ionize and NaCl does not dissociate completely in solvents.

15. Activity coefficient is essential for quite dilute solutions
Some facts about strong electrolytes
solution
present species
0.52 mol·dm-3 KCl
95% K+ + 5% KCl
0.25 mol·dm-3 Na2SO4
76 % Na+ + 24% NaSO4¯
0.1 mol·dm-3 CuSO4
44% CuSO4
For concentration-dependence of ion pair, see Levine p. 305, Figure 10.10
Degree of association
Activity coefficient is essential for quite dilute solutions

16. 5. Conducting mechanism of electrolyte
Electric transfer of ion in solution under electric field +  I E
Motion of ions in the solution:
1) diffusion: due to difference in concentration
2) convection: due to the difference in density
3) transfer: due to the effect of electric field
How can current cross the electrode / solution interface ?

17. Conducting mechanism:
Cl e
At cathode:
2H+ + 2e  H2 H+
At anode:
2Cl  2e  Cl2
Conducting mechanism:
Transfer of ion in solution under electric field;
electrochemical reaction at electrode/solution interface.

18. For quantitative electrolysis:
6. Law of electrolysis
For quantitative electrolysis:
Faraday’s Law
where m is the mass of liberated matter; Q the electric coulomb, z the electrochemical equivalence, F a proportional factor named as Faraday constant, M the molar weight of the matter.
Faraday’s constant
F = (    1023 ) C·mol-1
= C·mol-1 usually round off as C·mol-1, is the charge carried by 1 mole of electron.
Micheal Faraday
Great Britain
Invent the electric motor and generator, and the principles of electrolysis.

19. Current efficiency ()
Current efficiency is lower than 100% due to side-reactions.
For example, evolution of hydrogen occur during electro-deposition of copper.

20. Application of Faraday’s law
1) Definition of ampere:
IUPAC: constant current that would deposit g of silver per second from AgNO3 solution in one second: 1 ampere.
2) Coulometer: copper / silver / gas (H2, O2) coulometer
3) Electrolytic analysis – electroanalysis
Q ↔m ↔ n ↔ c

21. 7. Transfer of ion under electric field
How do we describe the motion of ions under electric field?
1) Ionic mobility
Rate of electric transfer: Ionic velocity
Ionic mobility (U) :
the ionic velocity per unit electric field, is a constant.

22. measure ionic mobility using moving boundary method
MA, MA’ have an ion in common.
The boundary, rather different in color, refractivity, etc. is sharp.

23. Supporting electrolyte?
8. Transference number
Transference number (transfer/ transport number), is the fraction of the current transported by an ion.
I = I＋ + I－
Q = Q＋ + Q－
plane A I－ I＋ I
t+ + t- = ?
Supporting electrolyte?

24. (1) Principle for measuring transference number
For time t:
Q+ = A U+t c+ Z+ F
Q  = A Ut c Z F
Owing to electric migration, on the left side of plane A, there are more anions, while on the right side, more cations. Is this real?

25. (1) Principle of Hittorf method (1853)
Example: Electrolysis of HCl solution
= 1 F +
anodic region
cathodic region
bulk solution + 
When 4 Faraday pass through the electrolytic cell + 
4Cl- -4e-  2Cl2
4H+ +4e-  2H2
3 mol H+
1 mol Cl- 

26. 4H+ +4e-  2H2 final result For anodic region: 4Cl- -4e-  2Cl2
4Cl- -4e-  2Cl2
4H+ +4e-  2H2
3 mol H+
1 mol Cl- 
final result
anodic region
cathodic region
bulk solution + 
For anodic region:

27. EXAMPLE What factors will affect the accuracy of the measurement?
Pt electrode, FeCl3 solution:
In cathode compartment:
Initial: FeCl mol·dm-3
Final: FeCl mol·dm-3
FeCl mol·dm-3
Calculate the transference number of Fe3+
Anode chamber
Cathode chamber
Cock stopper
What factors will affect the accuracy of the measurement?
Hittorf’s transference cell

29. (2) Principle for the moving-boundary method
When Q coulomb passes, the boundary moves x, the cross-sectional area of the tube is A, then:
xAcZ+F = Q+ = t+Q
Why is the moving-boundary method more accurate that the Hittorf method?
Are there any other methods for measuring transfer number?

30. (3) Influential factors
(1) Temperature and (2) Concentration
Transference number of K+ in KCl solution at different concentration and temperature
0.000
0.005
0.01
0.02 15
0.4928
0.4926
0.4925
0.4924 25
0.4906
0.4903
0.4902
0.4901 35
0.4889
0.4887
0.4886
0.4885
c /mol·dm-3
T /℃

31. Table transference number on co-existing ions
Electrolyte
KCl
KBr KI
KNO3 t+
0.4902
0.4833
0.4884
0.5084
LiCl
NaCl
HCl t–
0.6711
0.6080
0.5098
0.1749
Problem: Why does the transference number of certain ion differ a lot in different electrolytes?