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

Solvated (hydrated) ion

+

7.1.2 State of ion in solution

In what state do ions exist in solution?

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:

Ira N. Levine, Physical Chemistry, 5th Ed., McGraw-Hill, 2002.

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.

Outside class reading