electrochemistryclass12-180412053250.pptx

Chapter 3
Electrochemistry
Electrochemistry

introduction
Electrochemistry
• Electrochemistry is the study of
production of electricity from energy
released during spontaneous chemical
reactions and the use of electrical
energy to bring about non-spontaneous
chemical transformations

Oxidation and Reduction
• What is reduced is the oxidizing agent.
➢H+oxidizes Zn by taking electrons from it.
• What is oxidized is the reducing agent.
➢Zn reduces H+by giving it electrons.
Electrochemistry

Electrochemistry
Electrochemical cells
• The cell that converts chemical energy to
electrical energy is called daniell’s cell.
and has an electrical potential equal to
1.1 V when concentration of Zn2+and Cu2+
ions is unity (1 mol dm–3). Such a device
is called a galvanic or a voltaic cell.
If an external opposite potential is applied
in the galvanic cell [Fig. 3.2(a)] and
increased slowly, we find that the
reaction continues to take place till the
opposing voltage reaches the value 1.1 V
when, the reaction stops altogether and
no current flows through the cell.
Any further increase in the external
potential again starts the reaction but in
the opposite direction . It now functions
as an electrolytic cell, a device for using
electrical energy to carry non-
spontaneous chemical reactions.
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
•
•

Electrochemistry
Functioning of Daniell cell when
external voltage Eext opposing
the cell potential is applied.

Galvanic cells
Electrochemistry
• In this device the Gibbs energy of the spontaneous redox reaction is
converted into electrical work which may be used for running a motor or
other electrical gadgets like heater, fan, geyser, etc.
Daniell cell discussed earlier is one such cell in which the following redox
reaction occurs.
Zn(s) + Cu2
+(aq) → Zn2
+(aq) + Cu(s)
This reaction is a combination of two half reactions whose addition gives the
overall cell reaction:
(i) Cu2
++ 2e– → Cu(s) (reduction half reaction)
)(ii) Zn(s) → Zn2+ + 2e– (oxidation half reaction)
These reactions occur in two different portions of the Daniell cell. The
reduction half reaction occurs on the copper electrode while the oxidation
half reaction occurs on the zinc electrode. These two portions of the cell are
also called half-cells or redox couples. The copper electrode may be called
the reduction half cell and the zinc electrode, the oxidation half-cell.
•
•
•
•
•
•

•
Electrochemistry
•
•
•
•
Each half- cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are
connected by a metallic wire through a voltmeter and a switch externally. The electrolytes of the
two half-cells are connected internally through a salt bridge.
Sometimes, both the electrodes dip in the same electrolyte solution and in such cases we do
not require a salt bridge.
At each electrode-electrolyte interface there is a tendency of metal ions from the solution to
deposit on the metal electrode trying to make it positively charged. At the same time, metal
atoms of the electrode have a tendency to go into the solution as ions and leave behind the
electrons at the electrode trying to make it negatively charged.
• At equilibrium, there is a separation of charges and depending on the tendencies of the two
opposing reactions, the electrode may be positively or negatively charged with respect to the
solution.
A potential difference develops between the electrode and the electrolyte which is called
electrode potential. When the concentrations of all the species involved in a half-cell is unity
then the electrode potential is known as standard electrode potential.
• According to IUPAC convention, standard reduction potentials are now called standard
electrode potentials. In a galvanic cell, the half-cell in which oxidation takes place is called
anode and it has a negative potential with respect to the solution.
The other half-cell in which reduction takes place is called cathode and it has a positive
potential with respect to the solution. Thus, there exists a potential difference between the two
electrodes and as soon as the switch is in the on position the electrons flow from negative
electrode to positive electrode. The direction of current flow is opposite to that of electron flow.

Electrochemistry
• The potential difference between the two electrodes of a galvanic cell is called the
cell potential and is measured in volts. The cell potential is the difference between
the electrode potentials (reduction potentials) of the cathode and anode.
• It is called the cell electromotive force (emf) of the cell when no current is drawn
through the cell. It is now an accepted convention that we keep the anode on the left
and the cathode on the right while representing the galvanic cell. A galvanic cell is
generally represented by putting a vertical line between metal and electrolyte
solution and putting a double vertical line between the two electrolytes connected by
a salt bridge. Under this convention the emf of the cell is positive and is given by the
potential of the half- cell on the right hand side minus the potential of the half-cell on
the left hand side i.e.,
Equation:- Ecell = Eright – Eleft
This is illustrated by the following cell reaction example :
Cell reaction:
Cu(s) + 2Ag+(aq) → Cu2+ (aq) + 2 Ag(s)
Half-cell reactions:
Cathode (reduction): 2Ag+
(aq) + 2e–→ 2Ag(s)
Anode (oxidation): Cu(s) → Cu2+(aq) + 2e–
• It can be seen that the sum of the above reactions leads to overall reaction in the
cell and that silver electrode acts as a cathode and copper electrode acts as an
anode. The cell can be represented as:
Cu(s)|Cu2+(aq)|Ag+(aq)|Ag(s) and we have
Ec
e
ll= Erig
h
t– Ele
ft = EA
g
+
A
g – EC
u
2
+
C
u
•

Measurement of electrode potential
Electrochemistry
• The potential of individual half-cell cannot be measured. We can measure
only the difference between the two half-cell potentials that gives the emf of
the cell. If we arbitrarily choose the potential of one electrode (half-cell) then
that of the other can be determined with respect to this.
According to convention, a half-cell called standard hydrogen electrode
represented by Pt(s) H2(g) H+(aq), is assigned a zero potential at all
temperatures corresponding to the reaction
H+(aq)+e– → 1/2H2 .
The standard hydrogen electrode consists of a platinum electrode coated
with platinum black. The electrode is dipped in an acidic solution and pure
hydrogen gas is bubbled through it. The concentration of both the reduced
and oxidised forms of hydrogen is maintained at unity . This implies that the
pressure of hydrogen gas is one bar and the concentration of hydrogen ion
in the solution is one molar.
•
•

•
Electrochemistry
•
• If the concentrations of the oxidised and the reduced forms of
the species in the right hand half-cell are unity, then the cell
potential is equal to standard electrode potential.
• Sometimes metals like platinum or gold are used as inert
electrodes. They do not participate in the reaction but provide
their surface for oxidation or reduction reactions and for the
conduction of electrons. For example, Pt is used in the following
half-cells:
Hydrogen electrode: Pt(s)|H2(g)| H+(aq)
With half-cell reaction: H+ (aq)+ e– → 1
⁄2H2(g) Bromine electrode:
Pt(s)|Br2(aq)| Br–(aq)
With half-cell reaction: 1
⁄2Br2(aq)+ e– → Br–(aq)

• If the standard electrode potential of an electrode is greater than zero then
its reduced form is more stable compared to hydrogen gas.
• Similarly, if the standard electrode potential is negative then hydrogen gas is
more stable than the reduced form of the species.
• It can be seen that the standard electrode potential for fluorine is the highest
in the Table indicating that fluorine gas (F2) has the maximum tendency to
get reduced to fluoride ions (F–) and therefore fluorine gas is the strongest
oxidising agent and fluoride ion is the weakest reducing agent.
• Lithium has the lowest electrode potential indicating that lithium ion is the
weakest oxidising agent while lithium metal is the most powerful reducing
agent in an aqueous solution. It is observed that as we go from top to
bottom the standard electrode potential decreases and with this, decreases
the oxidising power of the species on the left and increases the reducing
power of the species on the right hand side of the reaction.
• Electrochemical cells are extensively used for determining the pH of
solutions, solubility product, equilibrium constant and other thermodynamic
properties and for potentiometric titrations.
Electrochemistry

Standard Reduction Potentials
Reduction
potentials for
many
electrodes
have been
measured and
tabulated.
Electrochemistry

Oxidizing and Reducing Agents
• The strongest
oxidizers have the
most positive
reduction potentials.
• The strongest
reducers have the
most negative
reduction potentials.
Electrochemistry

Oxidizing and Reducing Agents
The greater the
difference between
the two, the greater
the voltage of the
cell.
Electrochemistry

Electromotive Force (emf)
Electrochemistry
• The potential difference between the
anode and cathode in a cell is called
the electromotive force (emf).
• It is also called the cell potential, and is
designated Ecell.

Electrochemistry
Standard Cell Potentials
The cell potential at standard conditions
can be found through this equation:
Ecell = Er

ed (cathode) −Er
ed (anode)
Because cell potential is based on
the potential energy per unit of
charge, it is an intensive property.

Cell Potentials
Electrochemistry
• For the oxidation in this cell,
Er
ed = −
0.76 V
• For the reduction,
Ered= +0.34 V

Cell Potentials
Electrochemistry
Ecell = Ered (cathode) −
Er

ed (anode)
= +0.34 V −(−
0.76 V)
= +1.10 V

Voltaic Cells
In spontaneous
oxidation-reduction
(redox) reactions,
electrons are
transferred and
energy is released.
Electrochemistry

Voltaic Cells
• A typical cell looks
like this.
• The oxidation occurs
at the anode.
• The reduction
occurs at the
cathode.
Electrochemistry

Voltaic Cells
Once even one
electron flows from
the anode to the
cathode, the
charges in each
beaker would not be
balanced and the
flow of electrons
would stop.
Electrochemistry

Voltaic Cells
• Therefore, we use a
salt bridge, usually a
U-shaped tube that
contains a salt
solution, to keep the
charges balanced.
➢Cations move toward
the cathode.
➢Anions move toward
the anode.
Electrochemistry

Voltaic Cells
• In the cell, then,
electrons leave the
anode and flow
through the wire to
the cathode.
• As the electrons
leave the anode, the
cations formed
dissolve into the
solution in the
anode compartment.
Electrochemistry

Voltaic Cells
• As the electrons
reach the cathode,
cations in the
cathode are
attracted to the now
negative cathode.
• The electrons are
taken by the cation,
and the neutral
metal is deposited
on the cathode.
Electrochemistry

Nernst’s equation
Electrochemistry
• We have assumed in the previous
section that the concentration of all the
species involved in the electrode
reaction is unity. This need not be
always true. Nernst showed that for the
electrode reaction:
• Mn+(aq) + ne–→ M(s)

•
Electrochemistry
It can be seen that E(c
e
ll)depends on the concentration of both
Cu2
+and Zn2
+ions. It increases with increase in the concentration
of Cu2
+ions and decrease in the concentration of Zn2
+ions.
• By converting the natural logarithm to the base 10 and
substituting the values of R, F and T = 298 K, it reduces to
E0
(c
ell) (c
e
ll)
= E0 – 0.059 log [Zn2+]
2 [Cu2+]
For a general electrochemical Nernst's equation
• a A + b B → c C + d D (on passing it with ne-)
E0
(c
ell) (c
e
ll)
E0
(c
ell) (c
e
ll)
= E0 – RT ln Q
nF
= E0 – RT ln [C]c[D]d
nF [A]a[B]b

Equilbirum constant from Nernst's
equation
Electrochemistry
•
•
If the circuit in Daniell celis closed then we note that the reaction
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) takes place and as time passes, the
concentration of Zn2+keeps on increasing while the concentration of Cu2+
keeps on decreasing.
At the same time voltage of the cell as read on the voltmeter keeps on
decreasing. After some time, we shall note that there is no change in the
concentration of Cu2+and Zn2+ions and at the same time, voltmeter gives zero
reading. This indicates that equilibrium has been attained. In this situation
the Nernst equation may be written as:
•
E0
(ce
ll) (c
e
ll)
= 0= E0 – 2.303 RT log [Zn2+]
2F [Cu2+]
Therefore,
E0
(c
e
ll) = 2.303 RT log [Zn2+]
2F [Cu2
+
]

Electrochemistry
But at equilibrium,
[Zn2+] = Kc
[Cu2+]
and at T = 298K the above equation can be
written as
E0
(cell) =0.059V log Kc = 1.1V
2
Therefore , log Kc= 1.1V x 2 = 37.288
0.059
Therefore Kc =2×1037at 298K.
In general,
E0
(cell) c
=2.303RT log K
nF

Electrochemistry
• Thus, the above equation gives a relationship
between equilibrium constant of the reaction
and standard potential of the cell in which that
reaction takes place. Thus, equilibrium
constants of the reaction, difficult to measure
otherwise, can be calculated from the
corresponding E⊖ value of the cell.

Electrochemistry
Electrochemical cell and gibbs
energy of the reaction
• Electrical work done in one second is equal to
electrical potential multiplied by total charge
passed. If we want to obtain maximum work
from a galvanic cell then charge has to be
passed reversibly. The reversible work done by
a galvanic cell is equal to decrease in its Gibbs
energy and therefore, if the emf of the cell is E
and nF is the amount of charge passed and ∆rG
is the Gibbs energy of the reaction, then
∆rG = – nFE(cell)

Electrochemistry
• We know that E(cell)is an intensive parameter but ∆rG is an extensive
thermodynamic property and the value depends on n.
Thus, if we write the reaction
Zn(s) + Cu2
+(aq) → Zn2
+
(aq) + Cu(s)
∆rG = – 2FE(cell) but when we write the reaction as
2 Zn (s) + 2 Cu2
+(aq) →2 Zn2
+
(aq) + 2Cu(s)
Then the equation becomes
∆rG = – 4FE(cell)
•If the concentration of all the reacting species is unity, then
Ecell= E⊖
celland we have
∆rG⊖ = – nFE⊖
cell
Thus, from the measurement of E⊖
cell we can obtain an important
thermodynamic quantity, ∆rG⊖, standard Gibbs energy of the reaction. From
the latter we can calculate equilibrium constant by the equation:
∆rG⊖ = –RT ln K.

Electrochemistry
Conductance of electrolytic cells
• The electrical resistance is represented by the symbol ‘R’ and it is measured
in ohm ( Ω)which in terms of SI base units is equal to (kg m2)/(S3 A2).
It can be measured with the help of a Wheatstone bridge. The electrical
resistance of any object is directly proportional to its length, l, and inversely
proportional to its area of cross section, A. That is,
•
we also know,
The constant of proportionality, is called resistivity (specific resistance).
• Its SI units are ohm metre (Ωm) and quite often its submultiple, ohm
centimetre (Ωcm) is also.
• Physically, the resistivity for a substance is its resistance when it is one
metre long and its area of cross section is one m2. It can be seen that:
1 Ωm = 100 Ωcm or 1 Ωcm = 0.01 Ωm

• The inverse of resistance, R, is called conductance, G,
and we have the relation:
G = 1/R = A/ρ
l =κ
A/l
• The SI unit of conductance is siemens, represented by
the symbol S’ and is equal to ohm–
1(also The known as
mho) or Ω–
1
.
• The inverse of resistivity, called conductivity (specific
conductance) is represented by the symbol, κ
.
• The SI units of conductivity are S m–1but quite often, κ
is expressed in S cm–1.
• Conductivity of a material in S m–1is its conductance
when it is 1 m long and its area of cross section
is1m2.It may be noted that 1Scm–1 =100Sm–1
.
Electrochemistry

•
Electrochemistry
•
•
•
The magnitude of conductivity varies a great deal and depends on the nature of the material. It
also depends on the temperature and pressure at which the measurements are made. Materials
are classified into conductors, insulators and semiconductors depending on the magnitude of
their conductivity.
Metals and their alloys have very large conductivity and are known as conductors. Certain non-
metals like carbon-black, graphite and some organic polymers* are also electronically
conducting.
Substances like glass, ceramics, etc., having very low conductivity are known as insulators.
Substances like silicon, doped silicon and gallium arsenide having conductivity between
conductors and insulators are called semiconductors and are important electronic materials.
Certain materials called superconductors by definition have zero resistivity or infinite
conductivity.
Earlier, only metals and their alloys at very low temperatures (0 to 15 K) were known to behave
as superconductors, but nowadays a number of ceramic materials and mixed oxides are also
known to show superconductivity at temperatures as high as 150 K.
• Electrical conductance through metals is called metallic or electronic conductance and is due to
the movement of electrons. The electronic conductance depends on
(i) the nature and structure of the metal
(ii) the number of valence electrons per atom
(iii) temperature (it decreases with increase of temperature).

•
Electrochemistry
As the electrons enter at one end and go out through the other end, the
composition of the metallic conductor remains unchanged. The mechanism
of conductance through semiconductors is more complex.
pure water has small amounts of hydrogen and hydroxyl ions (~10–7M)
which lend it very low conductivity (3.5 × 10–5 S m–1). When electrolytes are
dissolved in water, they furnish their own ions in the solution hence its
conductivity also increases.
The conductance of electricity by ions present in the solutions is called
electrolytic or ionic conductance. The conductivity of electrolytic (ionic)
solutions depends on:
•
•
i) the nature of the electrolyte added
ii) size of the ions produced and their solvation
iii) the nature of the solvent and its viscosity
iv) concentration of the electrolyte
v) temperature (it increases with the increase of temperature).
• Passage of direct current through ionic solution over a prolonged period can
lead to change in its composition due to electrochemical reactions

Measurement of conductivity of
ionic solutions
• A conductivity cell consists of two platinum electrodes coated
with platinum black which has finely divided metallic Pt is
deposited on the electrodes electrochemically. These have
area of cross section equal to ‘A’ and are separated by distance
‘l’. Therefore, solution confined between these electrodes is a
column of length l and area of cross section A. The resistance
of such a column of solution is then given by the equation:
R=ρl = l
A kA
Electrochemistry

•
Electrochemistry
•
• The quantity l/A is called cell constant denoted by the symbol,
G*.
• It depends on the distance between the electrodes and their
area of cross-section and has the dimension of length–1 and can
be calculated if we know l and A.
Measurement of l and A is not only inconvenient but also
unreliable. The cell constant is usually determined by
measuring the resistance of the cell containing a solution
whose conductivity is already known.
For this purpose, we generally use KCl solutions whose
conductivity is known accurately at various concentrations and
at different temperatures. The cell constant, G*, is then given
by the equation:
G*= llA =Rκ

•
• Once the cell constant is determined, we can use it for measuring the resistance or conductivity
of any solution.
It consists of two resistances R3and R4, a variable resistance R1 and the conductivity cell having
the unknown resistance R2.
• The Wheatstone bridge is fed by an oscillator O which is a source of a.c. power in the audio
frequency range 550 to 5000 cycles per second.
• P is a suitable detector and the bridge is balanced when no current passes through the
detector. Under these conditions:
Unknown resistance R2 = R1R4
R3
• Once the cell constant and the resistance of the solution in the cell is determined, the
conductivity of the solution is given by the equation:
κ
=
c
e
l
lconstant= G*
R R
• The conductivity of solutions of different electrolytes in the same solvent and at a given
temperature differs due to charge and size of the ions in which they dissociate, the
concentration of ions or ease with which the ions move under a potential gradient.
• It, therefore, becomes necessary to define a physically more meaningful quantity called molar
conductivity denoted by the symbol Λm
.It is related to the conductivity of the solution by the
equation:
Molar conductivity = Λ
m= κ
C
Electrochemistry

In the above equation, if κ
is expressed in S m–
1 and the
concentration, c in mol m–
3 then the units of Λ
mare in S m2 mol–
1.
Electrochemistry
1 S m2mol–1 = 104 S cm2mol–1 or 1 S cm2mol–1 = 10–4 S m2mol–1.

Variation of conductivity and molar
conductivity with concentration
Electrochemistry
• Both conductivity and molar conductivity change with the
concentration of the electrolyte. Conductivity always decreases
with decrease in concentration both, for weak and strong
electrolytes.
This can be explained by the fact that the number of ions per
unit volume that carry the current in a solution decreases on
dilution.
The conductivity of a solution at any given concentration is the
conductance of one unit volume of solution kept between two
platinum electrodes with unit area of cross section and at a
distance of unit length.
If A and l are unity then G*=k
•
•
•

• Molar conductivity of a solution at a given concentration is the
conductance of the volume V of solution containing one mole of
electrolyte kept between two electrodes with area of cross
section A and distance of unit length. Therefore,
Λ
m=κ
A
=κ( A=V and l are containing I mole of electrolyte)
•
l
Therefore Λ
m
= kV.
Electrochemistry

•
• Molar conductivity increases with decrease in concentration. This is because the total
volume, V, of solution containing one mole of electrolyte also increases. It has been
found that decrease in κ
on dilution of a solution is more than compensated by
increase in its volume.
At a given concentration, Λ
mcan be defined as the conductance of the electrolytic
solution kept between the electrodes of a conductivity cell at unit distance but having
area of cross section large enough to accommodate sufficient volume of solution that
contains one mole of the electrolyte.
• When concentration approaches zero, the molar conductivity is known as
o
.
m
Electrochemistry

Strong electrolytes
Electrochemistry
• For strong electrolytes, Λmincreases slowly with dilution and
can be represented by the equation:
Λ
m m
°–Ac1
⁄2
magainst c1
/2
, we obtain a straight line with intercept
m
° and slope equal to ‘–A’. The value of the constant
‘A’ for a given solvent and temperature depends on the type of
electrolyte i.e., the charges on the cation and anion produced
on the dissociation of the electrolyte in the solution. Thus, NaCl,
CaCl2, MgSO4 are known as 1-1, 2-1and 2-2electrolytes
respectively. All electrolytes of a particular type have the same
value for ‘A’.

•
Electrochemistry
m
° values for a number of strong
electrolytes and observed certain regularities.
m
° of the electrolytes NaX and
KX for any X is nearly constant. For example at 298 K:
•
• m(K
C
l
) m(N
a
C
l) m(K
B
r) m(N
a
B
r) m(K
I) m(N
a
I) ≃23.4 S cm2 mol–
1
and similarly it was found that
m(N
a
B
r) m(N
a
C
l) m(K
B
r) m(K
C
l)≃1.8 S cm2 mol–
1

Law of independent migration of
ions
Electrochemistry
• The law 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. Thus, if λ
°N
a
+ and λ
°C
l
–
are limiting molar conductivity of the sodium
and chloride ions respectively, then the limiting
molar conductivity for sodium chloride is given
by the equation:
(
N
a
C
l
) =λ0
N
a
+ +λ0
c
l
–

•
Electrochemistry
In general, if an electrolyte on dissociation gives ν
+ cations and
ν
–anions then its limiting molar conductivity is given by:
• ++ ν
-λ
0
-
Here, λ
0
+and λ
0
−are the limiting molar conductivities of the cation
and anion respectively.

Electrochemistry
Weak electrolytes
•
• o
Weak electrolytes like acetic acid have lower degree of dissociation at
higher concentrations and hence for such electrolytes, the change in Λm
with dilution is due to increase in the degree of dissociation and
consequently the number of ions in total volume of solution that contains 1
mol of electrolyte.
mincreases steeply on dilution, especially near lower
mcannot be obtained by extrapolation of Λ
mto
zero concentration.
At infinite dilution i.e., concentration c → zero electrolyte dissociates
completely (
α=1), but at such low concentration the conductivity of the
solution is so low that it cannot be measured accurately.
•
•
independent migration of ions . At any concentration c, if αis the degree of
• α
=
Λ
m
Λ
o
m

Electrochemistry
But for weak electrolytes like acetic acid, the
equation of Ka becomes
o
A
p
p
lica
tio
n
so
fk
o
h
lra
u
s
c
h
’slaw
Using Kohlrausch law of independent migration
mfor any
electrolyte from the λ
o of individual ions.
Moreover, for weak electrolytes like acetic acid it
is possible to determine the value of its
Λ
mat a given concentration c.

Electrochemistry
Electrolytic cells and electrolysis
• In an electrolytic cell external source of voltage is used to bring about a chemical
reaction.
One of the simplest electrolytic cell consists of two copper strips dipping in an
aqueous solution of copper sulphate.
If a DC voltage is applied to the two electrodes, then Cu2+ ions discharge at the
cathode (negatively charged) and the following reaction takes place:
Cu2+(aq) + 2e–→ Cu (s) Copper metal is deposited on the cathode. At the anode,
copper is
converted into Cu2+ ions by the reaction:
Cu(s) → Cu2+(s) + 2e–
Thus copper is dissolved (oxidised) at anode and deposited (reduced) at cathode.
This is the basis for an industrial process in which impure copper is converted into
copper of high purity. The impure copper is made an anode that dissolves on passing
current and pure copper is deposited at the cathode.
Sodium and magnesium metals are produced by the electrolysis of their fused
chlorides and aluminium is produced by electrolysis of aluminium oxide in presence
•
•
•
•
•
•
of cryolite.

Faraday’s law of electrolysis
Electrochemistry
Faraday’s two laws of electrolysis are:
•i) First Law: The amount of chemical reaction which occurs
at any electrode during electrolysis by a current is
proportional to the quantity of electricity passed through the
electrolyte (solution or melt).
•(ii) Second Law: The amounts of different substances
liberated by the same quantity of electricity passing through
the electrolytic solution are proportional to their chemical
equivalent weights (Atomic Mass of Metal ÷ Number of
electrons required to reduce the cation).
According to faraday Q=It

Products of electrolysis
Electrochemistry
• Products of electrolysis depend on the nature of material being electrolysed
and the type of electrodes being used.
If the electrode is inert (e.g:-platinum or gold), it does not participate in the
chemical reaction and acts only as source or sink for electrons. On the other
hand, if the electrode is reactive, it participates in the electrode reaction.
Thus, the products of electrolysis may be different for reactive and inert.
electrodes.
The products of electrolysis depend on the different oxidising and reducing
species present in the electrolytic cell and their standard electrode
potentials.
Moreover, some of the electrochemical processes although feasible, are so
slow kinetically that at lower voltages these do not seem to take place and
extra potential which is called overpotential has to be applied, which makes
such process more difficult to occur.
•
•
•

Batteries
Electrochemistry
Any battery (actually it may have one or more
than one cell connected in series) or cell that we
use as a source of electrical energy is basically a
galvanic cell where the chemical energy of the
redox reaction is converted into electrical
energy .
There are 2 types of batteries
a)Primary batteries
b)Secondary batteries

Electrochemistry
Primary batteries
• In the primary batteries, the reaction occurs only once and after
use over a period of time battery becomes dead and cannot be
reused again.
The most familiar example of this type is the dry cell is also known
as Leclanche cell after its discoverer which is used commonly
in our transistors and clocks.
• The cell consists of a zinc container that also acts as anode
and the cathode is a carbon (graphite) rod surrounded by
powdered manganese dioxide and carbon . The space between
the electrodes is filled by a moist paste of ammonium chloride
(NH4Cl) and zinc chloride (ZnCl2). The electrode reactions are
complex, but they can be written approximately as follows :
Anode: Zn(s) → Zn2++ 2e–
•
• Cathode: MnO2 + NH4
+ + e- → MnO(OH)+NH3

•
Electrochemistry
In the reaction at cathode, manganese is reduced from the + 4 oxidation
state to the +3 state. Ammonia produced in the reaction forms a complex
with Zn2+to give [Zn (NH3)4]2+. The cell has a potential of nearly 1.5 V.
Mercury cell, suitable for low current devices like hearing aids, watches, etc.
consists of zinc – mercury amalgam as anode and a paste of HgO and
carbon as the cathode. The electrolyte is a paste of KOH and ZnO. The
electrode reactions for the cell are given below:
•
Anode:
Zn(Hg)+ 2OH → ZnO(s) + H2O + 2e–
Cathode:
• HgO + H2O + 2e → Hg(l ) + 2OH–
• The overall reaction is represented by
• Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)
• The cell potential is approximately 1.35 V and remains constant during its
life as the overall reaction does not involve any ion in solution whose
concentration can change during its life time.

Alkaline Batteries
Electrochemistry

Secondary batteries
Electrochemistry
•
•
A secondary cell after use can be recharged by passing current through it in the
opposite direction so that it can be used again. A good secondary cell can undergo a
large number of discharging and charging cycles.
The most important secondary cell is the lead storage battery commonly used in
automobiles and invertors. It consists of a lead anode and a grid of lead packed with
lead dioxide (PbO2 ) as cathode. A 38% solution of sulphuric acid is used as an
electrolyte.
• The cell reactions when the battery is in use are given below:
Anode: Pb(s) + SO4(aq) → PbSO4 (s) + 2e
2
– –
Cathode: PbO(s)+SO4 (aq)+4H (aq)+2e →PbSO4 (s)+ 2H2
O(l)
2- + –
•
• i.e., overall cell reaction consisting of cathode and anode reactions is: Pb(s) +
PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode
is converted into Pb and PbO2, respectively.

A lead storage battery (secondary
battery)
Electrochemistry

• Another important secondary cell is the nickel-cadmium cell
which has longer life than the lead storage cell but more
expensive to manufacture.
• The overall reaction during discharge is:
• Cd (s) + 2Ni(OH)3 (s) → CdO (s) + 2Ni(OH)2(s) + H2
O (l)
Electrochemistry

Galvanic cells that are designed to convert the energy of combustion of fuels
like hydrogen, methane, methanol, etc. directly into electrical energy are
called fuel cells.
One of the most successful fuel cells uses the reaction of hydrogen with
oxygen to form water.
In the cell, hydrogen and oxygen are bubbled through porous carbon
electrodes into concentrated aqueous sodium hydroxide solution. Catalysts
like finely divided platinum or palladium metal are incorporated into the
electrodes for increasing the rate of electrode reactions. The electrode
reactions are given below:
Cathode: O2 (g) + 2H2O(l) + 4e–→ 4OH–(aq)
Anode: 2H2 (g) + 4OH–(aq) → 4H2O(l) + 4e–
Overall reaction being:
2H2(g) + O2(g) → 2 H2O(l)
The cell runs continuously as long as the reactants are supplied. Fuel cells
produce electricity with an efficiency of about 70 % compared to thEelercmtroachlemistry
plants whose efficiency is about 40%.
Fuel cells
•
•
•
•
•
•
•

Electrochemistry
Hydrogen Fuel Cells

Electrochemistry
corrosion
•
•
•
•
•
•
Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of
the metal.
Corrosion of iron (commonly known as rusting) occurs in presence of water and air.
The chemistry of corrosion is quite Complex but it may be considered essentially as
an electrochemical phenomenon. At a particular spot of an object made of iron,
oxidation takes place and that spot behaves as anode and we can write the reaction
E 0
(F
e
2
+
/F
e
)= – 0.44 V
Anode: 2 Fe (s) → 2 Fe2
++ 4 e–
Electrons released at anodic spot move through the metal and go to another spot on
the metal and reduce oxygen in presence of H+ (which is believed to be available from
H2CO3 formed due to dissolution of carbon dioxide from air into water. Hydrogen ion in
water may also be available due to dissolution of other acidic oxides from the
atmosphere). This spot behaves as cathode with the reaction
Cathode: O2(g)+4H+(aq)+4e- →2H2O(l)
The overall reaction being:
2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2 H2O (l) E(
H2
O
)=1.23V
• E0 =1.67 V
cell

• The ferrous ions are further oxidised by atmospheric oxygen to ferric ions
which come out as rust in the form of hydrated ferric oxide (Fe2O3. xH2O) and
with further production of hydrogen ions.
Prevention of corrosion is of prime importance. It not only saves money but
also helps in preventing accidents such as a bridge collapse or failure of a
key component due to corrosion.
One of the simplest methods of preventing corrosion is to prevent the
surface of the metallic object to come in contact with atmosphere. This can
be done by covering the surface with paint or by some chemicals (e.g.
bisphenol).
Another simple method is to cover the surface by other metals (Sn, Zn, etc.)
that are inert or react to save the object. An electrochemical method is to
provide a sacrificial electrode of another metal (like Mg, Zn, etc.) which
corrodes itself but saves the object.
•
•
•
Electrochemistry

How to Prevent Corrosion ?
Electrochemistry

electrochemistryclass12-180412053250.pptx

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