Upload
vukhanh
View
242
Download
3
Embed Size (px)
Citation preview
§7.1 Electrolyte and electrolytic solution
Out-class reading:
Levine, pp. 294-310
Section 10.6 solutions of electrolytes
Section 10.9 ionic association
pp. 512-515
Section 16.6 electrical conductivity of
electrolyte solutions.
Contents of solution chemistry
(1) non-electrolytic solution: Colligative Property and activity
(2) electrolytic solution: conductivity and mean activity
(3) Solution reaction: kinetics and mechanism
Main contents:
1) Electrolyte: origin of the concept
2) Existence of ions in solution
3) Ion-dipole interaction--Hydration theory
4) Interionic interaction
5) Motion under electric field
6) Conducting mechanism
7) Faraday’s law and its application
§7.1 Electrolyte and electrolytic solution
An electrolyte is a substance that, when dissolved in solvent,
produces a solution that will conduct electricity.
1) Definition of electrolyte
Progress of the definition:
(1) molten salt;
(2) solid-state electrolyte: Al2O3, PEO and Nafion;
(3) room-temperature ionic liquids (RTIL).
§7.1 Electrolyte and electrolytic solution
In 1886, van’t Hoff published hisquantitative research on the colligativeproperties of solution.
For sucrose, the osmotic pressure () canbe expressed as:
= c R T
But for some other kind of solvates suchas NaCl, the osmotic pressure had to bemodified as:
= i c R T
i , van’t Hoff factor, is larger than 1.
2) Dissociation of substance
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.
§7.1 Electrolyte and electrolytic solution
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.
+ +
3) Dissociation theory for weak electrolytes
§7.1 Electrolyte and electrolytic solution
New definitions:
Ion, cation, anion;
Dissociation, ionization
Weak / strong electrolyte?
True / potential electrolyte?
Theory of Electrolytic Dissociation
Acid-base theory
Cf. Levine p.295
§7.1 Electrolyte and electrolytic solution
Solvated (hydrated) ion
2. Ions in solution
In what state do ions exist in solution?
ion
Primary hydration shell
secondary hydration shell
Disordered layer
Bulk solution
Solvation shells
§7.1 Electrolyte and electrolytic solution
Hydration of ion
Coordination number:
Li+: 4, K+: 6
Primary solvation shell:
4-9, 6 is the most common number
Secondary solvation shell:
6-8, for Al3+ and Cr3+: 10-20
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.
§7.1 Electrolyte and electrolytic solution
3. Hydration Theory / Solvation Theory
H/
kJ
mo
l-1
4NaCl(s)
Na+(aq) + Cl(aq)
Na+(g) + Cl(g)
788 784
hydration energy:
784 kJ mol-1
1948, Robinson and Storks
Why does NaCl only melt at higher temperature, but dissolve
in water at room temperature?
§7.1 Electrolyte and electrolytic solution
The interionic distance for NaCl crystal is 200 pm, while for 0.1
moldm-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.
2
0
21
4 r
qqF
r Long-range forces
4. Interionic interaction
§7.1 Electrolyte and electrolytic solution
2
0
21
4 r
qqF
r
At low
concentration
At medium
concentration
At high
concentration
+ + +
Cf. Levine, p. 304 In equilibrium -- Bjerrum
§7.1 Electrolyte and electrolytic solution
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.
§7.1 Electrolyte and electrolytic solution
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
Some facts about strong electrolytes
Degree of association
Activity coefficient is essential even for quite dilute solutions
For concentration-dependence of ion pair, see Levine p. 305, Figure 10.10
§7.1 Electrolyte and electrolytic solution
•Electric transfer of ion in
solution under electric field
+
+
+
+
+
+
+
+
+
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 ?
I
E
5. Conducting mechanism of electrolyte
§7.1 Electrolyte and electrolytic solution
Cl
e
e
e
At cathode:
2H+ + 2e H2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
H+
e
H+
e
H+
e
H+
H+
H+
H+
H+
H+
Cl
At anode:
2Cl 2e Cl2
H+
Cl
Conducting mechanism:
1) Transfer of ion in solution under electric field;
2) electrochemical reaction at electrode/solution interface.
§7.1 Electrolyte and electrolytic solution
6. Law of electrolysis
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.
MzF
Qm
For quantitative electrolysis:
Micheal Faraday
Great Britain 1791-1867
Invent the electric motor
and generator, and the
principles of electrolysis.
Faraday’s Law
Faraday’s constant
F = (1.6021917 10-19 6.022169 1023 )
C·mol-1 = 96486.69 C·mol-1 usually round off
as 96500 C·mol-1, is the charge carried by 1
mole of electron.
§7.1 Electrolyte and electrolytic solution
Current efficiency ()
effective
ltheoretica
Q
Q
ltheoretica
effective
m
m
Current efficiency is lower than 100% due to side-reactions.
For example, evolution of hydrogen occur during electro-
deposition of copper.
§7.1 Electrolyte and electrolytic solution
1) Definition of ampere:
IUPAC: constant current that would deposit 0.0011180 g of
silver per second from AgNO3 solution in one second: 1
ampere.
Application of Faraday’s law
2) Coulometer: copper / silver / gas (H2, O2) coulometer
3) Electrolytic analysis – electroanalysis
Q ↔m ↔ n ↔ c
§7.1 Electrolyte and electrolytic solution
7. Transfer of ion under electric field
1) Ionic mobility
d
d
E
l
d
d
EU
l
Ionic mobility (U) :
the ionic velocity per unit electric field, is a constant.
Rate of electric transfer: Ionic velocity
How do we describe the motion of ions under electric field?
§7.1 Electrolyte and electrolytic solution
MA, MA’ have an ion in common.
The boundary, rather different in
color, refractivity, etc. is sharp.
measure ionic mobility using moving boundary method
xv
t
v xU
V tE
l
§7.1 Electrolyte and electrolytic solution
I = I+ + I-
Q = Q+ + Q-
j j
j
I Qt
I Q
t+ + t- = ?
8. Transference number
Transference number (transfer/ transport number), is the
fraction of the current transported by an ion.
plane A
I-
I+
I
+ ++
I Qt
I Q - -
-
I Qt
I Q
Supporting electrolyte?
§7.1 Electrolyte and electrolytic solution
For time t:
Q+ = A U+t c+ Z+ F
Q = A Ut c Z F
(1) Principle for measuring transference number
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?
A
I+
BC
tU
§7.1 Electrolyte and electrolytic solution
(1) Principle of Hittorf method (1853)
Example: Electrolysis of HCl solution
When 4 Faraday pass through the electrolytic cell
anodic region cathodic regionbulk solution
+ + + + + + + + + + + + + + + + + +
+ = 1 F
+ + + + + + + + + + + + + + + + + +
4Cl- -4e- 2Cl2 4H+ +4e- 2H23 mol H+1 mol Cl-
3 mol H+1 mol Cl-
§7.1 Electrolyte and electrolytic solution
anodic region cathodic regionbulk solution
+ + + + + + + + + + + + + +
For anodic region:
transferedreactedinitialresidual cccc
final
result
+ + + + + + + + + + + + + + + + + +
4Cl- -4e- 2Cl2 4H+ +4e- 2H23 mol H+1 mol Cl-
3 mol H+1 mol Cl-
§7.1 Electrolyte and electrolytic solution
EXAMPLE
Pt electrode, FeCl3 solution:
In cathode compartment:
Initial: FeCl3 4.00 mol·dm-3
Final: FeCl3 3.150 mol·dm-3
FeCl2 1.000 mol·dm-3
Calculate the transference
number of Fe3+
Hittorf’s transference cell
Anode
chamber
Cathode
chamber
Cock stopper
What factors will affect the
accuracy of the measurement?
§7.1 Electrolyte and electrolytic solution
§7.1 Electrolyte and electrolytic solution
(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?
§7.1 Electrolyte and electrolytic solution
(1) Temperature and (2) Concentration
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
Transference number of K+ in KCl solution at different
concentration and temperature
T /℃
c /mol·dm-3
(3) Influential factors
§7.1 Electrolyte and electrolytic solution
(3) Co-existing ions
Electrolyte KCl KBr KI KNO3
t+ 0.4902 0.4833 0.4884 0.5084
Electrolyte LiCl NaCl KCl HCl
t– 0.6711 0.6080 0.5098 0.1749
Table transference number on co-existing ions
Problem:
Why does the transference number of certain ion differ a lot in
different electrolytes?
§7.1 Electrolyte and electrolytic solution