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Chemistry
Session
Electrochemistry - 1
Session
Objectives
• Conductance of electrolytic solution
•
•
Specific conductance, Equivalent
conductance, Molar conductance
Kohlrausch's law
INTRODUCTION:
Electro chemistry is the branch of
chemistry which deals with
transformation
of electrical energy into chemical energy
vice versa.
Electricity is a flow of electrons generated
by a battery when the circuit is completed
Types of Electrolytes
Strong electrolyte are highly ionized in the
solution.
Examples are HCl, H2SO4, NaOH, KOH etc
Weak electrolytes are only feebly ionized in the
solution.
Examples are H2CO3, CH3COOH, NH4OH etc
Conductors:
 A substance which allows electric current to pass
through it is called a conductor.
 These are 2 types:
1) Metallic conductors
2) Electrolytic conductors
1.Metalic conductors:
 The substances which conduct electricity under the
influence of an applied electric potential through a
flow of electrons.
 The flow of electricity does not cause any physical
or chemical change in the conductors.
 Eg: all metals, graphite, human body

2.Electrolytic conductors:
Electrolyte solutions and
molten salts conduct electricity
through the migration of ions.

When the current is passed
through an electrolyte solutions
decomposition and changes occur
in the composition of electrolytes

Resistance
Resistance refers to the opposition
to the flow of current.
For a conductor of uniform cross section(a)
and length(l); Resistance R,
l
R l and R 
a
l
R  
a
a
Where  is called resistivity or
specific resistance.
l
Conductance
The reciprocal of the resistance is
called conductance. It is denoted by C.
C=1/R
Conductors allows electric current to pass
through them. Examples are metals, aqueous
solution of
acids, bases and salts etc.
Unit of conductance is ohm-1 or mho or Siemen(S)
Insulators do not allow the electric current to pass
through them.
Examples are pure water, urea, sugar etc.
Specific Conductivity
It is the reciprocal of specific resistance
of an electrolyte.
Specific conductance
a
1


But ρ = R
K 
a.R
 
K    x Conductance
a
l/a is known as cell constant
Unit of specific conductance is ohm–1cm–1
SI Unit of specific conductance is Sm–1 where S is Siemen
Equivalent conductance : Equivalent conductance is defined
as the conductance of all the ions produced by one gram
equivalent of an electrolyte in a given solution.

(To understand the manning of equivalent conductance, imagine a rectangular
trough with two opposite sides made of metallic conductor (acting as
electrodes) exactly 1 cm apart, If 1 cm3 (1 mL) solution containing 1 gram
equivalent of an electrolyte is places in this container is measured. )

/\ eq =v x specific conductance of 1cm3 solution (k)
/\ eq= KV

/\ eq =
k × 1000/N
Where N = normality
The unit of equivalent conductance is ohm-1 cm-2 equi-1.
Representation of Equivalent conductance
1 cc
m
1c
1 cm
Molar conductance
The molar conductance is defined as the conductance of all
the ions produced by ionization of 1 g mole of an
electrolyte when present in V mL of solution. It is denoted
by.
Molar conductance
Λ m = k ×V
Where V is the volume in mL containing 1 g mole of the
electrolyte. If c is the concentration of the solution in g
mole per litre, then
Λ m = k × 1000/M
It units are ohm- cm2 mol-1.
Effect of Dilution on
Conductivity
Specific conductivity decreases on dilution.
Equivalent and molar conductance both increase with
dilution and reaches a maximum value.
The conductance of all electrolytes increases with
temperature.
KCl (strong electrolyte)
CH3COOH (weak electrolyte)
concentration, (mole L–1)1/2
Ionic Mobility (u):
 Ionic mobility is defined as the velocity of
an ion when the potential gradient is
1v/cm.
 Hence the units of u are cm2/v.sec
Kohlrausch’s law of independent ionic
mobilities

At infinite dilution when dissociation complete (m) , the molar
conductivity of an electrolyte is expressed as the sum of the
contributions from its individual ions
Λ∞m = v+ λ∞ + + v- λ∞v+ and v- are the number of cations and anions
per formula
unit of electrolyte respectively and,
λ∞+ and λ∞- are the molar conductivities of the
cation and anion at infinite dilution respectively
Applications of Kohlrausch's law

Determination of Λ∞m for weak electrolytes

Determination of the degree of dissociation of a
weak electrolyte

Determination of the solubility of a sparingly
soluble salt
APPLICATIONS OF KOHLRAUSCH LAW
 1) Determination of molar conductivities of weak
electrolytes:
It is not possible to determine value of Λ∞m for weak
electrolyte like CH3COOH,NH4OH ETC. BY THE
EXTRAPOLATION OF THE MOLAR CONDUCTIVITY VALUES
TO ZERO CONCENTRATION. From the value of of Λ∞m HCl,
Λ∞m CH3COONa and Λ∞m NaOH the value of Λ∞m CH3COOH
can be calculated.
Λ∞CH3 COOH = Λ∞CH3COONa + Λ∞HCI - Λ∞NaCI
= λ∞m (H+)+ λ∞m (Cl-) + λ∞m ( CH3COO-)+ λ∞m ( Na+) - λ∞m (
Na+) +λ∞m (Cl-)
= λ∞m (H+)+ λ∞m ( CH3COO-)
Λ∞CH3 COOH = Λ∞CH3 COOH
2)Determination of degree of dissociation :
Degree of dissociation is the fraction of the total number of
molecules dissociated into ions.
Degree of dissociation (∞) = No. of molecules dissociated in to ions
Total no. of molecules present
No. of molecules dissociated is directly proportional to
conductivity of the molecules.
No. of molecules dissociated in to ions Λ∞m ( molar conductivity
at a particular concentration.
Total no. of molecules Λ∞m (molar conductivity at infinite dilution)
∞(degree of dissociation) = Λ∞m / Λ∞m
3) Determination of solubility of sparingly soluble salt :
The solubility of a sparingly soluble salts such as silver chloride,
silver chromate, lead sulphate, barium sulphate etc…can be
determined from conductance values.
The solubility S in gram equivalent/ liter is related to equivalent
conductance Λ∞ and specific conductivity k
The concentration of sparingly soluble salt is the solubility of the
salt.
hence Λ∞ = 1000K
S
Illustrative Example
Equivalent conductance of NaCl, HCl
and C2H5COONa at infinite dilution are
126.45, 426.16 and 91 ohm–1 cm2
respectively.Calculate the equivalent conductance
of C2H5COOH.
Solution:
 C2H5COOH   C2H5COONa   HCl   NaCl
= 91 + 426.16 – 126.45
= 390.71 ohm–1 cm2
GALVANIC or ELECTROCHEMICAL CELLS

Galvanic cell is a device which converts chemical
energy into electrical energy.
ex: Daniel cell
Daniel cell consists of zinc and copper
electrodes. Zn electrode is dipped in
ZnSO4 solution & Cu is dipped in
CuSO4 solution.
Cu2+ + 2e- --> Cu
Zn --> Zn2+ + 2e-
Reduction
Cathode
Positive
Oxidation
Anode
Negative
<--Anions
Cations-->
•Electrons travel thru external wire.
Salt bridge allows anions and
cations to move between electrode
compartments.






The 2 solutions separated by a porous
membrane, a current is seen to be flow on
connecting the two wires externally.
The cell function due to dissolution of zinc
and the simultaneous deposition of copper.
The over all reaction is:
Zn + CuSO4  ZnSO4 + Cu
The Danial cell may be represented as:
Zn/ZnSO4 // CuSO4/Cu
E.M.F:
 The potential difference or electrode between
the two electrodes of the cell which is a
driving for the fllow of electrons is called the
E.M.F of the cell.
 Units electron volts
NERNST EQUATION:









The theoretical relation ship b/w the electro chemical reaction
and the corresponding cell e.m.f, this relation ship is generally
known as nernest equation.
Consider a galvanic cell
a A +bB  c C + d D here a,b,c,d are represent the
number of moles of A,B,C,D respectively, the nernest equation is
Ecell =RT/n F ln K - RT/n F ln [C]c[D]d / [A]a[B]b
Here Ecell = e.m.f of the cell,
R= gas constant,
T = Temperature,
n= no. of faraday of current F passed,
K = equilibrium constant,
RT/n F ln k = standard e.m.f of the Eocell

Ecell = Eo cell
-
RT/n F ln [C]c[D]d / [A]a[B]b (or)

Ecell = Eo cell
-
2.303 RT/ nF log [C]c[D]d / [A]a[B]b

at R.T T=298 K, R=8.314 K-1, F=96457 C
substitute the values in above equation

Ecell = Eo cell

Standard cell e.m.f equal to cell e.m.f when the
activities of both reactants and products is equal to
unity.
-
0.05916/ n log [C]c[D]d / [A]a[B]b
Cell formulation:

A short hand notation for representing a cell is called cell
formulation

In this notation the state of the element ,a single stroke for
separation of two different phases, a double stroke for separation
of the two electrodes.
Eg; H2(g) / Pt/H+(1M) // Cu+2(1M) / Cu (s)
SHE anode
SCE cathode
Classification of electrodes :
a) metal-metal ion electrode. Eg: Cu+2 /Cu
b) Metal-metal insoluble salt electrode. Eg: calomel electrode.
c) Gas electrode. Eg: hydrogen electrode.
d) Redox electrode.
Eg: Pt(s)/Fe+2 (1M),Fe +3(1M)
A)
metal-metal ion electrode. Eg: Cu+2 /Cu
it consists of a pure metal (M) in contact with
a solution of its ion(Mn+)
It is represented as Mn+ (aq) + ne-  M(S)
B)
Metal-metal insoluble salt electrode. Eg: calomel
electrode.
It consists of a metal (M) covered by layer of sparingly
soluble salt(MX) immersed in a solution containing a
common ion (X-)
it is represented as X-(aq)// MX/ M(S)
MX(s) + ne-  M(s) + X-(aq)
c) Gas electrode. Eg: hydrogen electrode.
It is represented as X+(aq)/ X2(P = atm) Pt
X2(p) + 2e-  2X+(aq)
Reference electrodes:
Reference electrodes are electrodes at which the
oxidation or reduction occurs reversibly.
 Eg: standard hydrogen electrode, calomel electrode.
1)STANDARD CALOMEL ELECTRODE (S.C.E):
 The calomel electrode undergoes the spontaneous
process of reduction with respect to the hydrogen
electrode represented as
The cell is represented as:
Pt/Hg,Hg2Cl2(s) /KCl

Hg2Cl2+2e-  2 Hg (l) + 2Cl–(aq)
The emf of the calomel electrode
varies with the concentration of the
chloride ions and three concentrations
of chloride ions are normally used.
The decinormal calomel electrode with
0.1N KCl having a potential of 0.3338v
Normal calomel electrode with 1.0 N
KCl having a potential of 0.28 v
Saturated calomel electrode with
saturated KCl having a potential of
0.2415 v on the SHE scale at R.T
The electrode can be coupled with hydrogen
electrode containing solution of unknown PH
The emf of the cell
Ecell = E right- Eleft = 0.2422V + 0.0592V PH
PH = Ecell- 0.2422V
0.0592V
Quinhydrone electrode: (Redox electrode)




This is a redox electrode reversible to protons &
often replaces the hydrogen electrode
This is 1:1 molar mixture of quinone &
hydroquinone
Electrode consist of a Pt electrode dipped in a
test solution which is saturated with quinhydrone
The electrode reaction is given by


Q + 2H+ + 2e-  QH2

The electrode potential at 25oc is given by
E Pt/Q,,H+,QH2 =
E0 Pt/Q,H+,QH2 – 0.0592/2 log aQH2/aQa2 H+
Since Q,,QH2 are in equimolar amounts i.e. a Q=a QH2
So, E=E0 + 0.0592 log a H+
EQ,QH2 = E0 Q,QH2 – 0.0592 p H
Quinhydrone electrode can be measure p H of a
solution.
This electrode can’t be used at p H >8
Even this electrode fails in the presence of strong
oxidizing & reducing agents







(quinone)
( hydroquinone)
ION SELECTIVE ELECTRODES





This electrodes consist of specially prepared
membranes placed between two electrolytes.
Ecell=k- (0.059/n)log(a1/a2)
a1,a2 are the activities of the ion to be measured in
the external and internal solutions respectively.
The ion selective electrodes is coupled to a SCE
and immersed in the sample or test solution
containing the ion to be monitored.
The potential developed across the membrane is
related to the activities of the ion of interest in the
gel and sample solution.




The response of the membrane is usually highly
selective to only one ion or a small number of
ions.
The construction is similar to that of the glass
electrode and consists of a tube, one end of
which is fused to an electrically conducting
membrane.
The tube consists with a gel incorporating the
ion to which the electrode is sensitive and inert
electrolyte such as KCl.
A silver wire in contact with the gel together with
the inert electrolyte constitutes the Ag-AgCl
reference electrode.
Different types of ion-selective electrodes have been
developed as the above principle.
1) glass electrodes for the determination of cations
other than H+
2) Solid state electrodes
3) liquid-ion exchange membrane electrodes &
heterogeneous membrane electrodes
4) Gas sensing electrodes
glass electrodes with high selectivity for
cations such as Na+,NH4+,Ag+ & Li+ consists glass
membranes whose composition determines the
selectivity to individual cations.

POTENTIOMETRIC TITRATIONS:

The detection of the end point of a volumetric
titration by the use of potentiometer is an important
application of measurement of emf.

The emf of cell consisting of an indicator electrode
responsive to the analyte ions and a reference
electrode is measured as a function of the volume of
titrant added to the analyte solution.
BATTERIES





A device which stores Chemical energy for later release
as electricity is called battery.
It is an electrochemical cell or often electrochemical cells
connected in series ,can be used as a source of direct
electric current at a constant voltage.
Batteries are classified into two categories depending on
their recharging capacities.
1) Primary batteries
2) secondary batteries.
1)PRIMARYCELL:




In primary cells, a chemical reaction proceeds spontaneously
and the free energy of the reaction is converted into electrical
energy.
The production of electrical energy at the expense of the free
energy of the cell reaction is called DISCHARGEING of the cell.
In a primary cell the chemical reactions can’t be reversed by
passing electricity through the cell and hence a discharged cell
can’t be used again and the battery become dead.
In a primary cell the cathode at which reduction occurs is
designated positive by conversion. Eg: voltaic cell, Daniel cell,
leclanche cell (or) dry cell, lithium cell….
LITHIUM CELL:
Li battery chemistry comprises a number
of cell designs, in that Li is used as anode
due to its light weight and highest standard
potential greater than 3V.
Types of lithium cells:
Three types
1)Lithium primary cell with Liq cathode
2) Lithium primary cell with solid cathode
3) Lithium primary cell with solid electrolyte
Liq cathode cells:
Anode : Li
Cathode : SOCl2
Electrolyte : LiAlCl2
Anode : Li --Li+ + eCathode : 4Li+ + 4e- + SOCl2 -- 4 LiCl + SO2
+S
Overall rxn: 4Li+ + 2SOCl2 -- 4 LiCl + SO2 + S
They perfom best in low current applications
and have a very long service life. For this
reason they are used in pacemaker.
These cells offer higher discharge rates due to
the rxns occur at the cathode surface.
The direct contact between the liq cathode n
the Li forms a film over the Li,called solid
electrolyte interface.(SEI)
This prevents further chemical rxn when not in
use,thus preserving the shells life.
The thick film causes an initial voltage delay.
solid cathode (LiMnO2) cell:
Anode : Li
Cathode : MnO2
Electrolyte : propylene carbonate n 1,2-dimethoxy
ethane
Anode : Li --Li+ + e+4
+3
Cathode : Li+ + e- + MnO2 -- MnO2 (Li+)
Li + MnO2 -- MnO2 (Li+)
+4
+3
Uses:
Low rate cells are used commercially for small
electronics and memory back up.
solid electrolyte cells:
Anode : Li
Cathode : poly -2- vinyl pyridine(P2VP)
Electrolyte : solid Li
Anode : 2Li --2Li+ + 2eCathode : 2Li+ + 2e- + P2VP.n I2 -- P2VP.(n-1) I2 +
2LiI
Overall rxn: 2Li+ + P2VP.n I2 -- P2VP.(n-1) I2 + 2LiI
These cells can’t be used in high drain
applications and don’t perform well under low
temp conditions.
They are used generally for memory back up
,watches and portable electronic devices
2)SECONDARY CELLS (ACCUMULATORS) :




In secondary cells chemical reactions proceeds both in the
forward and reverse directions depending on weather electrical
energy is supplied from an external source
Electrical energy is passed into the cell to induce a chemical
reaction and the products remained at the electrodes, this
process is called charging the cell.
Secondary cells can accumulate electrical energy in the form of
chemical reaction and later on the reaction is reversed to liberate
electrical energy. Hence these cells are called accumulators or
storage batteries. The cathode at which reduction occurs during
the discharge of the cell is designated +ve. while it becomes
anode during charging.
Eg: lead-acid battery, alkaline storage battery, nickel –cadmium
battery.
LEAD ACID BATTERY:
 This battery consists of a number of spongy
lead anodes and a grid of lead dioxide coated
lead-antimony alloy cathode.
 The electrode pairs separated by inert porus
partitions are kept immersed in the electrolyte
sulphuric acid.







At the anode Pb  Pb2+ +2ePb2+ + SO42-PbSO4
At the cathode
PbO2 + 4H+ +2e- Pb2+ + 2H2O
Pb2+ + SO4 2- PbSO4
The net reaction of the cell is
PbO2 + 4H+ + 2e- + SO42- PbSO4 +2H2O
The lead sulphate formed precipitates on the cathode
.
Pb/PbSO4(s)/H2SO4(aq),PbSO4(s)/Pb
The net reaction of the cell is
PbO2+Pb+2H2SO42 PbSO4 +2H2O

NICKEL-CADMIUM CELL:
It consists of a steel grid containing cadmium powder as the
anode and cathode made of Ni2O3 mixed with Ni supported on
steel grid.
 An aqueous solution of KOH placed in an inert steel container is
used as the electrolyte. The cell generates the voltage of 1.35v
and the reaction may be represented as
Cd +2OH-  Cd(OH)2 +2 e- (at anode)
2NIO(OH) + 2H2O +2e- 2 Ni(OH)2 +2OH- (at cathode)
The net reaction is
Cd +2Ni(OH)3  Cd(OH)2 + 2NI(OH)2
 The disadvantage of this battery is the reaction can be reversed,
b’cos the reaction products Cd(OH)2 & Ni(OH)2 remain adhered
to the electrodes called MEMORY EFFECT or FALSE BOTTOM.

Nickel-Cadmium battery
FUEL CELLS




A fuel cell is an electrochemical cell which converts
chemical energy contained in a easily available fuel
oxidant system into electrical energy.
The basic principle of a fuel cell is the chemical energy is
provided by a fuel and an oxidant stored outside the cell.
The fuel and the oxidizing agent are continuously and
separately supplied to the electrodes of the cell at which
they undergo reactions.
These are also primary cells & they are capable of
current as long as the reactants are supplied.
HYDROGEN – OXYGEN FUEL CELL







It consists of two inert porous electrodes made of either graphite
impregnated with finely divided Pt or a 75/25 alloy of Pd with Ag(Ni)
and an electrolyte solution which is 25% KOH solution.
Through the anode, H2 gas is bubbled and through the cathode O2
gas bubbled.
At anode: 2H2(g) + 4OH-(g)  4H2O(l) + 4eAt cathode : O2(g) + 2H2O(l) + 4e-  4OH-(aq)
Net rxn : 2H2(g) + O2(g)
 2H2O(l)
The emf of the cell is 0.8 to 1.0v
A no of such cells are stacked together in series to make a battery,
called fuel cell battery.
Hydrogen-oxygen fuel cell
Uses of H2- O2 fuel cell
They are used as auxiliary energy source in space
vehicles (ex: Apollo space craft), submarines & other
military vehicles.
 For space craft, they are preferred due to their lightness
& product water is available as source of fresh water for
astronauts.
 Fuel cells are categorized on the basis of electrolyte
used:
1)Proton exchange membrane fuel cell
2)Alkaline Fuel cell
3) Molten carbonate Fuel cell
4) Phosphoric acid fuel cell
5) Solid oxide fuel cell

Thank you