Chapter 7 Electrochemistry What is electrochemistry? A science that studies the relation between...
Transcript of Chapter 7 Electrochemistry What is electrochemistry? A science that studies the relation between...
Chapter 7 Electrochemistry
What is electrochemistry?
A science that studies the relation between electric and
chemical phenomena and the disciplines that govern the
conversion between electric and chemical energies.
Main contents
• Section 1: Electrolyte and electrolytic solution
• Section 2: Electrochemical Thermodynamics:
• Section 3: Irreversible electrochemical system
• Section 4: Applied electrochemistry
Chapter 7 Electrochemistry
§7.1 Electrolyte and electrolytic solution
Main contents:
1) Electrolyte: origin of the concept
2) Existence of ions in solution
3) Hydration theory:
4) Interionic interaction
5) Motion under electric field
6) Conducting mechanism
7) Faraday’s law and its application
7.1.1 Origin of the concept – electrolyte
An electrolyte is a substance that, when dissolved in
solvent, produces a solution that will conduct
electricity.
1) Definition of electrolyte
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 expressed 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.
The equation for freezing point depression and boiling po
int elevation contains the letter i. i stands for the van’t Hoff
Factor.
∆T = imKf
Since freezing point depression and boiling point elevation
depend only on the number of particles ( it does not matter w
hat the particles are), we need only determine the total m of th
e particles.
If a solution is 0.2 m NaCl, the i would be about 2. The tru
e van’t Hoff factor is not exactly 2, but is close enough to call
it 2.
http://en.wikipedia.org/wiki/Van_'t_Hoff_factor
In 1887, Svant August Arrhenius postul
ated that, when dissolved in adequate so
lvent, some substances can split into sm
aller particles, the process was termed a
s 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
Therefore, the number of particles present in solution is
actually larger than that predicted by van’t Hoff, which
resulted van’t Hoff factor.
New definitions:
Dissociation, ionization
Weak / strong electrolyte? True and potential?
Theory of Electrolytic Dissociation
Acid-base theory
Greenhouse effect
Cf. Levine p.295
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.
ionPrimary hydration shell
secondary hydration shell
Disordered layer
Bulk solution
Solvation shells The interaction between ions and water molecules disturb the structure of liquid water.
Hydration of ion
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
7.1.3 Hydration Theory / Solvation TheoryH
/ kJ
mol
-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?
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.
20
21
4 r
qqF
r Long-range forces
20
21
4 r
qqF
r
At low concentration
At medium concentration
At high concentration
+ + +
Cf. Levine, p. 304 In equilibrium -- Bjerrum
7.1.4 Interaction between cation and anion
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.
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 for quite dilute solutions
For concentration-dependence of ion pair, see Levine p. 305, Figure 10.10
(1) Category of conductor:
Charge carriers:
7.1.5 Conducting mechanism of electrolyte
PbO2, NiOOHIon and electronMixed conductor
Superconductors electron pair5th
Conducting polymerspolaron4th
SemiconductorElectron and hole3rd
Electrolytic solution, solid-state electrolyte (Al2O3, ZrO2)
ion2nd
Metals, carbonous materials, some metal oxides
electron1st
samplesCharge carrierConductor
electron; ion; hole; Cooper electron pair; polaron.
(2) Conducting mechanism
•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
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.
7.1.7 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 1791-1867Invent 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.
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.
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
A 0.100 molality (mol/kg) solution of NaCl has a freezing-poi
nt depression of -0.348 oC, whereas the expected decrease in the f
reezing point is -0.186 oC. The van’t Hoff factor in this case is 1.
87. If there were no ion pairing, we would expect the van’t Hoff f
actor for NaCl to be 2.00. Similarly, acetic acid in a 0.100 molal
solution has a van’t Hoff factor of 1.05. Calculate the concentrati
on of NaCl ion pairs and also the percent ionization of acetic acid
form the above information.
Exercise-1:
A current of 2.34 A is delivered to an electrolytic cell for 85
min. how many grams of (a) Au from AuCl3, (b) Ag form
AgNO3, and (c) Cu from CuCl2 will be plated out?
Exercise-3
Levine: p.317 10. exercise 48
Exercise -4
Yin: p. 217 exercise 1.
Exercise-2: