Chapter 7 Electrochemistry

§7.12 Basic principal and application of electrolysis

Electrolysis:

The chemical reactions which accompany the

passage of a current through an electrolytic solution.

Electrode

Positive electrode Negative electrode

Anode Cathode

Reaction Oxidation Reduction

Electrode and reaction:

1. Cathode reaction

Suppose a solution in an electrolytic cell containing Ag+, Cu2+, H+, and Pb2+ of 1 molarity. If the potential is initially very

high and is gradually turned down, in which order will the m

etals be plated out onto the cathode?

1) Order of liberation

Ag+ + e Ag:

⊖ Ag+/Ag = 0.799 V

Cu2+ + 2e Cu:

⊖ Cu2+/Cu = 0.337 V

2H+ + 2e H2:

⊖ H+/H2 = 0.000 V

Pb2+ + 2e Pb:

⊖ Pb2+/Pb = -0.126 V

For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration.

Ag+, Cu2+, H+, and Pb2+ will liberates at 0.799 V; 0.337 V; 0.000 V; -0.126 V, respectively without consideration of overpotential;

Overpotential of hydrogen liberation on Cu is

0.6 V, on Pb is 1.56 V

0.337

⊖ Cu2+/Cu

-0.126

⊖ Pb2+/Pb

0.799

⊖ Ag+/Ag

0.000

⊖ H+/H2

For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential.

a(Ag+) = 1.510-8

0.799 V

a(Cu2+) = 2.210-16

0.337 V

a(Pb2+) = 3.310-49

-0.126 V -1.56 V

The liberation order and the residual concentration of the ions upon negative shift of potential of cathode

Potential sweep: polarization curve

2) Application

1) Separation of metal

2) Quantitative and qualitative analysis

3) Electroplating of single metal and alloy

4) Electrolytic metallurgy

5) Electrorefining of metal

6) Electrosynthesis

⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V;

When Zn begins to plate out, the residual concentration of Cu2+ in the solution can be calculated according to:

2+ 2+ 2+Cu /Cu Cu /Cu Culn

RTa

nF y

(1) Separation of metal

-0.763 = 0.337 + 0.05916lg aCu2+

CCu2+ aCu2+ = 2.5410-19 mol·dm-3

When Zn begins to deposit, Cu has deposited completely.

When the difference between liberation potential of two metals is larger than 0.2 V, the two metal can be separated completely.

(2) Quantitative and qualitative analysis

Polarograph

Polarographic wavenDC

I d (max)

C

CII

s

dd 1(max)

Dropping mercury cathode

N2

A

+

Hg anode

Cu2+

Tl+

E1/2Imax

Polarograph

Jaroslav Heyrovský1959 Noble Prize

Czechoslovakia

1890/12/20 ~ 1967/03/27

Polarography

Progress of the sensitivity of polarogr

aphy

1935: 10-2 ~ 10-5 mol·dm-3

1957: 10-8 ~ 10-9 mol·dm-3

1957: 210-10 mol·dm-3

At present: 1010~1012 moldm-3

(3) Electroplating of single metal and alloy

Anode: Ag Ag++ e

Cathode: Ag+ + e Ag

A silver-plated teapot

Alloy electroplating:

Zn-Fe, Cu-Zn

Composite electroplating:

Ni-PTFE, Ni-Diamond

Electroplating of non-metals:

Plastic, wood, flowers

⊖ Cu2+/Cu = 0.337 V; ⊖ Zn2+/Zn = -0.763 V;

When Zn begins to plate out, the residual concentration of Cu2+ in the solution is

Principle of alloy deposition

Brass can’t deposit from the solution containing Cu2+ and Zn2+.

⊖ Cu(CN)3/Cu = 1.03 V; ⊖ Zn(CN)4

2/Zn = 1.12 V;

< 0.2 V

Zn co-deposits with Cu and form brass. When tin is added, a alloy coating with gold luster can be plated out.

2+ 2+

19 -3

Cu Cu2.54 10 mol dmc a

(4) Electrolytic metallurgy

Many most active metals, such as Li, Na, K, Mg, Ca, Al, Ti, rare earth metal, etc. can be only produced electrochemically.

Electroreduction of aluminum

Charles Martin Hall (1863-

1914), who first produced

metal aluminum cheaply by

electrolysis of molten mixture

of Al2O3/Na3AlF6.

Production of metal sodium

The man who discovered the largest number of elements

Davy, on knowing the electrolysis of water by Nicholson a

nd Carlisle, set out his element finding trip using electrolys

is as his powerful tools, he discovered 8 elements including:

K, Na, Ma, Ca, Sr, Ba, B, and Si.

Titanium

(5) electrorefining of metal

From 95% to 99.99%, which is suitable for electric usage.

CuZnAgAu

Cu2+

Zn2+

Cu2+

Cu2+

Cu2+

Cu2+

Industrial electrorefining of copper

(6) Electrosynthesis

Advantages of Electrochemical Synthesis

1) The oxidative or reductive ability can be easily adjusted.

2) The most powerful oxidation or reduction methods.

3) Without introduction of impurities.

2. Anode reaction

When inert material such as

Platinum and graphite was used,

the species in the solution discharge

on the electrode in the order of

liberation potential.

F < Cl < Br < I Henri Moissan1906 Noble PrizeFrance1852/09/28 ~ 1907/02/20 Investigation and isolation of the element fluorine

1) Reaction over inert anode

(1) Active dissolution;

(2) Anodic passivation

(3) Anodic oxidation

2) Reaction over active anode

Pourbaix diagram of iron-water system

(1) Active dissolution:

At pH=4 and low current density, active dissolution occurs.

Fe Fe2+ + 2e

Fe2+

Fe2O3

Fe

pH

/ V

2 4 6 8 10 12 140

Fe3O4

Fe3+

FeO22

We usually judge the reaction

based on Porbaix diagram

(2) Anodic passivation:

At pH= 12 and high potential,

upon polarization, dense thin la

yer of Fe3O4 forms and passivat

ion of iron takes place.

3Fe + 4H2O – 8e

Fe3O4 + 8 H+

Passivation curve of iron

Fe2+

Fe2O3

Fe

pH

/ V

2 4 6 8 10 12 140

Fe3O4

Fe3+

FeO22

Active dissolution

passivation

Trans-passivation

Anodic oxidation of aluminum

(3) Anodic oxidation

t / h

E /

V

Barrier layer

Porous layer

Initiation of pores

SEM photograph of the AAM

top surface

Cross-section

Application of anodic alumina membrane (AAM)

1) Coloring of aluminum and aluminum alloys

2) Corrosion protection

3) Template synthesis of nanomaterials.

Nanomaterials synthesized using AAM template

Nanotubes