Chemistry

Session

Electrochemistry - 1

Session Objectives• Conductance of electrolytic solution

• Specific conductance, Equivalent conductance, Molar conductance

• Kohlrausch's law

INTRODUCTION:INTRODUCTION:

Electro chemistry is the branch of Electro chemistry is the branch of chemistry which deals with chemistry which deals with transformationtransformationof electrical energy into chemical energy of electrical energy into chemical energy vice versa.vice versa.

Electricity is a flow of electrons generated Electricity is a flow of electrons generated by a battery when the circuit is completedby a battery when the circuit is completed

Types of ElectrolytesStrong electrolyte are highly ionized in the solution.

Examples are HCl, H2SO4, NaOH, KOH etc

Weak electrolytes are only feebly ionized in the solution.

Examples are H2CO3, CH3COOH, NH4OH etc

Conductors: A substance which allows electric current to pass

through it is called a conductor. These are 2 types:1) Metallic conductors2) Electrolytic conductors

1.Metalic conductors: The substances which conduct electricity under the

influence of an applied electric potential through a flow of electrons.

The flow of electricity does not cause any physical or chemical change in the conductors.

Eg: all metals, graphite, human body

2.Electrolytic conductors:2.Electrolytic conductors:

Electrolyte solutions and Electrolyte solutions and

molten salts conduct electricitymolten salts conduct electricity

through the migration of ions.through the migration of ions.

When the current is passedWhen the current is passed

through an electrolyte solutionsthrough an electrolyte solutions

decomposition and changes occur decomposition and changes occur

in the composition of electrolytesin the composition of electrolytes

Resistance refers to the opposition to the flow of current.

For a conductor of uniform cross section(a)and length(l); Resistance R,

a

l

l lR l and R R

a a

Where is called resistivity orspecific resistance.

Resistance

ConductanceThe reciprocal of the resistance is called conductance. It is denoted by C.

C=1/R

Conductors allows electric current to pass through them. Examples are metals, aqueous solution of acids, bases and salts etc.

Insulators do not allow the electric current to pass through them.

Examples are pure water, urea, sugar etc.

Unit of conductance is ohm-1 or mho or Siemen(S)

Specific conductance 1

Specific Conductivity It is the reciprocal of specific resistance of an electrolyte.

K

ax Conductance

Unit of specific conductance is ohm–1cm–1

SI Unit of specific conductance is Sm–1 where S is Siemen

aBut ρ = R

Ka.R

l/a is known as cell constant

Equivalent conductance : Equivalent conductance is defined as the conductance of all the ions produced by one gram equivalent of an electrolyte in a given solution.

(To understand the manning of equivalent conductance, imagine a rectangular trough with two opposite sides made of metallic conductor (acting as electrodes) exactly 1 cm apart, If 1 cm3 (1 mL) solution containing 1 gram equivalent of an electrolyte is places in this container is measured. ) /\ eq =v x specific conductance of 1cm3 solution (k)

/\ eq= KV

                                         /\ eq = k × 1000/N

         Where N = normality                The unit of equivalent conductance is ohm-1 cm-2 equi-1.

1 cm

1 cc

1 cm1 cm

1 cc

Representation of Equivalent conductance

Molar conductanceThe molar conductance is defined as the conductance of all the ions produced by ionization of 1 g mole of an electrolyte when present in V mL of solution. It is denoted by. Molar conductance     Λ m = k ×V                                 Where V is the volume in mL containing 1 g mole of the electrolyte. If c is the concentration of the solution in g mole per litre, then Λ m = k × 1000/M

It units are ohm- cm2 mol-1.

Effect of Dilution on ConductivitySpecific conductivity decreases on dilution.

Equivalent and molar conductance both increase with dilution and reaches a maximum value.

The conductance of all electrolytes increases with temperature.

concentration, (m ole L )– 1 1 /2

CH COO H (weak electrolyte)3

KCl (strong electrolyte)

Ionic Mobility (u): Ionic mobility is defined as the velocity of

an ion when the potential gradient is 1v/cm.

Hence the units of u are cm2/v.sec

Kohlrausch’s law of independent ionic mobilities

At infinite dilution when dissociation complete (m) , the molar

conductivity of an electrolyte is expressed as the sum of the contributions from its individual ions

Λ∞m = v+ λ∞ + + v- λ∞

-

v+ and v- are the number of cations and anions per formula unit of electrolyte respectively and, λ∞+ and  λ∞- are the molar conductivities of the cation and anion at infinite dilution respectively

Applications of Kohlrausch's law

Determination of Λ∞m for weak electrolytes

Determination of the degree of dissociation of a weak electrolyte

Determination of the solubility of a sparingly soluble salt

APPLICATIONS OF KOHLRAUSCH LAW 1) Determination of molar conductivities of weak

electrolytes:

It is not possible to determine value of Λ∞m

for weak electrolyte like CH3COOH,NH4OH ETC. BY THE EXTRAPOLATION OF THE MOLAR CONDUCTIVITY VALUES TO ZERO CONCENTRATION. From the value of of Λ∞

m HCl, Λ∞

m CH3COONa and Λ∞m NaOH the value of Λ∞

m CH3COOH can be calculated.

Λ∞CH3 COOH =  Λ∞CH3COONa +  Λ∞HCI  -  Λ∞NaCI

= λ∞m (H+)+ λ∞m (Cl-) + λ∞m ( CH3COO-)+ λ∞m ( Na+) - λ∞m ( Na+) +λ∞m (Cl-)

= λ∞m (H+)+ λ∞m ( CH3COO-)

Λ∞CH3 COOH = Λ∞CH3 COOH

2)Determination of degree of dissociation :

Degree of dissociation is the fraction of the total number of molecules dissociated into ions.

Degree of dissociation (∞) = No. of molecules dissociated in to ions

Total no. of molecules present

No. of molecules dissociated is directly proportional to conductivity of the molecules.

No. of molecules dissociated in to ions Λ∞m ( molar conductivity

at a particular concentration.

Total no. of molecules Λ∞m (molar conductivity at infinite dilution)

∞(degree of dissociation) = Λ∞m / Λ∞

m

3) Determination of solubility of sparingly soluble salt :

The solubility of a sparingly soluble salts such as silver chloride, silver chromate, lead sulphate, barium sulphate etc…can be determined from conductance values.

The solubility S in gram equivalent/ liter is related to equivalent conductance Λ∞ and specific conductivity k

The concentration of sparingly soluble salt is the solubility of the salt.

hence Λ∞ = 1000K

S

Equivalent conductance of NaCl, HCl and C2H5COONa at infinite dilution are 126.45, 426.16 and 91 ohm–1 cm2 respectively.Calculate the equivalent conductanceof C2H5COOH.

2 5 2 5C H COOH C H COONa HCl NaCl

= 91 + 426.16 – 126.45

= 390.71 ohm–1 cm2

Solution:

Illustrative Example

GALVANIC or ELECTROCHEMICAL CELLS

Galvanic cell is a device which converts chemical energy into electrical energy.

ex: Daniel cellex: Daniel cell

Daniel cell consists of zinc and copper Daniel cell consists of zinc and copper electrodes. Zn electrode is dipped in electrodes. Zn electrode is dipped in ZnSO4 solution & Cu is dipped in ZnSO4 solution & Cu is dipped in CuSO4 solution.CuSO4 solution.

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn --> Zn2+ + 2e-Zn --> Zn2+ + 2e-

OxidationOxidationAnodeAnodeNegativeNegative

Cu2+ + 2e- --> CuCu2+ + 2e- --> Cu

<--Anions<--AnionsCations-->Cations-->

RedReductionuctionCatCathodehodePositivePositive

••Electrons travel thru external wire.Electrons travel thru external wire.Salt bridge Salt bridge allows anions and allows anions and cations to move between electrode cations to move between electrode compartments.compartments.

The 2 solutions separated by a porous membrane, a current is seen to be flow on connecting the two wires externally.

The cell function due to dissolution of zinc and the simultaneous deposition of copper.

The over all reaction is: Zn + CuSO4 ZnSO4 + Cu The Danial cell may be represented as: Zn/ZnSO4 // CuSO4/Cu

E.M.F: The potential difference or electrode between

the two electrodes of the cell which is a driving for the fllow of electrons is called the E.M.F of the cell.

Units electron volts

NERNST EQUATION: The theoretical relation ship b/w the electro chemical reaction

and the corresponding cell e.m.f, this relation ship is generally known as nernest equation.

Consider a galvanic cell a A +bB c C + d D here a,b,c,d are represent the

number of moles of A,B,C,D respectively, the nernest equation is

Ecell =RT/n F ln K - RT/n F ln [C]c[D]d / [A]a[B]b Here Ecell = e.m.f of the cell, R= gas constant, T = Temperature, n= no. of faraday of current F passed, K = equilibrium constant, RT/n F ln k = standard e.m.f of the Eocell

Ecell = Eo cell - RT/n F ln [C]c[D]d / [A]a[B]b (or)

Ecell = Eo cell - 2.303 RT/ nF log [C]c[D]d / [A]a[B]b

at R.T T=298 K, R=8.314 K-1, F=96457 C substitute the values in above equation

Ecell = Eo cell - 0.05916/ n log [C]c[D]d / [A]a[B]b

Standard cell e.m.f equal to cell e.m.f when the activities of both reactants and products is equal to unity.

Cell formulation: A short hand notation for representing a cell is called cell

formulation In this notation the state of the element ,a single stroke for

separation of two different phases, a double stroke for separation of the two electrodes.

Eg; HEg; H22(g) / Pt/H(g) / Pt/H+(+(1M) // Cu1M) // Cu+2+2(1M) / Cu (s)(1M) / Cu (s)

SHE anode SCE cathodeSHE anode SCE cathode

Classification of electrodes :a) metal-metal ion electrode. Eg: Cu+2 /Cub) Metal-metal insoluble salt electrode. Eg: calomel electrode.c) Gas electrode. Eg: hydrogen electrode.d) Redox electrode. Eg: Pt(s)/Fe+2 (1M),Fe +3(1M)

A) metal-metal ion electrode. Eg: Cu+2 /Cuit consists of a pure metal (M) in contact with a solution of its ion(Mn+)It is represented as Mn+ (aq) + ne- M(S)

B)B) Metal-metal insoluble salt electrode. Eg: calomel Metal-metal insoluble salt electrode. Eg: calomel electrode.electrode.

It consists of a metal (M) covered by layer of sparingly It consists of a metal (M) covered by layer of sparingly soluble salt(MX) immersed in a solution containing a soluble salt(MX) immersed in a solution containing a common ion (Xcommon ion (X--))it is represented as Xit is represented as X--(aq)// MX/ M(S)(aq)// MX/ M(S)

MX(s) + neMX(s) + ne-- M(s) + X M(s) + X--(aq)(aq)

cc) Gas electrode. Eg: hydrogen electrode.) Gas electrode. Eg: hydrogen electrode.

It is represented as XIt is represented as X++(aq)/ X(aq)/ X22(P = atm) Pt(P = atm) Pt

XX22(p) + 2e(p) + 2e-- 2X 2X++(aq)(aq)

Reference electrodes:

Reference electrodes are electrodes at which the oxidation or reduction occurs reversibly.

Eg: standard hydrogen electrode, calomel electrode.1)STANDARD CALOMEL ELECTRODE (S.C.E): The calomel electrode undergoes the spontaneous

process of reduction with respect to the hydrogen electrode represented as

The cell is represented as: Pt/Hg,Hg2Cl2(s) /KCl

HgHg22ClCl22+2e+2e-- 2 Hg (l) + 2Cl 2 Hg (l) + 2Cl––(aq) (aq)

The emf of the calomel electrode The emf of the calomel electrode varies with the concentration of the varies with the concentration of the chloride ions and three concentrations chloride ions and three concentrations of chloride ions are normally used.of chloride ions are normally used.The decinormal calomel electrode with The decinormal calomel electrode with 0.1N KCl having a potential of 0.3338v0.1N KCl having a potential of 0.3338vNormal calomel electrode with 1.0 N Normal calomel electrode with 1.0 N KCl having a potential of 0.28 vKCl having a potential of 0.28 vSaturated calomel electrode with Saturated calomel electrode with saturated KCl having a potential of saturated KCl having a potential of 0.2415 v on the SHE scale at R.T 0.2415 v on the SHE scale at R.T

The electrode can be coupled with hydrogen electrode containing solution of unknown PH

The emf of the cell

Ecell = E right- Eleft = 0.2422V + 0.0592V PH

PH = Ecell- 0.2422V

0.0592V

Quinhydrone electrode: (Redox electrode) This is a redox electrode reversible to protons &

often replaces the hydrogen electrode This is 1:1 molar mixture of quinone &

hydroquinone Electrode consist of a Pt electrode dipped in a

test solution which is saturated with quinhydrone The electrode reaction is given by

Q + 2H+ + 2e- QH2 (quinone) ( hydroquinone)

The electrode potential at 25oc is given by

E Pt/Q,,H+,QH2 = E0

Pt/Q,H+,QH2 – 0.0592/2 log aQH2/aQa2 H+

Since Q,,QH2 are in equimolar amounts i.e. a Q=a QH2

So, E=E0 + 0.0592 log a H+

EQ,QH2 = E0 Q,QH2 – 0.0592 p H

Quinhydrone electrode can be measure p H of a solution.

This electrode can’t be used at p H >8 Even this electrode fails in the presence of strong

oxidizing & reducing agents

ION SELECTIVE ELECTRODES

This electrodes consist of specially prepared membranes placed between two electrolytes.

Ecell=k- (0.059/n)log(a1/a2) a1,a2 are the activities of the ion to be measured in

the external and internal solutions respectively. The ion selective electrodes is coupled to a SCE

and immersed in the sample or test solution containing the ion to be monitored.

The potential developed across the membrane is related to the activities of the ion of interest in the gel and sample solution.

The response of the membrane is usually highly selective to only one ion or a small number of ions.

The construction is similar to that of the glass electrode and consists of a tube, one end of which is fused to an electrically conducting membrane.

The tube consists with a gel incorporating the ion to which the electrode is sensitive and inert electrolyte such as KCl.

A silver wire in contact with the gel together with the inert electrolyte constitutes the Ag-AgCl reference electrode.

Different types of ion-selective electrodes have been developed as the above principle.

1) glass electrodes for the determination of cations other than H+

2) Solid state electrodes

3) liquid-ion exchange membrane electrodes & heterogeneous membrane electrodes

4) Gas sensing electrodes

glass electrodes with high selectivity for cations such as Na+,NH4

+,Ag+ & Li+ consists glass membranes whose composition determines the selectivity to individual cations.

POTENTIOMETRIC TITRATIONS:

The detection of the end point of a volumetric titration by the use of potentiometer is an important application of measurement of emf.

The emf of cell consisting of an indicator electrode responsive to the analyte ions and a reference electrode is measured as a function of the volume of titrant added to the analyte solution.

BATTERIES A device which stores Chemical energy for later release

as electricity is called battery. It is an electrochemical cell or often electrochemical cells

connected in series ,can be used as a source of direct electric current at a constant voltage.

Batteries are classified into two categories depending on their recharging capacities.

1) Primary batteries 2) secondary batteries.

1)PRIMARYCELL:

In primary cells, a chemical reaction proceeds spontaneously and the free energy of the reaction is converted into electrical energy.

The production of electrical energy at the expense of the free energy of the cell reaction is called DISCHARGEING of the cell.

In a primary cell the chemical reactions can’t be reversed by passing electricity through the cell and hence a discharged cell can’t be used again and the battery become dead.

In a primary cell the cathode at which reduction occurs is designated positive by conversion. Eg: voltaic cell, Daniel cell, leclanche cell (or) dry cell, lithium cell….

LITHIUM CELL:

Li battery chemistry comprises a number

of cell designs, in that Li is used as anode

due to its light weight and highest standard

potential greater than 3V.

Types of lithium cells:

Three types

1)Lithium primary cell with Liq cathode

2) Lithium primary cell with solid cathode

3) Lithium primary cell with solid electrolyte

Liq cathode cells:

Anode : Li

Cathode : SOCl2Electrolyte : LiAlCl2Anode : Li --Li+ + e-

Cathode : 4Li+ + 4e- + SOCl2 -- 4 LiCl + SO2 + S

Overall rxn: 4Li+ + 2SOCl2 -- 4 LiCl + SO2 + S

They perfom best in low current applications and have a very long service life. For this reason they are used in pacemaker.

These cells offer higher discharge rates due to the rxns occur at the cathode surface.

The direct contact between the liq cathode n the Li forms a film over the Li,called solid electrolyte interface.(SEI)

This prevents further chemical rxn when not in use,thus preserving the shells life.

The thick film causes an initial voltage delay.

solid cathode (LiMnO2) cell:

Anode : Li

Cathode : MnO2

Electrolyte : propylene carbonate n 1,2-dimethoxy ethane

Anode : Li --Li+ + e-

+4 +3

Cathode : Li+ + e- + MnO2 -- MnO2 (Li+) Li + MnO2 -- MnO2 (Li+)

+4 +3

Uses:

Low rate cells are used commercially for small electronics and memory back up.

solid electrolyte cells:

Anode : Li

Cathode : poly -2- vinyl pyridine(P2VP)

Electrolyte : solid Li

Anode : 2Li --2Li+ + 2e-

Cathode : 2Li+ + 2e- + P2VP.n I2 -- P2VP.(n-1) I2 + 2LiI

Overall rxn: 2Li+ + P2VP.n I2 -- P2VP.(n-1) I2 + 2LiI

These cells can’t be used in high drain applications and don’t perform well under low temp conditions.

They are used generally for memory back up ,watches and portable electronic devices

2)SECONDARY CELLS (ACCUMULATORS) :

In secondary cells chemical reactions proceeds both in the forward and reverse directions depending on weather electrical energy is supplied from an external source

Electrical energy is passed into the cell to induce a chemical reaction and the products remained at the electrodes, this process is called charging the cell.

Secondary cells can accumulate electrical energy in the form of chemical reaction and later on the reaction is reversed to liberate electrical energy. Hence these cells are called accumulators or storage batteries. The cathode at which reduction occurs during the discharge of the cell is designated +ve. while it becomes anode during charging.

Eg: lead-acid battery, alkaline storage battery, nickel –cadmium battery.

LEAD ACID BATTERY: This battery consists of a number of spongy

lead anodes and a grid of lead dioxide coated lead-antimony alloy cathode.

The electrode pairs separated by inert porus partitions are kept immersed in the electrolyte sulphuric acid.

At the anode Pb Pb2+ +2e-

Pb2+ + SO42-PbSO4

At the cathode PbO2 + 4H+ +2e- Pb2+ + 2H2O Pb2+ + SO4 2- PbSO4

The net reaction of the cell is PbO2 + 4H+ + 2e- + SO4

2- PbSO4 +2H2O

The lead sulphate formed precipitates on the cathode . Pb/PbSO4(s)/H2SO4(aq),PbSO4(s)/Pb The net reaction of the cell is

PbO2+Pb+2H2SO42 PbSO4 +2H2O

NICKEL-CADMIUM CELL: It consists of a steel grid containing cadmium powder as the

anode and cathode made of Ni2O3 mixed with Ni supported on steel grid.

An aqueous solution of KOH placed in an inert steel container is used as the electrolyte. The cell generates the voltage of 1.35v and the reaction may be represented as

Cd +2OH- Cd(OH)2 +2 e- (at anode)

2NIO(OH) + 2H2O +2e- 2 Ni(OH)2 +2OH- (at cathode)The net reaction is

Cd +2Ni(OH)3 Cd(OH)2 + 2NI(OH)2

The disadvantage of this battery is the reaction can be reversed, b’cos the reaction products Cd(OH)2 & Ni(OH)2 remain adhered to the electrodes called MEMORY EFFECT or FALSE BOTTOM.

Nickel-Cadmium battery

FUEL CELLS

A fuel cell is an electrochemical cell which converts chemical energy contained in a easily available fuel oxidant system into electrical energy.

The basic principle of a fuel cell is the chemical energy is

provided by a fuel and an oxidant stored outside the cell. The fuel and the oxidizing agent are continuously and

separately supplied to the electrodes of the cell at which they undergo reactions.

These are also primary cells & they are capable of current as long as the reactants are supplied.

HYDROGEN – OXYGEN FUEL CELL It consists of two inert porous electrodes made of either graphite

impregnated with finely divided Pt or a 75/25 alloy of Pd with Ag(Ni) and an electrolyte solution which is 25% KOH solution.

Through the anode, H2 gas is bubbled and through the cathode O2 gas bubbled.

At anode: 2H2(g) + 4OH-(g) 4H2O(l) + 4e- At cathode : O2(g) + 2H2O(l) + 4e- 4OH-(aq)

Net rxn : 2H2(g) + O2(g) 2H2O(l) The emf of the cell is 0.8 to 1.0v

A no of such cells are stacked together in series to make a battery, called fuel cell battery.

Hydrogen-oxygen fuel cell

Uses of H2- O2 fuel cell

They are used as auxiliary energy source in space vehicles (ex: Apollo space craft), submarines & other military vehicles.

For space craft, they are preferred due to their lightness & product water is available as source of fresh water for astronauts.

Fuel cells are categorized on the basis of electrolyte used:

1)Proton exchange membrane fuel cell2)Alkaline Fuel cell3) Molten carbonate Fuel cell4) Phosphoric acid fuel cell5) Solid oxide fuel cell

Thank you