Module1.Thermodynamics, Electrochemistry& corrosion (12 h)
Electrochemistry: Electrode potentials: Origin of electrode potential using Nernst
electrolytic pressure theory, Introduction to single electrode potential, Derivation
of Nernst equation, Electrochemical cell: Construction and working of Daniel cell
using Zn and Cu electrodes, electrochemical series, types of electrodes (Hydrogen,
Calomel electrode, glass electrode). Numerical problems based on determination
of EMF.
Corrosion- causes- factors- electrochemical theory of corrosion, types-chemical,
electrochemical corrosion (galvanic, differential aeration), corrosion control -
material selection and design aspects - electrochemical protection – sacrificial
anode method and impressed current cathodic method.
Thermodynamic functions: Energy, entropy and free energy. Estimations of
entropy and free energies. Use of free energy considerations in metallurgy through
Ellingham diagrams.
ELECTROCHEMIST
RY
Chemical reactions involves oxidation and reduction reaction or redox
reactions
Redox reaction are those involving the oxidation and reduction of species.
LEO – Loss of Electron is Oxidation
GER – Gain of electrons Is Reduction
Oxidation and reduction must occur together.
They cannot exist alone.
For example: If Zinc metal is placed in a solution of copper sulphate,
immediate precipitation of Cu takes place.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Here in the above case:
Zinc atom (Zn) is oxidised to Zinc Ion (Zn2+), since it loses electrons
Whereas copper Ion (Cu2+) is reduced to copper atom, since it gains electrons
,1.1 Electrochemistry
Terms involved
SINGLE ELECTRODE POTENTIAL (E):
It can be defined as “the potential developed at the interface between the
metal and solution, when it is in contact with a solution of its own ions”
STANDARD REDUCTION POTENTIAL (Eo):
“Standard electrode potential is the electrode potential when the electrode
is in contact with a solution of unit concentration; at 298K. If the electrode
involves a gas then the gas at one atmosphere pressure”.
EMF OF A CELL:
“Emf is the potential difference between the two electrodes of a galvanic
cell which causes the flow of current from one electrode to other”
Measurement of electrode potential:
For example, to measure the potential of the zinc electrode is coupled with SHE through
a salt bridge and voltmeter, as in diagram.
In the above cell, it was found that the given Zn electrode acts as anode
and SHE acts as cathode.
Emf of above cell was found to be 0.76V
Ecell= Ecathode-Eanode
Ecell= E0SHE - Ezn/zn2+
0.76= 0 - EZn/Zn2+ E0 SHE = 0
Ezn/zn2+ = -0.76V
Hence the electrode potential of zinc is –ve
Origin of electrode potential
When a metal is in contact with a solution of its own ions, two types of reactions
are possible
, Metal as the tendency to undergo dissolution
M → Mn+ + ne − ------------(1)
This is due to solution pressure or solution tension.
On the other hand metal ions in the solution have a tendency to get deposited on
the metal.
M n+ + ne − →M ------------(2)
This tendency is due to the osmotic pressure of ions in the solution.
Suppose at equilibrium the forward reaction has occurred to a greater extent than the
backward reaction. As a result the electrode surface develops a layer of +ve charges,
which attracts a layer of –vely charged ions at the interface developing an electrical
double layer or Helmholtz double layer.
If the backward reaction has occurred to a greater extent than the forward reaction at
equilibrium electrode develops a layer of –ve charges, which attracts a layer of +vely
charged ions at the interface, again establishing an electrical double layer or
Helmholtz double layer.
Formation of Helmholtz double layer gives rise to a potential difference across the
layer. This potential difference between the metal and the solution at the interface is
the single electrode potential
NERNST EQUATION
In electrochemical cells, the potential of an electrode is found to depend on these
factors:
1) Nature of electrode
2) Concentration of Metal Ion
3) Temperature
Single electrode potential can be calculated from Nernst equation and the Nernst
equation can be derived on thermodynamic consideration.
Consider a general redox reaction
Electrochemistry: Electrode potentials: Origin of electrode potential using Nernst
electrolytic pressure theory, Introduction to single electrode potential, Derivation
of Nernst equation, Electrochemical cell: Construction and working of Daniel cell
using Zn and Cu electrodes, electrochemical series, types of electrodes (Hydrogen,
Calomel electrode, glass electrode). Numerical problems based on determination
of EMF.
Corrosion- causes- factors- electrochemical theory of corrosion, types-chemical,
electrochemical corrosion (galvanic, differential aeration), corrosion control -
material selection and design aspects - electrochemical protection – sacrificial
anode method and impressed current cathodic method.
Thermodynamic functions: Energy, entropy and free energy. Estimations of
entropy and free energies. Use of free energy considerations in metallurgy through
Ellingham diagrams.
ELECTROCHEMIST
RY
Chemical reactions involves oxidation and reduction reaction or redox
reactions
Redox reaction are those involving the oxidation and reduction of species.
LEO – Loss of Electron is Oxidation
GER – Gain of electrons Is Reduction
Oxidation and reduction must occur together.
They cannot exist alone.
For example: If Zinc metal is placed in a solution of copper sulphate,
immediate precipitation of Cu takes place.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Here in the above case:
Zinc atom (Zn) is oxidised to Zinc Ion (Zn2+), since it loses electrons
Whereas copper Ion (Cu2+) is reduced to copper atom, since it gains electrons
,1.1 Electrochemistry
Terms involved
SINGLE ELECTRODE POTENTIAL (E):
It can be defined as “the potential developed at the interface between the
metal and solution, when it is in contact with a solution of its own ions”
STANDARD REDUCTION POTENTIAL (Eo):
“Standard electrode potential is the electrode potential when the electrode
is in contact with a solution of unit concentration; at 298K. If the electrode
involves a gas then the gas at one atmosphere pressure”.
EMF OF A CELL:
“Emf is the potential difference between the two electrodes of a galvanic
cell which causes the flow of current from one electrode to other”
Measurement of electrode potential:
For example, to measure the potential of the zinc electrode is coupled with SHE through
a salt bridge and voltmeter, as in diagram.
In the above cell, it was found that the given Zn electrode acts as anode
and SHE acts as cathode.
Emf of above cell was found to be 0.76V
Ecell= Ecathode-Eanode
Ecell= E0SHE - Ezn/zn2+
0.76= 0 - EZn/Zn2+ E0 SHE = 0
Ezn/zn2+ = -0.76V
Hence the electrode potential of zinc is –ve
Origin of electrode potential
When a metal is in contact with a solution of its own ions, two types of reactions
are possible
, Metal as the tendency to undergo dissolution
M → Mn+ + ne − ------------(1)
This is due to solution pressure or solution tension.
On the other hand metal ions in the solution have a tendency to get deposited on
the metal.
M n+ + ne − →M ------------(2)
This tendency is due to the osmotic pressure of ions in the solution.
Suppose at equilibrium the forward reaction has occurred to a greater extent than the
backward reaction. As a result the electrode surface develops a layer of +ve charges,
which attracts a layer of –vely charged ions at the interface developing an electrical
double layer or Helmholtz double layer.
If the backward reaction has occurred to a greater extent than the forward reaction at
equilibrium electrode develops a layer of –ve charges, which attracts a layer of +vely
charged ions at the interface, again establishing an electrical double layer or
Helmholtz double layer.
Formation of Helmholtz double layer gives rise to a potential difference across the
layer. This potential difference between the metal and the solution at the interface is
the single electrode potential
NERNST EQUATION
In electrochemical cells, the potential of an electrode is found to depend on these
factors:
1) Nature of electrode
2) Concentration of Metal Ion
3) Temperature
Single electrode potential can be calculated from Nernst equation and the Nernst
equation can be derived on thermodynamic consideration.
Consider a general redox reaction
