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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.
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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Amperes: The unit of strength of current is known as amperes. The current which
deposits 0.001118 gms of silver per second.
Ampere = volt/ohm
Coloumb: The quantity of current is measured in Coloumb.
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:
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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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 λ v. 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 ionisation increase and specific conductance decreases. This happens
because specific conductance is the conductance of the ions present in one centimetre
cube of the solution. On dilution the number of current carrying particles (or) ions
present per one centimetre cube of the solution decreases.
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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Equivalent conductance or molecular conductance of an electrolyte increases on
dilution because
 The above conductance are 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.Ionisation 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 ionisation:
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 ionised, even at low concentration such solution have 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 ionise 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 ionised 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
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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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 consists of platinum discs coated with finely divided platinum black. These
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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are called Platinised platinum electrodes. Platinum black surface catalyse 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 millimetres. 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 ionisation and the abnormal behaviour 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
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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 charged particles are called ions. When dissolved in water the weak electrostatic
forces of attraction of the charged ions in the molecules are weekened by the
dielectric constant of water and ionisation 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 ionisation 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 Ionisation
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 ionisation.
Example: Strong electrolytes ionise completely whereas weak electrolyte are less
ionised.
 Nature Of the Solvent: The solvent influences the rate of ionisation to a greater
extent. The solvent possess dielectric constant, which is
its capacity to weaken the forces of attraction between the electrical charge of the
ions present.
Example: The dielectric constant of water is 80; alcohol is 25and ether is 4.1.
The higher the value of dielectric consant the greater the ionisation.
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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 Concentration: The extent of ionisation of an electrolyte is inversely
proportional to the concentration of its solution. As the
dilution increases the ionisation increases.
 Temperature: The ionisation 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 Law
 Calculation of molar 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 kohlraush’s law.
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
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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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 λ v/λα
 Calculation of ionic product of water
The observed specific conductivity of the purest water at 25oc is 5.54x10-8 ohm cm-1
With the help of specific conductivity of water, the ionic product of water can be
determined. The ionization of water may be represented as,
H2O ↔ H+ + OH
The product of the concentrations of H+ and OH- ions expressed in mol/L is called ionic
product of water and is represented by Kw.
Kw = [H+] [OH-]
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 Daniell cell:
Original equation
Zn + Cu2+ → Zn2+ + Cu
Half cell (anode) of Zn
Zn → Zn2+ + 2e−
Half cell (cathode) of Cu
Cu2+ + 2e− → Cu
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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
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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
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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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
M1A + m2B n1X + n2Y + .... .......(i)
occurring in the cell, 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., ∆G o
= -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 Nearnst 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 of single electrode (Anode): Consider the general oxidation reaction,
M --> Mn+ + ne-
Applying Nernst equation,
Eox = Eoxo - 0.0591/n log10 [Mn+]/[M]
where Eox is the oxidation potential of the electrode (anode), is the standard oxidation
potential of the electrode.
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SUBJECT: Engg.Chemistry
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[Note: The concentration of pure solids and liquids are taken as unity.]
Eox = Eoxo - 0.0591/n log10 [Mn+]
Let us consider a Daniell cell to explain the above equations. The concentrations of the
electrolytes are not 1 M.
Zn(s)+Cu2+(aq) <=> Zn2+(aq) + Cu(s)
Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu
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+]
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.
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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
QUINHYDRONE ELECTRODE:
The quinhydrone electrode is essentially an oxidation-reduction electrode
and is based on the equilibrium set up in aqueous solution between quinone
and quinol, namely,
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Quinone and hydroquinine form a reversible redox system in the presence of hydrogen
ions.
Hydroquinone ↔ Quinone + 2H+ +2e-
The potential E developed when an innert electrode example platinum is immersed in
this system is given by the NERNST equation
R T [C]c [D]d
DE = DE° - ----- ln --------n F [A]a [B]b
0.0592 V [C]c [D]d
DE = DE° - --------- log --------n
[A]a [B]b
Please note that log is the logrithm function based 10, and ln, the natural logrithm
function.
Advantages and limitations of quinhydrone electrode


The electrode is very easy to set up.
The pH value obtained is very accurate.
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry
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ION selective electrode
Ion selective electrodes use a membrane which is sensitive to a particular chemical
species. The potential develop is a measure of the concentration of the species of
interest.
Concentration cells
A concentration cell is a Galvanic cell in which electrical energy is produced by the
transfer of material from a system of hight concentration to one low concentration.
There are two types of concentration cells:
Electrode concentration cell:
NAME OF THE FACULTY: Shaik.Mehboob Subhani
SUBJECT: Engg.Chemistry