1

Electrochemistry is the branch of chemistry that deals with the

relation between chemical action and electricity. It deals with the study of
electrical properties of electrolytes and also the chemical changes brought
about by passage of electricity.
Or
Electrochemistry is the study of production of electricity from energy
released during spontaneous chemical reactions and also the use of
electrical energy to bring about non-spontaneous chemical transformations.
Electrical Conductance
 Substances that allow the passage of current through them are called
conductors and the phenomenon is called electrical conductance.
 Conductors are further divided into two categories, namely, metallic
and electrolytic conductors.
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Metallic vs electrolytic conductors

1. In metallic conductors, the conduction is only due to the presence of
free mobile electrons. Hence, these are referred to as electronic
conductors.
2. The substances which conduct electricity both in the fused state and in
the aqueous solution are called electrolytic conductors or
electrolytes. These substances dissociate in molten state or aqueous
solution to furnish ions. The conduction is due to the presence of these
mobile ions. For example, molten or aqueous solution of sodium
chloride.
 3

Types of Electrolytes

Electrolytes are classified into two types :

1. Strong electrolytes : Those which are almost completely dissociated in their
aqueous solutions and have a large value of conductance. For example, HCl,
HNO3, NaOH, KOH, NaCl and KCl

2. Weak electrolytes : Those which do not undergo complete dissociation even in

their dilute aqueous solutions and have low value of conductance. For example,
CH3COOH, HCOOH and NH4OH.
Electrolytic cell and Galvanic cell:
Differences
Electrolytic Cell Galvanic Cell

• Converts electrical energy • Converts chemical energy

to chemical energy to electrical energy
• Reactions are non- • Reactions are
spontaneous spontaneous
• Cathode is negatively • Cathode is positively
charged and anode is charged and anode is
positively charged negatively charged
Electrolytic cell and Galvanic cell: Similarity
and trick to remmeber
• In Cathode, there is always Reduction
• In Anode there is always Oxidation
• C for R or Consonant for Consonant
• A for O or Vowel for Vowel
Conductance (G):
Resistance is a measure of the opposition to current flow in a substance.
Resistance is measured in ohms.

Conductance measures how easily electricity flows through a substance.

The reciprocal of solution resistance (1/R) is called Conductance G.

G=1/R
It is expressed in the unit called reciprocal ohm (ohm-1 or Ω-1)
In SI system, the unit of conductance is Siemen, S
Specific Conductance
The resistance of any conductor varies directly as its length (l) and inversely as its
cross-sectional area (a), i.e R  l/a
or
R =  (l/a)
• Where  is a constant depending upon the nature of the material and is called
specific resistance of the material If, l =1cm and a =1cm2, then  = R
• Specific resistance is defined as the resistance in ohms of a specimen 1cm in
length and 1cm2 cross-section (1cm3 of the material).
• The reciprocal of specific resistance, i.e., 1/ is called specific conductance (ĸ).

The cross-sectional area is the area that is obtained

when a three-dimensional object - such as a cylinder -
is sliced perpendicular to some specified axis at a point.
Specific Conductance (ĸ): Continued………..
From equation of specific resistance
R =  (l/a) or  = R (a/l)
ĸ = 1/  = (l/a) (1/ R)
ĸ = (l/a) x G
l/a=known as cell constant, G=Conductance
Since, conductance is measured in Ω-1, length in cm and area in
cm2, hence, Units of specific conductance ĸ = Ω-1 x (cm/cm2) = Ω-
1 cm-1, in SI system, the units of specific conductance are Sm-1

where S stands for Siemen.

Equivalent Conductance (Λ):

It is defined as the conducting power of all the ions produced
by one gram equivalent of an electrolyte in a given solution.
 Relation Between Specific Conductance and Equivalent Conductance

Imagine 1cc (1 cm cube) of a solution of an electrolyte placed

between two large electrodes those are 1 cm apart. The cross-
sectional area of the solution will be 1 cm2.
The conductance of the solution will evidently be its specific
conductance because we are having 1 cm cube of solution.
Further, If 1 cc of the solution contains 1 gm equivalent of
electrolyte dissolved in it.
Then according to the definition in the prevoius slide, the
conductance of the solution will be equal to the equivalent
conductance (Λ).
Conductance (G) = Specific conductance (k) = Equivalent
conductance (Λ)
 Molar Conductance (Λm ):
Conducting power of all the ions produced by one mole of the electrolyte in a
given solution
Molar conductance is related to specific conductance by the relation
Λm = k / c
Where c = concentration of the solution in moles/m3
Λm = 1000k / cm
Where cm = concentration of the solution in molarity or moles/dm3 or moles/l
Since units of k is Sm-1 and those of c is mol.m3, the units of Λm are Sm2 mol-1
Degree of dissociation (α) and dissociation constant (Kd) of a Weak Electrolyte

Initial number of moles 1 0 0

Number of moles at equilibrium 1−α α α
Concentration at equilibrium c(1−α) cα cα

𝟐 𝟐
𝒅

𝟐
𝒅

𝟐
𝒅

𝒅
Degree of dissociation (α) and dissociation constant (Kd) of a Weak Electrolyte

 The degree of dissociation of a weak electrolyte increases with dilution.

 At infinite dilution the weak electrolyte will be completely dissociated.
 Its conductance will be maximum.

α = Λc/Λ0
Where Λc and Λ0 are the molar conductance at a certain concentration c and
at infinite dilution respectively
 13

Ionic Mobility

• Although, at infinite dilution, all electrolytes are completely

dissociated, their molar conductance differ vastly from one another
• This is because of differences in speeds of the ions.

• Ex. The molar conductance at infinite dilution of HCl is more than

three times as high as that of NaCl. Since chloride ion is common, it
follows that the speed of hydrogen ion is more than three times of
the speed of sodium ion.
• Speed of an ion varies with the potential applied.

• Ionic mobility is defined as the distance travelled by an ion per

second under potential gradient of 1 volt per meter
 Variation of Molar Conductance with dilution

Dilution increase  degree of

dissociation of the electrolyte increases 
Molar conductance will increases.
Degree of dissociation is defined as the fraction of
the total electrolyte in solution which exists in the
form of its ions.
On dilution, the same amount of electrolyte is
capable of furnishing a larger number of ions

However, the increase in number of ions on dilution

is much lesser than increase in the volume of the
solution

Therefore, the number of ions per unit volume (per cc) actually decreases.
Hence, the specific conductance decreases although with molar conductance
increases on progressive dilution.
Questions:
1. Define conductance, molar conductance and specific conductance. Give their units.

2. Solutions of two electrolytes A & B each having concentration of 0.2M have

conductivities 2×10-‒2 and 4 × 10‒4 S cm‒1. Which will offer greater resistance to the
flow of current and why?

3. The specific conductance of 0.01 M solution of acetic acid was found to be 0.0163 S
m‒1 at 25 °C. Calculate the degree of dissociation of the acid. Molar conductance of
acetic acid at infinite dilution is 390.7 × 10‒4 S m2 mol‒1 at 25 °C.

4. Specific conductance of a decimolar solution of KCl at 18°C is 1.12 Sm‒1. The

resistance of a conductivity cell containing the solution at 18°C was found to 55 Ω.
Calculate the cell constant.

5. What is the effect of dilution on (a) the specific conductance of CH3COOH (b) the
equivalent conductance of CH3COOH and on (c) the equivalent conductance of NaOH

6. Molar conductivity of a solution is 1.26 x 102 Ohm‒1 cm2 mol‒1. Its molarity is 0.01.
What will be its specific conductivity?
Conductance of Alkali Metal Ions in Water
LiCl <NaCl <KCl
Lithium and sodium ions have comparatively lower ionic
mobilities
This is due to the higher charge density around these
ions because of their small radii
The higher density causes these ions to be more highly
hydrated by ion-dipole interactions than the larger ions
Since hydrated ions has to drag along a shell of water as
it moves through the solution, its mobility is naturally less
than that of an unhydrated ion
• Ionic Mobility
• The ionic mobility is extremely small as compared to the speed of
gaseous molecules which is about 102 ms-1. The low mobility of
ions is due to the fact that there are frequent collisions between
the ions and the solvent molecules since the mean free path of
molecules in the liquid is very small.
• The ionic mobility of H+ ion is found to be five to ten times that of
other ions, except OH- ion
• H+ ion in aqueous solutions is hydrated to form H9O4+ ion, i.e., a
trihydrate of hydronium ion, viz., H3O+. 3H2O, having the following
structure
H

H H O H
O H
+O
H H O H

H
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• Ionic Mobility
• The high mobility in hydroxylic solvents such as water can be
explained by Grotthus type mechanism in which the proton moves
rapidly from H3O+ to a hydrogen bonded water molecule and is
transferred further along a series of hydrogen bonded water
molecules by a rearrangement of hydrogen bonds.
• This accounts to high mobility of hydrogen ions in water.

Grotthus-type O

H H
Mechanism for H + H
O O H
High mobility of
H
+O
H O H
H+ ions H
H H
O

H 18
• Ionic Mobility
• Grotthus model also explains as to why H+ ions move about 50
times more rapidly through ice than through liquid water
• Ice has tetrahedral structure with each oxygen atom surrounded
by four hydrogen atoms

H H

O 1

The central oxygen atom A is

H
Covalent bond
surrounded tetrahedrally by
Hydrogen bond
O A
the atoms marked 1,2,3 and 4
H H
H
H 4
O O
2
H O 3 H

H
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 Each hydrogen atom lies on the H H

O 1
center of line joining of the oxygen
atoms. H

 When water molecules are O A

oriented properly, as in the case of
H H
ice, the hydrogen ions can move H
O
H 4
2 O
rapidly through its tetrahedral H O 3 H

structure H

20
• Ionic mobilities increase with temperature, the temperature

coefficients being very nearly the same for all the ions in a given
solvent.

• Thus, ionic mobilities increase by about 2% per degree in the

vicinity of 25 °C

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 22

Conductometric Titrations
Principle:

The principle of conductometric titration is based on the

fact that during the titration, one of the ions is replaced by
the other and invariably these two ions differ in the ionic
conductivity with the result that conductivity of the solution
varies during the course of titration. The equivalence point
may be located graphically by plotting the change in
conductance as a function of the volume of titrant added.

Conductance depends upon:

I. Nature of ion
II. Number of ions present (concentration)
 • Conductometric titrations
• Conductance measurements are frequently employed to find the
end points of acid-alkali and other titrations.
• Ex. Titration of strong acid (HCl) with a strong base (NaOH) with acid
in conducting vessel and alkali in burette – the conductance of HCl is
due to the presence of hydrogen and chloride ions
• H+ (aq) + Cl- (aq) + [Na+ (aq) + OH- (aq)]  Na+ (aq) + Cl- (aq) + H2O (liq)

• On plotting conductance
against volume of alkali
Conductance

added, the point of

intersection X of these two
X lines gives the volume of alkali
required for the neutralization.
Equivalence point
Volume of alkali added
• Conductometric titrations: CH3COOH against NaOH
• If a weak acid (such as acetic acid) is titrated against a strong alkali
(such as NaOH), the conductance of acid will be low on account of
poor dissociation.
• On adding alkali, highly ionized sodium acetate is formed and hence
the conductance begins to increase
• CH3COOH (aq) + [Na+ (aq) + OH- (aq)]  Na+ (aq) + CH3COO- (aq) + H2O (l)
• When the acid is completely neutralized, further addition of alkali
introduces excess of fast moving OH ions
• The conductance of solution
begins to increase even more
sharply than earlier case.
Conductance

• On plotting conductance
X against volume of alkali
added, the point of
intersection X of these two
Equivalence point
lines gives the volume of alkali
Volume of alkali added required for the neutralization
• Conductometric titrations: Mixture of HCl+CH3COOH against
NaOH: When a mixture of a strong and a weak acid is to be
titrated against a strong alkali a combination of earlier curves
is obtained.

• If a mixture of HCl and CH3COOH is to be titrated against

NaOH, HCl will get titrated first, and the titration of acetic acid
will commence only after HCl has been completely
neutralized.

• Point B corresponds to the

neutralization of HCl, the
Conductance

point C corresponds to the

neutralization of CH3COOH

HCl CH3COOH
B C
Volume of NaOH added
26
• Conductometric titrations:

• If a strong acid like HCl is titrated against a weak base, like

NH4OH, the conductance will fall at first due to replacement of fast
moving H+ ions by slow moving NH4+ ions
• H+ (aq) + Cl- (aq) + [NH4OH (aq)]  NH4+ (aq) + Cl- (aq) + H2O (l)

• After neutralization of the acid, further addition of weakly ionised

ammonium hydroxide will not cause any appreciable change in the
conductance
Conductance

X
Equivalence point

Volume of NH4OH added

• Conductometric titrations: Silver nitrate against potassium
chloride
• Ag+ (aq) + NO3- (aq) + [K+ + Cl-]  K+ (aq) + NO3-(aq) + AgCl (s)

• Since the mobility of K ion in nearly the same as that of silver ion
which it replaces, the conductance will remain more or less
constant and will begin to increase only after the equivalence
point.
Conductance

X
Equivalence point

Volume of KCl added

 Advantages of Conductometric titrations:
• Colored solutions which cannot be titrated by ordinary volumetric
methods with help of indicators, can be successfully titrated
conductmetrically
• The method can also employed in the case of very dilute solutions
and also for weak acids and bases.
• No special care is necessary near the end point as it is
determined graphically.
Q 1. What is ionic mobility? A potential of 12.0 volts was applied to two
electrodes placed 20 cm apart. A dilute solution of NH4Cl was placed between
the electrodes when NH4+ ion was found to cover a distance of 1.60 cm in one
hour. What is the mobility of NH4+ ion?
2. Why do hydrogen ions (H+) have a very large ionic mobility (molar
conductance) when compared to all other common ions?

3. Why the mobility of Li+ ion is less than that of Na+ ion?

4. What is meant by conductometric titration? Explain with the help of plots,

how does conductance varies when (a) CH3COOH acid is titrated against
NaOH and (b) AgNO3 is titrated against KCl.

5. In conductometric titration, more concentrated solution is added from the

burette, why?

6. What are the advantages of conductometric titrations over volumetric

titrations?