FUTURE BHUBANESWAR SCHOOL

CLASS XII # SUBJECT – CHEMISTRY

CHAPTER 2 # ELECTROCHEMISTRY
Now let us learn about the following in lesson 2
 Electrical resistance
 Types of conductor
 Kohlrausch law

Lesson 2
Electrical resistance
o It is denoted by the symbol ‘R’.
o It is measured in a unit called ohm (Ω) with the help of a Wheatstone bridge
o The electrical resistance is:
i) Directly proportional to its length, l
ii)Inversely proportional to its area of cross section, A.

o In the equation the constant of proportionality denoted by  (called rho) is called

Specific Resistance or resistivity.
o It is measured in units called ohm metre.
o When length = 1m.
Cross sectional area = 1m2
o Then resistivity becomes the resistance.
1 Ω m = 100 Ω cm

Electrical conductance
o The inverse of resistance is known as conductance.

o It is measured in a unit called Siemens represented by the symbol ‘S’

o It is equal to ohm–1 , also known as mho or Ω –1.

The electrical conductance depends on:

1. Nature and structure of the metal
2. Number of valence electrons per atom

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY
3. Electrical conductance decreases with increase of temperature and vice
versa.

The inverse of resistivity is termed as Specific Conductance or

conductivity.
κ = 1/ ρ
o It is represented by the symbol K (Greek, kappa).
o It is measured in a unit called Sm–1.
o When length = 1m.
Cross sectional area = 1m2
Then conductivity becomes the conductance.

(Fig 1)

o The free ions Na+ and Cl- present in the solution are responsible for the
conductance in a solution.

The conductivity of an electrolytic solution depends on:

1.Nature of the electrolyte added

2.Size of the ions produced and their solvation
3.Nature of the solvent and its viscosity
4.Concentration of the electrolyte
5.It increases with the increase of temperature.

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY
Conductors and Non Conductors
Substances around us can be divided into two classes based on their ability of
conduct electricity:

 Non-Conductors: Those substances which do not allow electric current to pass

through them are called non-conductors or insulators. Example: - wood, plastic
glass, rubber etc.
 Conductors: Those substances which allow electric current to flow through them
are called conductors. Examples: Copper, Iron, Gold, Silver, Graphite, salt solution
etc.
Conductors can further be divided into two groups

 Metallic Conductors: These conductors conduct electricity or electric current by

movement of electrons without undergoing any chemical change during the
process. These conduct electricity in both solid as well as molten state. Example:
All the metals and Graphite
 Electrolytes: Those substances which conduct electricity only when they are
present in aqueous solution and not in solid form are called electrolytes. These
conduct electricity by movement of ions in solutions.

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY
Comparison of Electrolytic and Metallic Conduction

S.No Metallic Conduction Electrolytic Conduction

1 Electric current flows by movement of Electric current flows by movement of ions.

electrons.

2 No chemical change occurs. Ions are oxidized or reduced at the electrodes.

3 It does not involve the transfer of any It involves transfer of matter in the form of ions.
matter.

4 Ohm's law is followed. Ohm's low is followed.

5 Resistance increases with increase of Resistance decreases with increase of

temperature. temperature.

6 Faraday law is not followed. Faraday law is followed.

o Matters can also be classified into conductors, insulators and semiconductors

depending on the magnitude of their conductivity.

Conductors:
o Solids with conductivities ranging between 104 to 107 ohm–1m–1 are conductors.
o Metals have conductivities in the order of 107 ohm–1m–1 is good conductors.
For example, Iron, Copper, Aluminum.

(Fig 2)

Fig. Metal is used at the tip of the plug that is inserted into the socket

Insulators:
o Solids with very low conductivities ranging between 10–20 to 10–10 ohm–1m–1.
For example, Wood, plastic, cloth, glass.

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 FUTURE BHUBANESWAR SCHOOL
CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY

(Fig 3)

Fig. Wood and plastics are also solids but are insulators

(Fig 4)

A man touching the electric pole with a metal rod will get an electric shock because
metal rod is a conductor whereas a man touching the same with a wooden plank will
be safe because wood is an insulator.

Semiconductors:
o Solids with conductivities in the intermediate range from 10–10 to 104 ohm–1 m–1.
For example, Gallium, Germanium, Silicon
Measurement of resistance of a solution of an electrolyte
o In the formula

o is called cell constant

o The cell constant depends on the distance between the electrodes (l) and their
area of cross-section (A).
o It is determined by measuring the resistance of the cell containing a solution of
known conductivity.

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY

E xam ple 1:

The resist ance of a conduct ivity cell cont aining 0.001M KCl solution at
298 k is 1500 Ω . W hat is t he cell constant if conductivity of 0.001M

K Cl solut ion at 298 k is 0.146 x 10 - 3 s cm - 1 .

S olution:
Conductivity, k = 0.146 × 10 - 3 S cm - 1
Resistance, R = 1500 Ω

cell constant = k × R
= 0.146 × 10 - 3 × 1500 = 0.219 cm –

Arrangement for measurement of Resistance

(Fig 5)

o The arrangement consists of two resistances R3 and R4.

o There is a variable resistance R1 and a conductivity cell with unknown
resistance R2.
o The Wheatstone bridge is provided with an oscillator O that acts as source of
a.c. power.
o The arrangement has a suitable detector P.

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY
o The Wheatstone bridge is balanced when there is no flow of current through the
detector.
Unknown resistance = R2 = R1R4 /R3
o After calculating the resistance the conductivity can be easily calculated using
the formula:
o κ = G' /R

Equivalent Conductance (Λ):

Conducting power of all the ions produced by one gram-equivalent i.e. one
equivalent of an electrolyte in a given solution

Λ = 1000 x

here, N be the normality of solution i.e. concentration of electrolytic solution in

equivalent/L.

Units: ohm–1 cm2(g eq)-1 or S cm2(g eq)-1

Molar Conductance (Λm):

Conductance of solution due to all the ions produced by one mole of the dissolved
electrolyte in a given solution.

Λm = 1000 x

Where, M is molarity of solution.

Units: ohm–1 cm2 mol-1 or S cm2 mol-1

Relation between Λ and Λm

Λm = n Λ

Where n = n-factor of the electrolyte = total charge carried by either ion = M/E

Example 2:
The conductiv it y of 0.20 M solution of KCl at 298 k is 0. 0248 scm - 1 .
Calc ulat e its m olar conductivity.
Solution:
k = 0.0248 S cm – 1
c = 0.20 M
Molar conductivity,Λm = 1000 x

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY
= 0.0248 x1000 / 0.20

= 124 S cm2 mol - 1

Variation of Conductivity and Molar Conductivity with

Concentration
o They depend on the concentration of the electrolyte. The Conductivity and
Molar Conductivity of both weak and strong electrolytes decreases with
decrease in concentration as the number of ions per unit volume carrying the
current in a solution decreases on dilution.
o When the concentration approaches to zero, the molar conductivity is referred
to as limiting molar conductivity. It is represented by the symbol om .
Strong electrolytes
o A solute or substances that completely ionize or dissociates in a solution are
known as strong electrolyte. These ions are good conductors of electricity in the
solution.
For example, HCl, HBr, HI, HNO3, NaOH, KOH, etc.
o For strong electrolytes, Λm increases slowly with dilution and can be
represented by the equation:
Λm c = Λm 0 – A 𝐶

This equation is called Debye Huckel-Onsager equation

o It dissociates completely at moderate concentrations.

o The conductance of the solution increases with dilution of solution.
o There exist strong inter -ionic forces of attraction at moderate concentrations.
o The slope for Λm vs c1/2 is linear at low concentrations.

Fig. Molar conductivity Λm Vs. c1/2 for acetic acid and potassium chloride

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CLASS XII # SUBJECT – CHEMISTRY
CHAPTER 2 # ELECTROCHEMISTRY

Kohlrausch law of independent migration of ions

The Kohlrausch law of independent migration of ions states that limiting molar
conductivity of an electrolyte can be represented as the sum of the individual
contributions of the anion and cation of the electrolyte.

Limiting molar conductivity of an electrolyte is the algebraic sum of limiting

molar equivalent conductivities of its constituent ions.

Mathematically, Λm0 = + 

Where  = limiting equivalent conductivities of cation

 = limiting equivalent conductivities of anion .

Weak electrolytes
o A substance which forms ions in an aqueous solution do not dissociate
completely at moderate concentrations is known as weak electrolyte.
o For example, CH3COOH (acetic acid), H2CO3 (carbonic acid), etc.
o The conductance of the solution increases with dilution of solution.
o The slope for Λm vs c1/2 is not linear even at lower concentrations.

o These electrolytes have lower degree of dissociation at higher concentrations.

o The value of Λm changes with dilution due to increase in the degree of
dissociation.
o Λm increases sharply on dilution exclusively at lower concentrations.
o At infinite dilution when concentration approaches to zero, the electrolyte
dissociates completely. But at lower concentration the conductivity of a solution
is low to an extent that cannot be even measured.

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