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Chemistry. Session Electrochemistry - 1 Session Objectives Conductance of electrolytic solution Specific conductance, Equivalent conductance, Molar conductance

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Text of Chemistry. Session Electrochemistry - 1 Session Objectives Conductance of electrolytic solution...

  • SessionElectrochemistry - 1

  • Session ObjectivesConductance of electrolytic solution Specific conductance, Equivalent conductance, Molar conductance

    Kohlrausch's law

  • INTRODUCTION:

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

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

  • Types of Electrolytes

    Strong 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:

    Electrolyte solutions and molten salts conduct electricity through the migration of ions.

    When the current is passed through an electrolyte solutionsdecomposition and changes occur in 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,Resistance

  • Conductance

    The 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 Conductivity It is the reciprocal of specific resistance of an electrolyte.Unit of specific conductance is ohm1cm1 SI Unit of specific conductance is Sm1 where S is Siemenl/a is known as cell constant

  • Equivalentconductance : Equivalentconductance 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.

  • Representation of Equivalent conductance

  • Molar conductance

    The 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 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 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.

  • 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

  • Kohlrauschs 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 ionsm = v+ + + v- - v+ and v- are the number of cations and anions performula unit of electrolyte respectively and, + and - are the molar conductivities of the cation and anion at infinite dilution respectively

  • Applications of Kohlrausch's lawDetermination of 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 LAW1) Determination of molar conductivities of weak electrolytes: It is not possible to determine value of m for weak electrolyte like CH3COOH,NH4OH ETC. BY THE EXTRAPOLATION OF THE MOLAR CONDUCTIVITY VALUES TO ZERO CONCENTRATION. From the value of of m HCl, m CH3COONa and m NaOH the value of m CH3COOH can be calculated.CH3 COOH = CH3COONa + HCI - NaCI = m (H+)+ m (Cl-) + m ( CH3COO-)+ m ( Na+) - m ( Na+) +m (Cl-) = m (H+)+ m ( CH3COO-) CH3 COOH = 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 m ( molar conductivity at a particular concentration. Total no. of molecules m (molar conductivity at infinite dilution)

    (degree of dissociation) = m / 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 etccan be determined from conductance values.

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

    The concentration of sparingly soluble salt is the solubility of the salt. hence = 1000K S

  • Equivalent conductance of NaCl, HCl and C2H5COONa at infinite dilution are 126.45, 426.16 and 91 ohm1 cm2 respectively.Calculate the equivalent conductance of C2H5COOH.= 91 + 426.16 126.45= 390.71 ohm1 cm2Solution:Illustrative Example

  • GALVANIC or ELECTROCHEMICAL CELLSGalvanic cell is a device which converts chemical energy into electrical energy. ex: Daniel cell

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

  • Zn --> Zn2+ + 2e-OxidationAnodeNegativeCu2+ + 2e- --> Cu

    ReductionCathodePositiveElectrons travel thru external wire.Salt bridge allows anions and cations to move between electrode 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 + CuThe 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; H2(g) / Pt/H+(1M) // Cu+2(1M) / Cu (s) SHE anode SCE cathode

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

  • 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)

  • Metal-metal insoluble salt electrode. Eg: calomel electrode.

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

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

  • c) Gas electrode. Eg: hydrogen electrode.

    It is represented as X+(aq)/ X2(P = atm) Pt

    X2(p) + 2e- 2X+(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 spontaneou

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