Electrochemistry deals with chemical reactions caused by electric currents or electric currents produced by chemical reactions. Galvanic cells convert chemical energy to electrical energy through redox reactions. Reversible cells like Daniel cells can undergo reactions in both directions while irreversible cells like zinc-silver cells cannot. Protective metal coatings through electroplating or electroless plating prevent corrosion by depositing a noble metal layer on a substrate.

1. UNIT- IV-ELECTROCHEMISTRY

2. Electrochemistry deals with the chemical
reactions produced by passing electric
current through an electrolyte or the
production of electric current through
chemical reactions

3. Types of Conductors
Metallic conductors (or) Electronic
conductors
Electrolytic Conductors

4. Differences between metallic conduction and
electrolytic conduction
Metallic conduction
• It involves the flow of
• electrons.
• It does not involve any
transfer
• of matter.
• Conduction decreases with
• increase in temperature.
Electrolytic conduction
• It involves the movement of
• ions present in the solution.
• It involves tranfer of
• electrolyte.
• Conduction increases with
• increase in temperature.

5. Cell Terminology
CURRENT
ANODE
CATHODE
ANODE COMPARTMENT
CATHODE COMPARTMENT
ELECTROLYTE
CELL

6. A metal (M) consists of metal ions (Mn+)
with valence electrons. When the metal
(M) is placed in a solution of its own salt,
any one of the following reactions will
occur.
Positive metal ions may pass into the
solution. M −−−−−>Mn+ + ne− (oxidation)
Positive metal ions from the solution may
deposit over the metal.
Mn+ + ne− −−−−−> M (reduction)

8. Thus, a sort of layer (positive (or) negative ions)
is formed all around the metal. This layer is
called Helmholtz electrical double layer. This
layer prevents further passing of the positive
ions from or to the metal.At equilibrium, the
potential difference becomes a constant value,
which is known as the electrode potential of a
metal.

9. Factors affecting electrode potential
The rate of the above reactions depend on
The nature of the metal.
The temperature.
The concentration of metal ions in solution.

10. Single electrode potential (E)
It is the measure of tendency of a
metallic electrode to lose or gain
electrons, when it is in contact with a
solution of its own salt.
Standard electrode potential (E°)
It is the measure of tendency of a
metallic electrode to lose or gain
electrons, when it is in contact with a
solutionof its own salt of 1 molar
concentration at 25°C.

11. Nernst equation for electrode
potential

12. Applications of Nernst equations
Nernst equation is used to calculate
electrode potential of unknown metal.
Corrosion tendency of metals can be
predicted.

13. REFERENCE ELECTRODES
(STANDARD ELECTRODES)

14. Primary Reference Electrode
(Standard Hydrogen electrode)

15. Construction
• Hydrogen electrode consists of platinum foil,
that is connected to a platinum wire and
sealed in a glass tube. Hydrogen gas is passed
through the side arm of the glass tube. This
electrode, when dipped in a 1N HCl and
hydrogen gas at 1 atmospheric pressure is
passed forms a standard hydrogen electrode.
The electrode potential of SHE is zero at all
temperatures.

16. It is represented as,
+
Pt , H2(1 atm)/H (1 M); E° = 0 V

17. Limitations
1) It requires hydrogen gas and is
difficult to set up and transport.
2)It requires considerable volume of test
solution.
3)The solution may poison the surface of
the platinum electrode.

19. Secondary Reference Electrode
• Construction
• Calomel electrode consists of a glass tube
containing mercury at the bottom over which
mercurous chloride is placed. The remaining
portion of the tube is filled with a saturated
solution of KCl. The bottom of the tube is sealed
with a platinum wire. The side tube is used for
making electrical contact with a salt bridge. The
electrode potential of the calomel electrode is +
0.2422 V.

21. It is represented as,
Hg , Hg2Cl2(s) , KCl(sat.solution);
E° = 0.2422 V

22. If the electrode acts as anode the
reaction is 2Hg(l) −−−−−> Hg2
2++ 2e−
Hg2
2+ + 2Cl−−−−−−>Hg2Cl2(s)
2Hg(l) + 2Cl− ------Hg2Cl2(s) +

23. The electrode potential
of the three calomel electrodes on
the hydrogen scale at
298 K are given as
0.1 N KCl=+ 0.3338 V
1.0 N KCl=+ 0.2800 V
Saturated KCl = + 0.2422 V.

24. If the electrode acts as cathode the
reaction is
Hg2 Cl2 (s) −−−−−>Hg2
2+ + 2Cl−
Hg2
2+ + 2e−−−−−−>2Hg(l)
Hg2Cl2(s) + 2e− −−−−−> 2Hg(l) + 2Cl−`

25. Measurement of single electrode
potential of Zn
using saturated calomel electrode

27. Ecell = E°right − E°left
Ecell = E°cal − E°Zn
E°Zn = E°cal − Ecell
= + 0.2422 − 1.0025
E°Zn = − 0.7603 volt

28. Internal Reference Electrode
Glass Electrode

30. Construction
• A glass electrode consists of thin-walled glass bulb (the
glass is a special type having low melting point and
high electrical conductivity) containing a Pt wire in
0.1M HCl (Fig.1.5).
• The glass electrode is represented as
• Pt, 0.1 M HCl / Glass
• HCl in the bulb furnishes a constant H+ ion
concentration

31. Determination of pH of a Solution
using Glass Electrode

32. the pH of the solution is calculated as
follows
Ecell = Eright − Eleft
Ecell = Ecal − EG
= Ecal − (E°G + 0.0592 pH)
= Ecal − E°G − 0.0592 pH
pH=( 0.2422-Ecell − E°G)/0.0592

33. Advantages of Glass Electrode
It can be easily constructed and readily
used.
The results are accurate.
It is not easily poisoned.
Equilibrium is rapidly achieved.

34. Disadvantages (Limitations)
Since the resistance is quite high, special
electronic potentiometers are employed
for measurement.
The glass electrode can be used in
solutions only with pH range of 0 to 10.
However above the pH 12 (high alkalinity),
cations of the solution affect the glass and
make the electrode useless

35. Applications of ISEs
• ISEs are used in determining the concentrations of cations likeH+,
Na+, K+, Ag+, Li+.
• ISEs are used for the determination of hardness Ca2+ and Mg2+
ions).
• Concentrations of anions like NO3
−, CN−, S2−, halides (X−) can be
determined.
• ISEs are used in the determination of concentration of a gas by
using gas-sensing electrodes.
•
• pH of the solution can be measured by using gas-sensing electrode
•

36. TYPES OF CELLS
ELECTROCHEMICAL CELLS OR
GALVANIC CELLS

37. GALVANIC CELLS
• .
• Electrochemical cells are entirely different from
electrolytic cells. The cells used for electrolysis (where
electrical energy is converted to chemical energy) are
called electrolytic cells, whereas in electrochemical
cells, chemical energy is converted to electrical energy.
Galvanic cells are electrochemical cells in which the
electrons, transferred due to redox reaction, are
converted to electrical energy.
•

39. Representation of a galvanic cell
• A galvanic cell consists of two electrodes anode and cathode.
• The anode is written on the left hand side while the cathode is written on
the right hand side.
• The anode must be written by writing electrode metal first and then electrolyte.
These two are separated by a vertical line or a semicolon. The electrolyte may be
written by the formula of the compound (or) by ionic species.
• The cathode must be written by writing electrolyte first and then the electrode
metal. These two are separated by a vertical line or a semicolon.
• The two half cells are separated by a salt bridge, which is indicated by two vertical
lines.

40. EMF OF A CELL
Definition
Electromotive force is defined as, "the
difference of potential which causes
flow of current from one electrode of
higher potential to the other electrode
of lower potential.

41. Measurement of emf of a cell
• The potential difference or emf of a cell can be
measured on the basis of poggendorff‟s
compensation principle. Here the emf of the
cell is just opposed or balanced by an emf of
standard cell (external emf), so that no
current flows in the circuit.

43. The potentiometer consists of a uniform
wire AB (Fig. 1.8). A storage battery (K) is connected
to the ends A and B of the wire through a rheostat (R).
The cell of unknown emf (x) is connected in the circuit
by connecting its positive pole to A and the negative
pole is connected to a sliding
contact (D) through a galvanometer G. The sliding
contact is freely moved along the wire AB till no
current flows through the galvanometer. Then the
distance AD ismeasured.

44. The emf of unknown cell is directly
proportional to the distance AD.
Exµ AD

45. Then the unknown cell (x) is replaced by a
standard cell (s) in the circuit. The sliding
contact is again moved till there is null
deflection in the galvanometer.
Then the distance AD′ is measured. The emf of
standard cell Es is directly proportional to the
distance AD′.
Esµ AD′

46. Factors affecting emf of a cell
Nature of the electrolytes and electrodes.
Concentration and composition of the
electrolytes.
pH and temperature of the solution.

47. Applications of emf measurements
• Determination of standard free energy change and equilibrium
constant.
• Determination of pH by using a standard hydrogen electrode.
• Solubility of a sparingly soluble salt can be determined.
• Valency of an ion can be determined.
• Potentiometric titrations can be carried out.
• Hydrolysis constant can also be determined

48. Differences between electrolytic cells and electrochemical cells
Electrolytic Cell
• Electrical energy is converted
• into electrical energy.
• The anode carries positive
• charge.
• The cathode carries negative
• charge.
• Here the electrons are supplied
• to the cell from the external
• battery. i.e., electrons move in
• through cathode and comes out
• from the cathode
Electrochemical Cell
• Chemical energy is converted into
• electrical energy.
• The anode carries negative charge.
• The cathode carries positive charge.
• But the electrons are drawn from the
• cell. i.e., electrons move from anode
• to cathode through the external
• circuit.

49. Differences
• Amount of the electricity
• passed during electrolysis
is
• measured by coulometer.
•
• The extent of chemical
reaction
• occurring at the
electrodes is
• governed by Faraday‟s
law of
• electrolysis.
• The e.m.f. produced in
the cell is
• measured by
potentiometer.
•
•
• The e.m.f. of the cell
depends on the
• concentration of the
electrolytes and
• the chemical nature of
the electrode.

50. REVERSIBLE AND IRREVERSIBLE
CELLS
Reversible Cells

51. Daniel cell, secondary batteries
(rechargeable batteries).Daniel cell is a
very good example for a reversible cell.
Its emf is 1.1 volt. It is represented as
Zn/ZnSO4(1 M)//CuSO4 (1 M)/Cu.

52. A cell which obey the following the
conditions of thermodynamic
reversiblity is called reversible cell.

53. If the daniel cell is connected to an external
source of emf equal to 1.1 volt, no current
flows and also no chemical reaction takes
place in the cell.
If the external emf is made slightly less
than 1.1 volt, small amount of current
flows from the cell and small chemical
reaction occurs.

54. If the external emf is made slightly
greater than 1.1 volt, the current will
flow in the opposite direction. Copper
will pass into the solution as copper
ions and zinc will get deposited on the
zinc electrode.

55. Irreversible Cells
• Zinc – silver cell, Dry cell (Primary Cells)
• Cells which do not obey the conditions of
thermodynamic reversibility are called
irreversible cells
• Zinc-Silver cell is an example for a irreversible cell.
It is represented as
Zn/H2SO4(aq)/Ag

56. The cell reactions occur at anode and
cathode are
Zn + H2SO4 −−−> ZnSO4 + H2↑
(at anode)
2Ag+ + 2e− −−−> 2Ag (at cathode)

57. When the two electrodes are
connected, zinc dissolves with the
liberation of hydrogen gas

58. When the external emf, slightly greater
than the actual emf of the cell, is applied to
it, the above reactions are not reversed.
Because one of the product, H2 gas, is
already escaped.
Such a cell, which does not obey the
conditions of thermodynamic reversibility,
is called an irreversible cell.

59. PROTECTIVE COATINGS
Types of protective coating

61. METALLIC COATINGS
Electroplating or Electro-deposition

62. Definition
• The basic principle of electroplating is
coating the coating material on the base
metal by passing direct current through an
electrolytic solution containing the soluble
salt of the coating material

64. Process
The copper object, to be plated, is first
treated with dil. HCl or dil. H2SO4.
The cleaned object is then made cathode
of an electrolytic cell and gold foil as the
anode.
AuCl3 solution is taken as electrolyte.
.

65. When the current is passed through
the solution from battery through the
solution, gold dissolves in the
electrolyte and deposits uniformly on
the copper object.

66. At Cathode: On passing current, Au3+
ions moves to the cathode and get
deposited there as Au metal.
At Anode: The free chloride ions
migrate to the gold anode and
dissolves an equivalent amount of Au
from AuCl3.

67. In order to get strong, adherent and
smooth deposit certain additive (glue,
gelatin, etc) are added to the electrolytic
bath. To improve the brightness of deposit,
brightening agents are added in the
electrolytic bath.
optimum temperature (60oC),
optimum current density (1-10 mA/cm2)

68. Electroless Plating
Electroless plating is a technique of
depositing a noble metal (from its salt
solution) on a catalytically active
surface of the metal, to be protected ,
by using a suitable reducing agent
without using electrical energy.

69. Metal ions + Reducing agents
Metal (deposited) +
oxidised products

70. Electroless Nickel plating

71. Nature of the Name of the Quantity (g/l) Function
compound compound
Coating solution NiCl2 20 Coating metal
Reducing agent
Sodium
20 Metal ions reduced
hypophosphite
Complexing agent
Sodium succinate 15
Improves the
cum exhaltant quality
Buffer Sodium acetate 10 Control the pH
Optimum pH 4.5 - -
Optimum 93oC - -
temperature

72. Step I: Pretreatment and activation of
the surface
Step II: Preparation of plating bath
Step III : Procedure for plating

73. Difference between electroplating and
electroless plating
Electroplating
• Carried out by passing
current
• Separate anode is
employed
• Object to be coated is
cathode
• It is not satisfactory
for the object having
irregular shape
CaCarried out
• on
• conducting
• materials
•
•
• rried out
• on
• conducting
• materials
•
•
Electroless plating
• Carried out by auto
catalytic method
• Catalytic surface on
the substrate acts as
anode
• Object to be coated,
after making its surface
catalytically active
• It is satisfactory for all
parts
•
•
•

74. Applications
Extensively used in electronic applications.
Used in domestic as well as automotive
industry.
Polymers can be coated and used in
decorative and functional works.
Electroless Ni & Cu coated plastic cabinets
are used in digital as well as electronic
instruments.

75. Advantages
• No electricity is required.
• Electroless plating on insulators and
semiconductors can be easily carried out.
• Complicated parts can also be plated uniformly.
• Electroless coating posses good mechanical,
chemical and magnetic properties.