Electrochemistry

Electrochemistry:
The branch of physical chemistry which deals about the study of all
chemical and physical process in which electricity is either produced or
consumed is called electrochemistry.
The production of electricity on the basis of redox reaction are
known as electrochemical cell or galvanic cell or voltaic cell.
On the other hand passing of electricity through the solution of
an electrolyte so as to bring about redox reactions gives rise to the
phenomenon of electrolysis and the arrangement is called an
electrolytic cell.

Faraday’s Law of electrolysis:
This law establish the quantitative relationship between the amount of
substance deposited (liberated) at the electrode and quantity of
electricity passed through the electrolyte. In 1830 Faraday had
formulated the two laws of electrolysis. They are as follows:

1. Faraday’s First law of electrolysis:
It states that, “The amount of any substance liberated or deposited on
the electrode during the electrolysis is directly proportional to the
quantity of charge (electricity) passed through the electrolytic solution.”
If ‘m’ be the mass deposited on or liberated from an electrode
and Q be the charge passed through the solution, the Faraday first law
is given by
Amount of substance deposited or liberated ∝
quantity of electricity passed

(ECE)ElectroChemical equivalent(Z):
we have, m=ZQ,
Or, m=ZIT (since Q=IT)
Or, Z=m/IT
If one ampere current is passed through the electrolytic solution for
one second then m=Z. so electrochemical equivalent is defined as the
mass of the substance deposited or liberated by the passage of one
ampere current for one second.
In other word, it is defined as the amount of substance deposited or
liberated at the electrode by the passage of one coulomb charge.

Faraday:
It is the quantity of electricity required to deposit or liberated one gram
equivalent of the substance. One faraday is the charge of one mole of
electrons.
Mathematically,
1 F= charge of one mole of electrons
=1 mole of electrons × charge of electron
=6.022 × 1023 × 1.602 × 10-19 coulombs
=96500 coulombs(approximately)

We know,
1 faraday charge deposits one gram equivalent of any substance.
Let ‘E’ be the equivalent weight of the substance, then
96500 coulomb deposit E gram of the substance.
Therefore, one coulomb deposit
E
96500
g of the substance.
By definition, mass deposited or liberated by one coulomb charge is
called electrochemical equivalent. Therefore,
Z=
E
96500

Faraday's Second Law of Electrolysis:

Figure: Experimental set up for the verification of the Second Law of
Electrolysis

This law states that,
“When the same quantity of electricity is passed through a number of
electrolytic solutions connected in series, then the masses of the
different substance liberated or deposited at the respective electrodes
are proportional to their chemical equivalent or equivalent weight”.
Mathematically,
Amount of the substance discharged ∝ chemical equivalent
m∝ E
or, m= KE
or,
m
E
=K

ie,
mass of the substance deposited
chemcial equivalent
= K
Verifications: figure
Let same quantity of electricity is passed through different
voltammeters connected in series containing aq. Solution of H2SO4,
CuSO4 and AgNO3 as shown in figure.
For cell X:
Mass of hydrogen displaced ∝ equivalent weight of hydrogen
therefore,
𝑚𝑎𝑠𝑠 𝑜𝑓 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑑
𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛
= K

For cell Y: class work
For cell Z : class work
From i, ii, and iii we get ……..in board

Class work
A current of 1.7 A is passed through 300 ml of 0.16 M solution of ZnSO4
for 230 second with the current efficiency of 90%. Find the resultant
concentration of Zn++ after deposition of Zn. Assume the volume of
solution remain constant during the electrolysis.( ANS: 0.154 M)
Atomic weight of zinc=65.4

Electrolytic conductance:
1. Conductance(L): The reciprocal of resistance is called conductance.
We know from Ohm’s Law,
R=
V
I
Mathematically, L=
1
R
Unit: 1/ohm i.e. ohm-1 or mho or Siemen(SI)

2. Specific resistance or resistivity (𝝆):
The resistance of a conductor is directly proportional to the length (l)
and inversely proportional to the area of cross section(A) of the
conductor.
Mathematically,
R ∝
l
A
R = ρ
l
A
where
𝜌 rho is proportionality constant and is called specific resistance

If l= 1 cm and A = 1cm2 then R=ρ
Thus, specific resistance of the conductor may be defined as the
resistance offered by a conductor having 1 cm length and 1 cm2 area of
cross section. In other words, it is the resistance of one centimetre
cube of the conductor.
Unit: ohm cm

3. Specific conductance or conductivity of the
solution (𝛋):
The reciprocal of specific resistance is called specific conductance or
conductivity of the solution and is denoted by κ( kappa)
we know,
R= 𝜌
l
A
1
𝜌
=
1
𝑅
×
l
A
κ = L ×
l
A

If l= 1 cm and A=1 cm2 then,
Specific conductance of an electrolytic solution is defined as the
conductance of a solution kept between two parallel electrodes each of cross
sectional area 1 cm2 and separated by 1 cm length. In other words, specific
conductance of an electrolytic solution is defined as the conductivity of one
centimetre cube(cc) of the solution.
Figure
Unit: Κ= L
l
A
=
1
R
×
l
A
Ohm-1 cm-1

Variation of specific conductance with dilution:
Specific conductance of the solution is the conductance due to one
centimeter cube of the solution. As the solution is diluted, the
following changes occur:
1. Degree of ionization increases therefore number of ions increases
2. The number of ions per centimeter cube decrease since volume
increases.

The effect of increase in number of ions due to increase in ionization
and increase in mobility due to decrease in interionic interaction is far
less than the decrease in density of ions due to dilution. Thus the
number of ions per unit volume decrease on dilution. So overall effect
is decrease in specific conductance with increase in dilution for strong
and weak electrolyte.

Equivalent conductance(𝛌 𝐞𝐪):
It is defined as “ The conductance of certain volume of solution
containing one gram equivalent of solute in which the whole solution is
placed between two parallel electrode separated by one cm.”
It is equal to the product of the specific conductance (κ) and the
volume V in cc containing one gram equivalent of the electrolyte.
let one gram equivalent of electrolyte is dissolved to form V ml of the
solution and κ be the specific conductance.
Now, conductance of 1ml of solution = K
Conductance of V ml of the solution = K × V

By definition,
equivalent conductance 𝝀eq = K × V
………….in board
Unit: ohm-1 cm-1 cm3/equiv

Cell constant:
It is the ratio of distance between two electrodes to the area of cross-
section of the electrode.
…………..board

Variation of equivalent conductance
conductance with concentration (or dilution)
We know equivalent conductance is the product of specific
conductance and the volume of the solution in cc containing one gram
equivalent of electrolyte. As the decreasing value of specific
conductance is far less than the increasing value of V, so the value of
equivalent conductance increases with dilution.
The equivalent conductance of the solution doesn’t vary linearly with
concentration. The effect of concentration on equivalent conductance
can be studied by plotting 𝝀eq values against the square root of
concentration. It has been found that the variation of equivalent
conductance with C depend upon the nature of electrolyte.

a) Effect of dilution on equivalent conductance
of strong electrolyte:
Strong electrolyte can ionise completely at all dilution. On further
dilution there is hardly increase in number of ions. So, the increase in
equivalent conductance is not due to increase in number of ions rather
due to the decrease in force of attraction between ions.
On dilution the ions become more and more far apart from each
other and inter ionic attraction decreases. This decrease in inter ionic
attraction value results the increase in mobility of ions. Due to increase
in mobility of ions, equivalent conductivity of the strong electrolyte
increases.

b) Effect of dilution on equivalent conductance
of weak electrolyte:
As the solution of weak electrolyte is diluted, ionisation also increases.
Therefore on dilution, equivalent conductance increase with dilution
due to increase in number of ions.

Difference between electrochemical cell and
electrolytic cell
Electrochemical cell Electrolytic cell
1. It is device to convert chemical energy into
electrical energy.
2. Two electrodes are set up in separate beakers
1. It is a device to convert electrical energy to
chemical energy.
2. Both the electrodes are suspended in the solution
of the electrolyte in same beaker.
3. The electrolyte taken in the two beakers are
different.
4. The electrodes taken are of different materials.
3. Only one electrolyte is taken.
4. The electrodes taken may be the same or different
materials.
5. The electrode on which oxidation take place is
called anode(-ve pole) and the electrode on which
reduction takes place is called cathode (+ve pole)
5. The electrode which is connected to the –ve
terminal of the battery is called cathode and the other
electrode is anode.
6. To set up this cell, a salt bridge is used. 6. No salt bridge is used.

The important functions of the salt bridge
1. Salt bridge completes the electrical circuit.
2. Salt bridge maintains electrical neutrality.

Electrode potential:
To understand the concept of electrode potential, consider a metal
rod(M) placed in contact with its own ions (Mn+). Then there is one of
the following possibilities.
i. Mn+ ions may collide with the metal rod and deflected back without
undergoing any change.
ii. Mn+ ions on collision with the metal rod may gain electros and
change into metal atoms i.e. Mn+ ions may reduce.
Mn+ + ne- → M … … … … … … … . (i)
iii. Metal atoms of the metal rod may loose electrons and change into
Mn+ ions i.e metal atoms get oxidised
M → Mn+ + ne- …………………………(ii)

If the metal has relatively higher tendency to get oxidised reaction(ii)
will occur, the electrons will accumulate on the metal rod which will
therefore develop a –ve charge. Thus in turn my attract some metal
ions from the solution which may change into metal atoms. Ultimately
an equilibrium is reached as follows.
M ⇌ Mn+ + ne-
figure in board

If metal ions have relatively higher tendency to get reduced reaction(i)
will occur. Metal ions (Mn+) will gain electrons from the metal rod. As a
result, metal rod will develop a +ve charge with respect to solution and
ultimately the following equilibrium will be attained.
Mn+ + ne- ⇌ M

Thus in either of the case, there is the separation of charges between
the metal rod and its ions in the solution. As a result potential
difference exists between them.
The electrical potential difference set up between the metal rod
and its ions in the solution is called electrode potential.

How is single electrode potential originated?

It is defined as the voltage measured under standard condition when
the half cell is connected into an electrochemical cell with the other
half cell being standard hydrogen electrode.

The standard electrode potential of Cu++/Cu is
+0.34V. What does it mean?
It means that the voltage measured is 0.34 V under standard condition
when the Cu electrode in its own ion is incorporated into an
electrochemical cell with the other half cell being a standard hydrogen
electrode.

Standard hydrogen electrode(SHE):
The absolute value of the electrode potential of a single electrode
cannot be determined because oxidation half reaction or reduction half
reaction cannot take place independently. It can only be measured by
using the some electrode as the reference electrode. The reference
electrode used is standard hydrogen electrode.
Standard hydrogen electrode is an electrode when hydrogen gas at
one atmospheric pressure is in contact with H+ ions of 1 molar
concentration at 25o c. Here Pt acts as inert electrode through which
inflow or outflow of electrons takes place.

• Figure in board
When in cell, if electrode acts as the anode, oxidation takes place, the
following reaction takes place
H2 → 2H
+
+ 2e
−
i. e. some hydrogen gas changes into H
+
ions which go into the solutions.

When this electrode acts as cathode i.e. reduction takes place, the
following reactions occurs
2H+ + 2e- → H2
i.e. some H+ ions from the solution changes into H2 gas.
The electrode potential of the standard hydrogen electrode is taken as
0.00 V at 298 k.

Standard hydrogen electrode can acts as
anode and cathode. Give reason.
The electrode potential of standard hydrogen electrode is assumed to
be zero and therefore can act as both anode and cathode while
determining the potential of the other electrode.
If the electrode potential value of the electrode (half cell) connected in
electrochemical cell, is less than electrode potential value of SHE then
SHE acts as cathode and if electrode potential value of the
electrode(half cell) is greater than electrode potential value of SHE then
SHE acts as anode.

Electromotive force:
The difference of potential of two half cell is known as electromotive
force of the cell or cell potential or cell voltage.
Eo
cell voltage = Eo
cathode – Eo
anode

Electrochemical series:
The series in which elements are arranged in increasing order of
standard reduction potential or decreasing order of standard oxidation
potential as compared to that of standard hydrogen electrode.

Electrochemistry

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