MODULE – II/II SEM BE , ENGINEERING

CHEMISTRY

BY

RASHMI MASSISTANT PROFESSORSKITBANGALORE

ELECTROCHEMISTRY

MODULE IELECTRODE POTENTIAL & CELLS

Electrochemistry: It is a branch of chemistry which deals with the study of transformation of chemical energy into electrical energy and vice versa”

Electrochemical cell and Classification with examples.An electrochemical cell is a device, which is used to convert

chemical energy into electrical energy and vice versa.These electrochemical cells are classified into two types as follows’

1) Galvanic or Voltaic cells: These are the electrochemical cells, which converts chemical energy into electrical energy.

Ex. Daniel cell, Dry cell, etc

2) Electrolytic cells-are devices which convert electrical energy into chemical energy.

Example: Electrolysis of molten NaCl, Recharge process of lead acid battery

 

Galvanic or Voltaic cells:Galvanic or Voltaic cells are again classified into three types as followsa) Primary cells: These are the cells which serve as a source of

energy only as long as the active chemical species are present in the cell. The cell reactions are irreversible. These are designed for only single discharge and cannot be charged again.

Ex: Dry Cell, Zn – Hgo cell, Zn-Ag2o cell etc. b) Secondary cells: These cells are chargeable and can be used

again and again. The cell reactions are reversible and are often called as reversible cells. During discharging the cells acts like voltaic cell converting chemical energy into electrical energy. During charging the cell acts like electrolytic cell by converting electric energy into chemical energy, hence these batteries are called as storage battery.

Ex: Lead acid Battery, Ni-cd cells. Lithium ion cells etc. c) Concentration of cells: These are the electrochemical cells consisting of same metal

electrodes dipped in same metal ionic solution in both the half cells but are different in the concentration of the metal ions.

Ex: Cu/Cu2+ (M1) || Cu2+ (M2)/CuEx: copper concentration cell, Zinc concentration cell

Oxidation: A species loses one or more electrons resulting in the increase in its oxidation number.

Reduction: A species gain one or more electrons resulting in a decreasing in its oxidation number.

Oxidation should accompanied by reduction, because if one losses electrons another must ready to accept electrons. This reaction is called redox reaction.

Single electrode Potential: It is defined as the potential developed at the interphase between the

metal and the solution, when a metal is dipped in a solution containing its own ions. It is represented as E

Standard reduction potential (Eo) : It is defined as potential developed at the interface between the metal

and the solution, when a metal is dipped in a solution containing its own ions of unit concentration at 298K. [If the electrodes involve gases then it is one atmospheric pressure] It is denoted as E0.

Electromotive force (EMF): It is defined as the potential difference between the two electrodes of

a galvanic cell which causes the flow of current from an electrode with higher reduction potential to the electrode with lower reduction potential.

It is denoted as E cell.E cell = E right –E left.

E cell = E cathode – E anode.

Electrochemical series: The arrangement of elements in the increasing order of their standard reduction potential is referred to as emf or electrochemical series. Such a arrangement of few elements given in the table

Mn+/M Eo (volts) Mn+/M Eo (volts)Li+/Li -3.05 H+/H2 0.00K+/K -2.93 Sn4+/ Sn2+ +0.15Mg+/Mg -2.37 Cu2+/Cu +0.16Al3+/Al -1.66 Cu2+/Cu +0.34Zn2+/Zn -0.76 Cu+/Cu +0.52Cr3+/Cr -0.74 I2/I- +0.54Fe2+/Fe -0.44 Fe3+/Fe2+ +0.77Cr3+/Cr2+ -0.41 Hg2+/Hg+ +0.79Cd2+/Cd -0.40 Ag+/Ag +0.80Ni2+/Ni -0.25 Hg2+/Hg +0.85Sn2+/Sn -0.14 Pt2+/Pt +1.20Pb2+/Pb -0.13 Cr7+/Cr3+ +1.31Fe3+/Fe2+ -0.041 Cl2/2Cl- +1.36

Au3+/Au +1.50

• A negative value indicates oxidation tendency and a positive value indicates reduction tendency with respect to hydrogen.

• The metal with lower electrode potential is more reactive and as the electrode potential increases, the reactivity decreases, and metals with higher electrode potentials are nobler.

• Metals with lower electrode potentials have the tendency to replace metals with higher electrode potential from their solutions for example, Zn displaces Cu, and Cu displaces Ag

• Metals with negative electrode potentials can liberate hydrogen from acidic solutions

Derivation of Nernst Equation for Electrode potential

  In 1889 Nernst derived a quantitative relationship between the electrode potential and the concentrations of metal ions are involved. The maximum work available from a reversible chemical process is equal to the maximum amount of electrical energy that can be obtained; it shows decrease in free energy. 

Wmax = – ∆G--------[1]And Wmax = difference in potential between two

electrode x total quantity of charge flowing through the cell

Total quantity of charge flowing through the cell = (No. of moles of electrons) x (Faradays constant)

So Wmax = nFEcell --------[2]

c0

0c

KlnGG ,isotherm'reaction hoffvant ' aby related are G andG ,K

]n[M

[M]cK

as written becan cKconstant equlibrium ,reaction above for theM -ne nM

reaction, electrode reversible aconsider

0nFE- 0G ion,std.conditunder

[3]-----nFE- G [2] & [1]eqn equate

RT

]6[]n[M

1log RT0EE

1[M] condition, standardunder

]n[M

[M]ln RT0EE

nF-by sides both the divide

]n[M

[M]ln

0-nFEnFE-

equation, above the to0

G andG ,cK of values thesubstitute

RT

Where,E = Electrode potentialE0 = standard electrode potentialn = no. of electrons[Mn+] = Concentration of metal ionsR = Universal gas constant = 8.314J K-1 mol-1 T = Temperature (In Kelvin) = 298K 

]nlog[M n

0.05910EE

eqn[6], the to values thesubstitute

Concentration Cells• Concentration cell is an electrochemical cell that has

two equivalent half-cells of the same material differing only in concentrations. One can calculate the potential developed by such a cell using the Nernst Equation similar to an electrochemical cell.

• In concentration cell the voltage comes from its attempts to reach equilibrium, which will stop when the concentration in both half-cells are equal.

Types of concentration cells: The concentration differences could be affected in the electrode material or in the electrolyte.1. Electrode concentration cells2. Electrolyte concentration cells

Electrolyte concentrations cells“A concentration cell is an electrochemical cell in which electrode materials and electrolytes of two half cells are composed of same material but the concentration of two solutions are different”

Ex- Cu/Cu2+(M1)|| Cu2+(M2)/Cu

A concentration cell consists of two same metal electrode dipped into their own ionic solutions of two different concentration. Thus in a concentration cell, the electrode with lower electrolyte concentration acts as anode and the one with higher electrolyte concentration acts as cathode. The concentration of ions at anode increases and at cathode decreases, when the cell is in operation.

Consider two copper rods are dipped into their own ionic solutions of M1 and M2 and it is represented as Cu/Cu2+

(M1) Cu2+ (M2)/Cu

By electrochemical conventions, if M2 > M1 then, we have the following reactions.At anode:At cathode:

eMCu

sCu 2)1(

2)(

)(2)2(

2s

CueMCu

1

2log

2

0591.0

1log2

0591.002log2

0591.00

be willcellion concentrat theof emf The

M

M

cellE

MEMEcell

E

anodecathodecell EEE

The emf of the cell is + ve only if M2 > M1

The following characteristics of concentration cell can be noted:

• When M2 = M1, the concentration cell does not generate electrical energy.

• When M2 > M1, the Ecell is + ve.• When M2<M1, Ecell is – ve.• Higher the ratio of M2/M1, greater is the cell

potential.

eCd

MCd 2

2)1(

)2(2

2M

CdeCd

Electrode concentration cellsIn these cells, the potential difference is developed between two

electrodes at different concentrations dipped in the same solution of the electrolyte. For example, Cd-Hg (M1) | CdSO4 | Cd-Hg (M2)Two Cd-Hg electrodes of different concentration immersed in a CdSO4 solution.

Electrode containing high concentration of metal acts as a anode , where oxidation occurs

Electrode containing low concentration of metal acts as a cathode, where reduction occurs

M2.M1only when ve is Ecell

]2[

]1[log

2

0591.0

]1log[2

0591.00]2log[2

0591.00

be willcellion concentrat theof emf The

M

M

cellE

MEMEcell

E

anodecathodecell EEE

Types of electrodes:• Metal-Metal ion electrode: An electrode of this type consists of

a metal dipped in a solution containing its ions. Ex- Zn/Zn2+, Cu/Cu2+ etc.

• Metal-Metal salt ion electrode: These electrodes consist of a metal is in contact with a sparingly soluble salt of the same metal dipped in a solution containing anion of the salt.Example-Calomel (Hg|Hg2Cl2|Cl- , Silver- Silver salt electrode (Ag| AgCl |Cl-

• Gas electrode: Gas electrode consists of a gas bubbling about an inert metal wire, immersed in solution containing ions to which the gas is reversible. The metal provides electrical contact and facilitates the establishment of equilibrium between the gas and its ions.Example-Hydrogen electrode (Pt|H2|H+) , Chlorine electrode (Pt|Cl2|Cl-)

• Oxidation-Reduction electrode: An oxidation-reduction electrode is a one in which the electrode potential arises from the presence of oxidized and reduced forms of the same substance in solution. The potential arises from the tendency of one form changes into the other more stable form. The potential developed is picked up by an inert electrode like platinum.Example-Pt|Fe2+, Fe3+ Pt|Ce3+, Ce4+

• Ion selective electrode: In ion selective electrode, a membrane is in contact with a solution, with which it can exchange ions.Example- Glass electrode.

• amalgam electrode: Metal- amalgum is in contact with a solution containing its own ions.Example: lead amalgum electrode ( Pb – Hg/Pb2+)

Reference Electrodes:• Reference electrode: “Reference electrode are the electrode with reference

to those, the electrode potential of any electrode can be measured.” It can acts both as an anode or cathode depending upon the nature of other electrode.

The Reference Electrodes can be classified in to two types 

i) Primary reference electrodes Ex: Standard hydrogen electrodeii) Secondary reference electrodes Ex: Calomel and Ag/Agcl electrodes

Construction and working of Standard Calomel electrode (SCE):1. Calomel electrode is a metal-metal salt Ion electrode. 2. It consists of mercury, mercurous Chloride and a solution of KCl. Mercury is placed at the bottom of a glass tube.3. A paste of mercury and mercurous chloride Is placed above the mercury. The space above the paste is filled with a KCl solution of known concentration. 4. A platinum wire is kept immersed into the mercury to obtain electrical contact. 5. Calomel electrode can be represented as,Hg | Hg2Cl2 | KCl(sat)

•The calomel electrode can acts as anode or cathode depending on the nature of the other electrode of the cell.

Saturated KCl

Mercury

Calomel paste

Pt wire

Porous disc

Therefore electrode potential of calomel electrode is depending upon the concentration of KCl. The electrode is reversible with chloride ions.

298Kat log0591.0

2n where,log303.20

2log

303.20

-2Cl2Hg(l)-2e)(2Cl2Hg

is,reaction electrode reversible cellnet The

0

ClEE

ClF

RTEE

ClnFRT

EE

s

Concentration of KCl

E0 [v]

saturated KCl 0.241

1M KCl 0.281

0.1M KCl 0.334

MEASUREMENT OF ELECTRODE POTENTIAL USING CALOMEL ELECTRODE:Electrode potential of a given electrode can be measured by using calomel electrode as a reference electrode.Example: To measure the electrode potential of zinc, Zinc electrode is coupled with SCE. So zinc acts as anode and SCE acts as cathode

cellZnZn

cellcathodeanode

anodecathodecell

EEEEE

EEE

241.02/

Applications: 1. It is used as secondary reference electrode in the measurement of single electrode.2. It is used as reference electrode in all potentiometer determinations and to measure pH of the given solution

Construction and working of Silver- Silver Chloride electrode:

• Silver-Silver chloride is also a metal-metal salt ion electrode.

• Silver and its sparingly soluble salt silver chlorides are in contact with a solution of chloride ions. Generally a silver wire is coated with AgCl and dipped in a solution of KCl .

• Cell representation is as follows Ag |AgCl | Cl-

Concentration of KCl E0 [v]

saturated KCl 0.241

1M KCl 0.281

0.1M KCl 0.334

298Kat log0591.0

1n where,log303.20

-ClAg-2e)(AgCl

is,reaction electrode reversible cellnet The

0

ClEE

ClnFRT

EE

s

Therefore electrode potential of calomel electrode is depending upon the concentration of KCl. The electrode is reversible with chloride ions.

Applications:•Used as secondary reference electrode in ion selective elctrode.

Construction and working of Ion selective electrode (ISE) :

“Ion selective electrode is one which selectively responds to a specific ion in a mixture and the potential developed at the electrode is a function of the concentration of that ion in the solution”

Glass electrode: A glass electrode is an ion selective electrode where potential depends upon the pH of the medium.

1. The glass electrode consists of a glass bulb made up of special type of glass (sodium silicate type of glass) with high electrical conductance.

2. The glass bulb is filled with a solution of constant pH (0.1MHCl) and insert with a Ag-AgCl electrode, which is the Internal reference electrode and also serves for the external electrical contact.

3. The electrode dipped in a solution containing H+ ions as shown in the figure.

The electrode representation is,Glass | 0.1M HCl | Ag-AgCl

WORKING: • The glass electrode works on the principle that when a

thin glass membrane is placed between two different concentration of a solution, a boundary potential Eb is developed at layers of the glass membrane. This potential arises due to difference in the concentration of H+ ion inside and outside the membrane.

External Solution glass membrane Internal solutionC2=[H+] C1=[CONSTANT]=k

E2 Eb E1

• Boundary potential, Eb = E2 – E1

12

10

20

2121

20

2

log0591.0log0591.0

1n wherelog0591.0log0591.0

get we(1),eqn into E & E of values theSubstitutemembrane.outer andinner ions +H ofion concentrat are C and C Where,

log0591.0

1C logn

0.05910E1E

CCE

Cn

ECn

EE

Cn

EE

b

b

10.0591logC- =K =Const Where

log0591.0

(H+)) = (C2) &constant is C1 constant, issolution inner theofion concentrat the(Since

Hn

ConstEb

asyAgClAgG

GG

asyAgClAgG

asyAgClAgbG

EEKEwhere

pHEE

EEpHKE

EEEE

/0

0

/

/

b

0591.0

0591.0

potential.Easy called exists is potential additional small a However, C2. = C1for 0 = Eb lly,Theoritica

Easy. potentialAsymmetry iii)

and EAg/AgCl potential electrode AgCl-Ag ii)

Eb, potentialboundary i)

theof sum is potential electrode glass The

pHn0.0591

KE

Determination of pH using glass electrode:

0591.0

0591.0

0591.0

evaluated. becan glass E

cell, theemf knowing is E Since

anodeEcathodeEcellE

0

0

0

SCE

SCEcellG

SCEG

GG

SCEGcell

EEEpH

EpHEEcell

pHEE

EEE

Advantages• This electrode can be used to determine PH in the

range 0-9, with special type of glass even up to 12 can be calculated.

• It can be used even in the case of strong oxidizing agents.

• The equilibrium is reached quickly.• It is simple to operate, hence extensively used in

various laboratories.

Limitations• The glass membrane though it is very thin, it offers

high resistance. Therefore ordinary potentiometers cannot be used; hence it is necessary to use electronic potentiometers.

• This electrode cannot be used to determine the PH

above 12

1. Two Copper electrodes placed in CuSO4 solutions of equal concentration are connected to form o concentration cell.

a) What is the cell voltage? b) If one of the solutions is diluted until the concentration of Cu2+ ions is

1/5th of its original value. What will be the cell voltage after dilution? Solution: • The cell potential of concentration cell is given as

• When the concentration of the species are gqual (C2 = C1) the cell voltage is zero.

• When one of the solution is diluted to of its original value, C2 =1M & C1

1

2log

0591.0

C

C

ncellE

VE

E

E

E

cell

cell

cell

cell

0206.0

699.002955.0

5log2

0591.0

5/11log

20591.0

C1C2log

n0.0591

cellE

2.Calculate the emf of the given concentration cell at 298K. Ag(s) /AgNO 3 (0.018M) AgNO3 (1.2M) /Ag.

Solution :

Vcell

E

cellE

cellE

C

C

ncellE

1708.0

66.66log1

0591.0

018.0

2.1log

1

0591.0

1

2log

0591.0

THANK YOU