Download Electrochemistry: It is a branch of chemistry that studies chemical

Electrochemistry: It is a branch of chemistry that studies chemical reactions which take
place in a solution at the interface of an electron conductor (a metal or a semiconductor)
and an ionic conductor (the electrolyte), and which involve electron transfer between the
electrode and the electrolyte.
Electrolyte: it is a substance which is an aqueous solution or molten state liberates ions
and allows current to flow through it.
Electrolysis: when electric current passes through the electrolyte which results in
chemical decomposition and this phenomenon is called electrolysis.
Electrolytic cells: The conversion of electrical energy to chemical energy takes place.
Electrolysis takes place here. It contains aqueous solution of an electrolyte in which two
metallic rods are dipped which is connected to a battery.
Anode: The electrode through which current enters the cell is known as anode. It is
denoted as the positive electrode.
Cathode: The electrode through which current leaves the cell. It is denoted as the
negative electrode.
Ohm’s Law: The current “I” flowing through a conductor is given by relation E/R where
E is the electromotive force and R is the resistance.
I =E/R
(OR)
Ohm's law states that the current passing through a conductor between two points is
directly proportional to the potential difference or voltage across the two points, and
inversely proportional to the resistance between them.
The mathematical equation that describes this relationship is
Where I is the current, Ampere in units
Electromotive Force: The potential which is required to move a unit charge from one
place to another place. It is also known as E.M.F It is measured in volts.
Volt: The volt is defined as the value of the voltage across a conductor when a current of
one ampere strength through a one ohm resistance.
Amperes: The unit of strength of current is known as amperes. The current which
deposits 0.001118 gms of silver per second from a 15% solution of silver nitrate in
Voltameter.
Ampere = volt/ohm
Coloumb: The quantity of current is measured in Coloumb.
The quantity of current which passes in one second with a current strength of one ampere.
Electrical resistance: The electrical resistance of an object is a measure of its
opposition to the passage of an electric current. The unit of electrical resistance is the
ohm (Ω). Resistance’s reciprocal quantity is electric conductance is measured in Siemens.
The resistance of an object can be defined as the ratio of voltage to current:
Three types of conductance

Specific Conductance

Equivalent Conductance

Molecular conductance
Specific Conductance:
The resistance offered by a conductor to the passage
of electricity through it is directly proportional to length and inversely proportional to the
area of cross section. The resistance R is given by the relation:
A certain weight of an electrolyte is dissolved in Vml of solvent and the conductance of
one ml of the resulting electrolyte solution at a given dilution V is called the Specific
Conductivity.
Equivalent conductance:
If one gram equivalent weight of an electrolyte is dissolved in Vml of the solvent, the
conductivity of all ions produced from one gram equivalent of an electrolyte at the
dilution V is known as Equivalent Conductance. It is denoted by λv. Hence the equivalent
conductance is equal to the product of specific conductance and volume.
λv = Kv x v
=
Kv x 1000/ N
Molar Conductance:
The conductance of all ions produced by dissolving one gram molecular weight of one
mole of an electrolyte when dissolved in a certain volume Vml. Molar conductance is
denoted by µv.
µv= Kv x V =
Kv x 1000/ M
Units are ohm-1 cm2 mole-1
EFFECTS OF DILUTION ON CONDUCTANCE:
Due to dilution ionization increase and specific conductance decreases. This happens
because specific conductance is the conductance of the ions present in one centimeter
cube of the solution. On dilution the number of current carrying particles (or) ions
presents per one centimeter cube of the solution decreases.
Equivalent conductance or molecular conductance of an electrolyte increases on dilution
because

The above conductance is the product of kv and the volume of the solution. When
volume increases equivalent conductance also increases.

The number of ions of the electrolyte solution increases on dilution contributing to
the increase of conductance. Ionization increases on dilution, till the whole electrolyte
substance has ionised. the limiting value is known as equivalent conductance at
infinite dilution and it is represented by the symbol λα.
The conductance ratio is called the degree of ionization:
α = λv /λα
Electrolytes can be divided into two types

Strong Electrolytes

Weak electrolytes
Strong Electrolyte:
A strong electrolyte is a substance that gives a solution in which almost all the molecules
are ionized, even at low concentration such solution has increasing value of equivalent
conductance at low dilution.
Strong Acids: HCl, H2SO4, HNO3
Strong Base: NaOH, KOH
The Salts: Practically all salts are strong electrolytes
Weak Electrolyte:
The electrolyte which ionize to a small extent on dilution are called weak electrolyte.
They have a low value of equivalent conductance even at a higher concentration and are
not completely ionized even at very great dilution.
Weak Acids: All organics acids like acetic acids, propionic acid and H2SO3
Weak Bases: Alkyl Amines, NH4OH
Salts: A few salts such as mercuric chloride and lead acetate
Measurement of conductance
Conductance is the reciprocal of resistance and the resistance can be determined by a
Wheatstone bridge circuit in which the conductivity cell forms one arm of the bridge.
Wheatstone bridge circuit for is used for measuring conductivity, Conductivity cell with
one arm of a resistance bridge for measurement of conductivity of an electrolyte.
The arms AH and HB represented by resistance R1 and R2 are usually in the form of a
single calibrated slide wire resistor with a sliding contact connected to the null detector.
The solution whose conductance is to be determined is placed in conductivity cell. When
the bridge is balanced, assuming that the conductivity cell behaves as a pure resistance,
then the voltage between ‘B’ and ‘D’ is equal to
Zero.
Conductance cell
If you put two electrodes into a solution that contains dissolved ions and apply a voltage
to the electrodes, the ions will move through the solution: the negative ions (anions) will
move toward the positive electrode (the anode); the positive ions (cations) will move
toward the negative electrode (the cathode). This simple apparatus is called a
conductance cell.
The conductance is made of highly resistant glass such as Pyrex or quartz. The electrodes
consist of platinum discs coated with finely divided platinum black. These are called
Platinised platinum electrodes. Platinum black surface catalyze the union of hydrogen
and oxygen which tend to be liberated by the successive pulse of the current and the
polarization E.M.F is thus eliminated. The electrodes are welded to platinum wires fused
in two glass tubes. The glass tube contains mercury and is firmly fixed in the ebonite
cover of the cell, so that the distance between the electrodes may not alter during the
experiment. The cell is connected to a Wheatstone bridge which consists of a wire of
platinoid or manganin AB having a uniform thickness so that the ratio of lengths read on
the scales gives the ratio of resistance. The wire of AB is stretched tightly over a meter
scale graduated in millimeters. A sliding contact H moves along the wire. R is the
resistance box and C is the conductance cell. Conductance cell containing electrolyte is
placed in the thermostat for maintaining constant temperature during the measurement of
conductance. The induction coil is used to pass alternate current in the circuit. The sliding
contact H is moved until the sound in the head phones is minimum. This gives the null
point where resistance of the resistance box R and resistance of the electrolyte in the cell
C are equal.
Determination of Cell Constant
The electrodes in the cell are not exactly 1 cm apart and may not have surface area of 1
sq. cm (1cm2). Thus the value of observed conductivity is not equal to specific
conductance but is proportional to it.
Where, x = l/a = cell constant
Cell constant, x = κ/G or Specific conductance,
k = cell constant ‘a’ X observed conductance ‘G’.
IONIC MOBILITIES
The electrolytic conductance is due to the mobility of ions .With the help of the theory of
electrolytic dissociation, the higher value of osmotic pressure, depression of freezing
point, elevation of boiling point, increase of ionization and the abnormal behavior of
electrolytes in solution can be explained. The main points of this theory are:

When dissolved in water, salts, bases and acids give two kinds of particles, one
carrying the positive charge and the other carrying the negative charge. These

Charged particles are called ions. When dissolved in water the weak electrostatic
forces of attraction of the charged ions in the molecules are weakened by the
dielectric constant of water and ionization takes place.

There is a state of dynamic equilibrium between the dissociated and undissociated
molecules.
A + B A' + B'
Thus applying law of mass action to the ionic equilibrium the ionization constant
or dissociation constant K is given by

When electric current is passed through the solution of the electrolyte, the positive
ions move towards the cathode and the negative ions move towards the anode
conducting electric current through the solution. Electric conductivity of solutions
depends on the number of ions and the mobility of ions.

The ions behave like molecules in elevating the boiling point, depressing the
freezing point, lowering the vapour pressure and osmotic pressure. If a substance
gives out two types of ions then twice the normal effect of the solute.

The properties of electrolytes are the properties of the ions.
Factors Influencing Ionization
Ionisation of an electrolyte in solution is influenced by the following:

Nature of the Solute: The nature of solute is a chief factor which influences the
rate of ionization.
Example: Strong electrolytes ionize completely whereas weak electrolyte is less
ionized.

Nature of the Solvent: The solvent influences the rate of ionization to a greater
extent. The solvent possess dielectric constant, which is its capacity to weaken the
forces of attraction between the electrical charges of the ions present.

Concentration: The extent of ionization of an electrolyte is inversely
proportional to the concentration of its solution. As the dilution increases the
ionization increases.

Temperature: The ionization of an electrolyte increase with increase in
temperature. As the temperature increases the velocities of the molecules increase.
KOHLARAUSH LAW
The equivalent conductance at infinite dilution of different electrolytes is the sum of the
ionic conductances of cations and anions.
λα = λa + λc
Ionic conductance is expressed in ohm-1 cm2 equiv-1
Ionic conductance is directly proportional to the transport numbers.
λa = K x v
λc = K x u
λα = λa + λc =K(u +v)
Applications of Kohlrausch’s Law

Calculation of conductivity of a weak electrolyte at infinite dilution
It is not possible to determine the value of ∧ α for weak electrolytes since we cannot
obtain the limiting value of the molar conductivity for a weak electrolyte. This is done
indirectly by the molar ionic conductance for the individual ions of the weak electrolyte
and by using Kohlrausch’s law.
ΛCH3COONa = ΛCH3COO- + ΛNa+ -----(i)
Λ HCl = Λ H+ + ΛCl- ------(ii)
ΛNaCl = ΛNa+ + ΛCl- ---------(iii)
From eq’s i, ii, iii conductivity of CH3COOH is calculated as
Λ CH3COOH = ΛCH3COO - + ΛH+ ,
Determination of Solubility of Sparingly Soluble Salts
Salts like AgCl, BaSO4, CaCO3, Ag2CrO4, PbSO4, PbS, Fe (OH)3 etc. are ordinarily
regarded as sparingly soluble and have a very small but definite solubility in water. The
solubility of such sparingly soluble salts is obtained by determining the specific
conductivity (κ) of a saturated salt solution.
λv = Kv x V = λα = λa + λc
 Calculation of Apparent Degree of ionisation or conductivity ratio
The apparent degree of ionisation or conductivity ratio of an electrolyte α is by λv/λα
Half cell
Electrode which is dipped in its salt solution is called half cell.
The standard hydrogen half cell:
2H+(aq) + 2e- → H2(g)
The half cells of a Daniel cell:
Original equation
Zn + Cu2+ → Zn2+ + Cu
Half cell (anode) of Zn
Zn → Zn2+ + 2e−
Half cell (cathode) of Cu
Cu2+ + 2e− → Cu
Galvanic cell
DANIELL CELL
It is designed to make use of the spontaneous redox reaction between zinc and cupric
ions to produce an electric current it consists of two half-cells. The half-cells on the left
contain a zinc metal electrode dipped in ZnSO4 solution.
The half-cell on the right consists of copper metal electrode in a solution CuSO4. The
half-cells are joined by a salt bridge that prevents the mechanical mixing of the solution.
When the zinc and copper electrodes are joined by wire, the following observations are
made:
(i) There is a flow of electric current through the external circuit.
(ii) The zinc rod loses its mass while the copper rod gains in mass.
(iii) The concentration of ZnSO4 solution increases while the concentration of copper
sulphate solution decreases.
(iv) The solutions in both the compartments remain electrically neutral.
During the passage if electric current through external circuit, electrons flow from the
zinc electrode to the copper electrode. At the zinc electrode, the zinc metal is oxidized to
zinc ions which go into the solution. The electrons released at the electrode travel through
the external circuit to the copper electrode where they are used in the reduction of Cu2+
ions to metallic copper which is deposited on the electrode. Thus, the overall redox
reaction is:
Zn(s) + Cu2+
--------> Cu(s) + Zn2+(aq)
Thus, indirect redox reaction leads to the production of electrical energy. At the zinc rod,
oxidation occurs. It is the anode of the cell and is negatively charged while at copper
electrode.
(i) Voltaic or Galvanic cell consists of two half-cells. The reactions occurring in halfcells are called half-cell reactions. The half-cell in which oxidation taking place in it is
called oxidation half-cell and the reaction taking place in it is called oxidation half-cell
reaction. Similarly, the half-cell occurs is called reduction half-cell and the reaction
taking place in it is called reduction half-cell reaction.
(ii) The electrode where oxidation occurs is called anode and the electrode where
reduction occurs is termed cathode.
(iii) Electrons flow from anode to cathode in the external circuit.
Single Electrode potential
The potential difference developed between metal electrode and the solution of its ions of
unit Molarity (1M) at 25°C (298 K) is called standard electrode potential.
Depending on the nature of the metal electrode to lose or gain electrons, the electrode
potential may be of two types:
(i) Oxidation potential:
When electrode is negatively charged with respect to solution, i.e., it acts as anode.
Oxidation occurs.
M --> Mn+ + ne(ii) Reduction potential:
When electrode is positively charged with respect to solution, i.e., it acts as cathode.
Reduction occurs.
Mn+ + ne- --> M
Emf of the cell = EAnode + ECathode
= Oxidation potential of anode + Reduction potential of cathode
Knowing the value of reference electrode, the value of other electrode can be determined.
Measurement of single electrode potential
ECell = EoAnode + EoCathode
NERNST EQUATION
The electrode potential and the emf of the cell depend upon the nature of the electrode,
temperature and the activities (concentrations) of the ions in solution. The variation of
electrode and cell potentials with concentration of ions in solution can be obtained from
thermodynamic considerations. For a general reaction such as
the Gibbs free energy change is given by the equation
G = ∆Go + 2.303RT log10 ....... (ii)
where 'a' represents the activities of reactants and products under a given set of conditions
and ∆Go refers to free energy change for the reaction when the various reactants and
products are present at standard conditions. The free energy change of a cell reaction is
related to the electrical work that can be obtained from the cell, i.e., ∆Go = -nFEcell and
∆Go = -nFEo. On substituting these values in Eq. (ii) we get
-nFEcell = -nFEo + 2.30eRT log10 ....... (iii)
or Ecell = Ecello - 2.303RT/nF log10 ....... (iv)
This equation is known as Nernst equation.
Putting the values of R=8.314 JK-1 mol-1, T = 298 K and F=96500 C, Eq. (iv) reduces to
E = Eo - 0.0591/n log10 ....... (v)
= Eo - 0.0591/n log10 ([Products])/([Reactants]) ....... (vi)
Potential at zinc electrode (Anode)
Eox = Eoxo - 0.0591/n log10 [Zn3+]
Potential at copper electrode (Cathode)
Ered = Eredo - 0.0591/n log10 [Cu2+]
Emf of the cell
Ecell = Eox + Ered
= (Eoxo + Eredo )- 0.0591/n [Zn2+/Cu2+
standard hydrogen electrode (SHE).
SHE has a cylindrical glass vessel dipped in 1 M HCl. A platinum wire is fitted
inside the glass vessel. Hydrogen gas is passed in it at 1 atmospheric pressure. SHE acts
as anode and cathode, both as per need.
At anode
At cathode
H2
2 H+ + 2e
2 H+ + 2e
H2
SHE is a reversible electrode and can be represented as shown in Fig.
Pt H2 (g)/H+ (1M)
The electrode potential of SHE is zero. To measure the electrode potential of any
electrode, it is connected to SHE and the system acts like a cell. The electro- motive
force (EMF) of the cell is measured with the potentiometer
Since the electrode potential of the SHE is zero, the reading of the potentiometer
will be the electrode potential of the electrode. Other reference electrodes are chlorine
electrode, calomel electrodes, etc.
EMF) of the cell is measured as follows
E=E0 -2.303RT/nF log10[H+]
Calomel Electrode
Calomel electrode: It consists of mercury at the bottom over which a paste of mercurymercurous chloride is placed. A solution of potassium chloride is then placed over the
paste. A platinum wire sealed in a glass tube helps in making the electrical contact. The
electrode is connected with the help of the side tube on the left through a salt bridge with
the other electrode to make a complete cell.
The potential of the calomel electrode depends upon the concentration of the potassium
chloride solution.
If potassium chloride solution is saturated, the electrode is known as saturated calomel
electrode (SCE) and if the potassium chloride solution is 1 N, the electrode is known as
normal calomel electrode (NCE) while for 0.1 N potassium chloride solution, the
electrode is referred to as decinormal calomel electrode (DNCE). The electrode reaction
when the electrode acts as cathode is:
1/2 Hg2Cl2 + e- <---> Hg + ClThe reduction potentials of the calomel electrodes on hydrogen scale at 298K are as
follows:
Saturated KC1 0.2415 V
1.0NKC1
0.2800 V
0.1NKC1
0.3338 V
The emf is calculated by
E=E0 -2.303RT/nF log10[H+]
1. E0= 0.2415 V for Saturated KCl
2. E0= 0.28 V for 1 N KCl
3. E0= 0.2415 V for 0.1N KCl
11. What is quinhydrone electrode? What are its advantages and disadvantages?
Ans: It is type of redox electrode which can be used to measure the H≈ ion
concentration of a solution. The electrode consists of an inert metal electrode (a
platinum wire) in contact with quinhydrone crystals and a water based solution.
Quinhydrone is slightly soluble in water, formed by equimolar mixture of
hydroquinone and quinone.
The each one of the two substances can be easily oxidised or reduced to the other.
In this case the electrode reaction may be represented as follows
Using Nernst equation, the potential of quinhydrone electrode EQ may be written n
as
E 0 = standard potential of the electrode
.
Since (Q) = [QH2], concentration of quinone and hydroquinone are equal
Advantages and limitations of quinhydrone electrode

The electrode is very easy to set up.

The pH value obtained is very accurate.
12. Explain why a glass electrode is preferred to quinhydrone electrode in measuring pH
of a solution?
Ans: Glass electrode is not easily oxidized and attains equilibrium rapidly. It can be
safely used up to pH of 10.But quinhydrone electrode can be used up to a pH of 8 only.
It cannot be used in a redox system.
13. What is the role of salt bridge in the galvanic cell?
Ans: Salt Bridge provides a mechanism by which the ions not directly involved in the
redox reaction can migrate to anodic and cathodic compartment to maintain electrical
neutrality in the solution.
14. A dry cell stops functioning if left idle for some time why?
Ans: As the product of the reaction accumulates near the electrodes, the electrode
reaction becomes slow and stops functioning. After leaving idle for sometime the
products diffuse away and the cell starts functioning again.
15. A fuel cell is considered better than an electrical power plant using same fuel
why?
Ans: The fuel cells are considered better since the reaction takes place under nearly
reversible conditions and the efficiency is higher.
16. Describe the construction and working of a glass electrode?
Ans: The glass electrode used to measure pH is the most common ion-selective
electrode. A typical pH combination electrode, incorporating both glass and reference
electrodes (Calomel) in one body. Glass combination electrode with a silver-silver
chloride reference electrode. The glass electrode is immersed in a solution of unknown
pH so that the porous plug on the lower right is below the surface of the liquid. The two
silver electrodes measure the voltage across the glass membrane.
A glass electrode is a type of ion selective electrode consists of a thin walled glass bulb
attached to a glass tube. A very low melting point and high electrical conductivity glass
is used for the construction of this bulb. The glass tube contains a dilute solution of
constant pH of HCl (0.1 N) solution. A silver-silver chloride electrode or platinum wire
is immersed as reference electrode in the HCl solution. The working of glass electrode is
based upon the observation that when a glass surface is in contact with a solution, there
exists a potential differencebetween the glass surface and the solution, the magnitude of
which depends upon the H+ ion concentration of the solution and the nature of glass.
The glass electrode may be represented as
Ag, AgCl(s) | 0.1 N HCl |
glass | H≈ = unknown
The electrode potential of the glass electrode depends
upon the concentration of H≈ ions contained in the
experimental solution and is given
is the standard electrode potential i.e. the potential of the glass electrode when the
solution contains unit concentration of H≈ ions. The value of E 0 depends on the
nature of the glass used in the construction of the glass bulb.
The pH of the solution can be determined if the potential of the glass electrode is known.
To determine the value of glass electrode (Eg), the glass electrode is combined with a
reference electrode such as calomel electrode. The EMF of this
Cell will be given by
BATTERIES
Definition:
 A battery is a storage device used for the storage of chemical energy and
for the transformation of chemical energy into electrical energy
 Battery consists of group of two or more electric cells connected together
electrically in series.
Battery acts as a portable source of electrical energy.
Energy produced by an electrochemical cell is not suitable for commercial
purposes since they use salt bridge which produces internal resistance which results in
drop in the voltage. The drop in voltage is negligible only for a small interval of time
during which it is being used.
Batteries are of 3 types. Namely
• Primary Batteries (or) Primary Cells
• Secondary Batteries (or) Secondary Cells
• Fuel Cells (or) Flow Batteries
I. Primary Batteries (or) Primary Cells:Primary cells are those cells in which the chemical reaction occurs only once and the cell
becomes dead after sometime and it cannot be used again. These batteries are used as
source of dc power.
Eg. Dry cell (Leclanche Cell) and Mercury cell, lithium cell.
Requirements of Primary cell:
It should satisfy these requirements
1) It must be convenient to use.
2) Cost of discharge should be low.
3) Stand-by power is desirable.
Dry cell (Leclanche Cell)
It consists of a cylindrical Zinc container that acts as an anode. A graphite rod placed
in the centre (but not touching the base) acts as a cathode. The space between anode and
cathode is packed with the paste of NH4Cl and ZnCl2 and the graphite rod is surrounded
by powdered MnO2 and carbon as shown in Figure. The cell is called dry cell because of
the absence of any liquid phase, even the electrolyte consists of NH4Cl ,ZnCl2 and
MnO2 to which starch is added to make a thick paste which prevents leakage. The
graphite rod is fitted with a metal cap and the cylinder is sealed at the top with a pitch.
The Zn-MnO2 cell (dry cell) is represented as Zn/Zn+2,NH4+/MnO2/C (EMF = 1.5V)
At anode: (Oxidation)
Zn(s) 
Zn+2(aq)+2eAt Cathode: (Reduction)
2MnO2(s) +H2o+2e-  Mn2O3(s) +2OH –
The net cell reaction is
Zn(s) +2MnO2(s) +H2O Zn2++ Mn2O3+ 2OHThe resulting OH- ions react with NH4Cl to produce NH3 which is not liberated as gas
but immediately combines with the Zn2+ and the Cl- ions to form a complex salt
[Zn(NH3)2Cl2] (diammine dichloro zinc).
2 NH4Cl + 2 OH-  2NH3 + Cl- +2 H2O
Zn2+ + 2NH3 + 2 Cl-  [Zn (NH3)2Cl2]
Advantages:
1) These cells have voltage ranging from 1.25v to 1.50v.
2) Primary cells are used in the torches, radios, transistors, hearing
aids, pacemakers, watch etc.
3) Price is low.
Disadvantages:
These cells do not have a long life, because the acidic NH4Cl corrodes the container
even when the cell is not in use.
Lithium cells:Lithium Cells are Primary cells in which lithium acts as anode and cathode may differ.
Lithium metal is used as anode because of its light weight, high standard oxidation
potential (>3v) and good conductivity.
As the reactivity of lithium in aqueous solution is more, Lithium cells use non aqueous
solvents as electrolyte.
Lithium cells are classified into two categories:
a) Lithium cells with solid cathodes
b) Lithium cells with liquid cathodes
(a) Lithium cells with solid cathode: The electrolyte in these systems is a solid
electrolyte most widely used cell is Lithium-Manganese dioxide cell(3V) MnO2 should
be heated to over 3000C to remove water before keeping it in the cathode, there by the
efficiency of the cell is increased.
Anode: Lithium metal
Cathode: MnO2 as an active material
Electrolyte: LiBF4 salt in a solution of propylene carbonate and dimethoxy ethane
• Reactions:
At Anode:
Li  Li+ +eAt Cathode: e-+ MnO2  MnO2Net reaction:
Li + MnO2  Li MnO2
Applications: 1) The coin type cells are used in watches and calculators
2) Cylindrical cells are used in fully automatic cameras.
(b) Lithium cells with Liquid cathode: Lithium- Sulphur dioxide cell is an example of
liquid cathode. The co-solvents used are acrylonitrile or propylene carbonate (or) mixture
of the two with SO2 in 50% by volume.
Cell reaction: 2Li + 2SO2 → LiS2O4
Lithium thionyl chloride cell is another example of liquid cathode. It consists of high
surface area carbon cathode, a non-woven glass separator. Thionyl chloride acts as
electrolyte and as cathode.
Cell Reaction:
At Cathode:
4Li → Li + 4eAt Anode:
4 Li + 4e- + 2 SOCl2 → 4 LiCl + SO2 +S
4 Li + 2 SOCl2 → 4 LiCl + SO2 +S
In this cell no co- solvent is required as SOCl2 is a liquid with moderate vapour pressure.
The discharging voltage is 3.3- 3.5 V.
Uses: 1) They are used for military and space application.
2) In Medicinal devices like neuro-stimulators drug delivery system lithium batteries are
widely used.
3) They are also used in electric circuit boards for supplying fixed voltage for memory
protection and standby functions.
II. Secondary Cells (or) Accumulator batteries:These cells can be recharged by passing an electric current through them and can be used
again and again.
Eg: A. Lead storage battery
B. Nickel-Cadmium battery
Secondary cells are widely used in cars, trains, motors, electric clocks, power stations,
laboratories, emergency lights, telephone exchange, digital cameras, laptops etc.
These are reversible cells; they behave as galvanic cell while discharging and as
electrolytic cell while charging.
To improve the performance of battery for commercial purpose






The anodes and cathodes with very small separation to conserve space are used.
Current discharge should be high at low temperature.
It should have less variation in voltage during discharge
It should be reliable.
It should have tolerance to shock, temperature etc.
It should have number of charging and discharging cycles before failure of battery
(Cycle life)
Lead –acid battery:
If a number of cells are connected in series, the arrangement is called a battery. The
lead storage battery is one of the most common batteries that are used in the automobiles.
A 12 V lead storage battery is generally used, which consists of six cells each providing 2
V. Each cell consists of a lead anode and a grid of lead packed with lead oxide as the
cathode. These electrodes are arranged alternately, separated by a thin wooden piece and
suspended in dil. H2SO4 (38%), which acts as an electrolyte (Fig. 1.13).Hence it is
called Lead-acid battery.
Anode: Pb
Cathode:PbO2
Electrolyte: H2SO4 (20.22%)
EMF=2V
To increase the current output of each cell, the cathode and the anode plates are
Joined together, keeping them in alternate positions. The cells are connected parallel
to each other (anode to anode and cathode to cathode). The cell is represented as
Pb | PbSO4 (s), H2SO4 (aq.) | PbSO4 (s), Pb
In the process of discharging, i.e. when battery produces current, the reactions at
the electrodes are as follows:
At anode:
Pb  Pb+2 + 2ePb (s) + SO4 (aq.)  PbSO4 (s)
At cathode:
PbO2 (s) + SO4 (aq.) + 4H+ (aq.) + 2e–  PbSO4 (s) + 2H2O
Therefore, overall reaction is
Pb (s) + PbO2 (s) + 4H2SO4 (aq.)  2PbSO4 (s) + 2H2O
During discharging the battery, H2SO4 is consumed, and as a result, the density of
H2SO4 falls; when it falls below 1.20 g/cm3, the battery needs recharging. In
Discharging, the cell acts as a voltaic cell where oxidation of lead occurs.
During recharging, the cell is operated like an electrolytic cell, i.e. electrical
energy is supplied to it from an external source. The electrode reactions are the
reverse of those that occur during discharge.
PbSO4 (s) + 2e– Pb (s) + SO4– – (aq.)
PbSO4 (s) + 2H2O  PbO2 (s) + 2H2SO4 + 2e–
2PbSO4 (s) + 2H2O  Pb (s) + PbO2 (s) + 2H2SO4 (aq.)
During this process, lead is deposited at the cathode; PbO2 is formed at the anode
and H2SO4 is regenerated in the cell.
Advantages: Lead acid batteries are used for supplying current to railways, mines,
laboratories, hospitals, automobiles, power stations, telephone exchange, gas engine
ignition, Ups (stand-by supplies). Other advantages are its recharge ability, portability
and its relatively constant potential & low cost.
Disadvantages: Use of Conc.H2SO4 is dangerous, Use of lead battery is fragile.
Nickel–cadmium cell (Nicad cell)
It is rechargeable secondary cell. It consists of cadmium as the negative electrode
(anode) and NiO2 acting as a positive electrode (cathode). Potassium hydroxide (KOH) is
used
as an electrolyte. The cell reaction during charging and discharging are as follows.
Anode: Cd
Cathode: NiO (OH)
Electrolyte: KOH
EMF=1.4V
At Anode
Cd(S) + 2OH(Aq)  Cd(OH)2 (s) + 2eAt Cathode
NiO(OH) (s) + 2H2O + 2e- 2 Ni(OH)2+ OH-(aq)
Overall reaction
Cd(s) + 2 Ni(OH) + 2H2O  Cd(OH)2 (s) + 2 Ni(OH)2(s)
Advantages and uses
 The Nickel-Cadmium cell has small size and high rate charge/discharge capacity,
which makes it very useful.
 It has also very low internal resistance and wide temperature range (up to 70°C).
 It produces a potential about 1.4 volt and has longer life than lead storage cell.
 These cells are used in electronic calculators, electronic flash units, transistors etc.
 Ni- Cd cells are widely used in medical instrumentation and in emergency
lighting, toys etc.
III)
Fuel Cell :
Definition: A Fuel cell is an electrochemical cell which converts
chemical energy contained in readily available fuel oxidant system into electrical energy.
Principle: The basic principle of the fuel cell is same as that of electrochemical cell.
The only difference is that the fuel & oxidant are stored outside the cell. Fuel and
Oxidant are supplied continuously and separately to the electrodes at which they undergo
redox reactions.
Fuel cells are capable of supplying current as long as reactants are replenished.
Fuel + Oxidant  Oxidation Products + electricity
Eg : 1)H2 -O2 fuel cell
2) Propane -O2 fuel cell
3) CH3OH-O2 fuel cell
A. Hydrogen – Oxygen fuel cell: One of the most successful fuel cell is H2 –O2 fuel cell.
The cell consists of two inert porous electrodes made of graphite impregnated with finely
divided ‘Pt’ (or) Ni (or) Pd – Ag alloy and a solution of 2.5% KOH as electrolyte.
H2 & O2 gases are bubbled through anode & cathode compartments respectively. The
following reactions take place.
Cell Reaction:
At anode:
2H2 (g) + 4OH-  4H2O + 4eAt Cathode: O2 (g) + 2H2O + 4e-  4OHNet Reaction: 2H2 (g) + O2 (g)  2H2O, Ecell = 1.23V
A large no of these cells connected in series form a fuel-cell battery. In the production of
electricity by this method, the byproducts are heat, CO2, water, which will not cause
pollution of the environment.
Applications:
1. These are used as auxiliary energy source in space vehicles, submarines and other
military vehicles.
2. The product water produced is a valuable source of fresh water for astronauts.
3. Fuel cell is preferred in spacecraft because of its lightness.
4. Advantages:
5. 1) Fuel cells have high efficiency. It is nearly 70% while other sources have
efficiency 15-20% (gasoline engine) and 30-35 %( diesel engine).
6. 2) The efficiency of the fuel cell does not depend on the size of the power plant.
7. 3) Maintience cost is very low.
8. 4) Fuel cells are more efficient in producing the mechanical power to drive the
vehicles and require less energy consumption.
9. Disadvantages:
10. 1) Initial cost of fuel cell is high.
11. 2) Life time of fuel cell is not known accurately.
12. 3)There is a problem of durability and storage of large amount of hydrogen
Distinction between Primary, Secondary & Fuel cells
Primary
Secondary
Fuel cells
1) It only acts as galvanic
or voltaic cell. i.e.,
produces electricity
2) Cell reaction is not
reversible.
3) Can’t be recharged
4) Can be used as long as
the active materials are
present
eg: Leclanche cell or Dry
cell, Lithium cell

1) It acts as galvanic or
voltaic cell while
discharging (produces
electricity) and acts as
electrolytic cell
(consumes electricity)
2) Cell reaction is
reversible.
3) Can be recharged
4) Can be used again and
again by recharging.
eg: Lead storage battery,
Ni-Cd battery, Lithium
ion cell
1) It is a simple galvanic
or voltaic cell. i.e.,
produces electricity
2) Cell reaction is
reversible
3) Energy can be
withdrawn continuously
4) Reactants should be
replenished continuously.
it does not store energy
eg: H2&O2 Fuel cell
CH3OH &O2 Fuel cell
Concentration cells
A concentration cell is a Galvanic cell in which electrical energy is produced by the
transfer of material from a system of high concentration to one low concentration.
There are two types of concentration cells:
Electrode concentration cell:
Electrolyte concentration cell