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1 electrochemistry is the study of production ofelectricity from energy released during spontaneouschemical reactions and the use of electrical energyto bring about non-spontaneous chemicaltransformations. The subject is of importance bothfor theoretical and practical considerations. A largenumber of metals, sodium hydroxide, chlorine,fluorine and many other chemicals are produced byelectrochemical methods. Batteries and fuel cellsconvert chemical energy into electrical energy and areused on a large scale in various instruments anddevices. The reactions carried out electrochemicallycan be energy efficient and less polluting. Therefore,study of electrochemistry is important for creating newtechnologies that are ecofriendly. The transmission ofsensory signals through cells to brain and vice versaand communication between the cells are known tohave electrochemical origin. electrochemistry , istherefore, a very vast and interdisciplinary subject.

2 Inthis unit , we will cover only some of its importantelementary studying this unit , you will beable to describe an electrochemical celland differentiate between galvanicand electrolytic cells; apply Nernst equation forcalculating the emf of galvanic celland define standard potential ofthe cell; derive relation between standardpotential of the cell, Gibbs energyof cell reaction and its equilibriumconstant; define resistivity ( ), conductivity( ) and molar conductivity (Lm) ofionic solutions; differentiate between ionic(electrolytic) and electronicconductivity; describe the method formeasurement of conductivity ofelectrolytic solutions andcalculation of their molarconductivity; justify the variation ofconductivity and molarconductivity of solutions withchange in their concentration anddefine m (molar conductivity atzero concentration or infinitedilution); enunciate Kohlrausch law andlearn its applications; understand quantitative aspectsof electrolysis; describe the construction of someprimary and secondary batteriesand fuel cells; explain corrosion as anelectrochemical reactions can be used to produce electrical energy,conversely, electrical energy can be used to carry out chemicalreactions that do not proceed (20/01/2015)64 ChemistryCuEext> CurrentCathode+veAnode veZnFig.

3 Of Daniellcell when externalvoltage Eext opposing thecell potential is Class XI, unit 8, we had studied the construction and functioningof Daniell cell (Fig. ). This cell converts the chemical energy liberatedduring the redox reactionZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)( )to electrical energy and has an electricalpotential equal to V when concentrationof Zn2+ and Cu2+ ions is unity (1 mol dm 3)*.Such a device is called a galvanic or avoltaic an external opposite potential is appliedin the galvanic cell [Fig. (a)] and increasedslowly, we find that the reaction continues totake place till the opposing voltage reachesthe value V [Fig. (b)] when, the reactionstops altogether and no current flows throughthe cell. Any further increase in the externalpotential again starts the reaction but in theopposite direction [Fig. (c)]. It now functionsas an electrolytic cell, a device for usingelectrical energy to carry non-spontaneouschemical reactions.

4 Both types of cells arequite important and we shall study some oftheir salient features in the following pages.*Strictly speaking activity should be used instead of concentration. It is directly proportional to concentration. In dilutesolutions, it is equal to concentration. You will study more about it in higher :Daniell cell having electrodes of zinc andcopper dipping in the solutions of theirrespective < +veI=0 ZnCuZnSO4 CuSO4E = Eext < V(i)Electrons flow from Zn rod toCu rod hence current flowsfrom Cu to Zn.(ii)Zn dissolves at anode andcopper deposits at Eext = V(i)No flow ofelectrons orcurrent.(ii)No Eext > V(i)Electrons flowfrom Cu to Znand current flowsfrom Zn to Cu.(ii)Zinc is depositedat the zincelectrode andcopper dissolves atcopper electrode.(a)(b)(c)2015-16(20/01/2015)65 ElectrochemistryAs mentioned earlier (Class XI, unit 8) a galvanic cell is anelectrochemical cell that converts the chemical energy of a spontaneousredox reaction into electrical energy.

5 In this device the Gibbs energy ofthe spontaneous redox reaction is converted into electrical work whichmay be used for running a motor or other electrical gadgets like heater,fan, geyser, cell discussed earlier is one such cell in which the followingredox reaction (s) + Cu2+(aq) Zn2+ (aq) + Cu(s)This reaction is a combination of two half reactions whose additiongives the overall cell reaction:(i)Cu2+ + 2e Cu(s)(reduction half reaction)( )(ii)Zn(s) Zn2+ + 2e (oxidation half reaction)( )These reactions occur in two different portions of the Daniell reduction half reaction occurs on the copper electrode while theoxidation half reaction occurs on the zinc electrode. These two portionsof the cell are also called half-cells or redox couples. The copperelectrode may be called the reduction half cell and the zinc electrode,the oxidation can construct innumerable number of galvanic cells on the patternof Daniell cell by taking combinations of different half-cells.

6 Each half-cell consists of a metallic electrode dipped into an electrolyte. The twohalf-cells are connected by a metallic wire through a voltmeter and aswitch externally. The electrolytes of the two half-cells are connectedinternally through a salt bridge as shown in Fig. Sometimes, boththe electrodes dip in the same electrolyte solution and in such cases wedo not require a salt each electrode-electrolyte interface there is a tendency of metalions from the solution to deposit on the metal electrode trying to makeit positively charged. At the same time, metal atoms of the electrodehave a tendency to go into the solution as ions and leave behind theelectrons at the electrode trying to make it negatively charged. Atequilibrium, there is a separation of charges and depending on thetendencies of the two opposing reactions, the electrode may be positivelyor negatively charged with respect to the solution. A potential differencedevelops between the electrode and the electrolyte which is calledelectrode potential.

7 When the concentrations of all the species involvedin a half-cell is unity then the electrode potential is known as standardelectrode potential. According to IUPAC convention, standardreduction potentials are now called standard electrode potentials. In agalvanic cell, the half-cell in which oxidation takes place is called anodeand it has a negative potential with respect to the solution. The otherhalf-cell in which reduction takes place is called cathode and it has apositive potential with respect to the solution. Thus, there exists apotential difference between the two electrodes and as soon as theswitch is in the on position the electrons flow from negative electrodeto positive electrode. The direction of current flow is opposite to thatof electron Galvanic Galvanic Galvanic Galvanic Galvanic Cells2015-16(20/01/2015)66 ChemistryThe potential difference between the two electrodes of a galvaniccell is called the cell potential and is measured in volts.

8 The cellpotential is the difference between the electrode potentials (reductionpotentials) of the cathode and anode. It is called the cell electromotiveforce (emf) of the cell when no current is drawn through the cell. Itis now an accepted convention that we keep the anode on the left andthe cathode on the right while representing the galvanic cell. A galvaniccell is generally represented by putting a vertical line between metaland electrolyte solution and putting a double vertical line betweenthe two electrolytes connected by a salt bridge. Under this conventionthe emf of the cell is positive and is given by the potential of the half-cell on the right hand side minus the potential of the half-cell on theleft hand side ,Ecell = Eright EleftThis is illustrated by the following example:Cell reaction:Cu(s) + 2Ag+(aq) Cu2+(aq) + 2 Ag(s)( )Half-cell reactions:Cathode (reduction): 2Ag+(aq) + 2e 2Ag(s)( )Anode (oxidation): Cu(s) Cu2+(aq) + 2e ( )It can be seen that the sum of ( ) and ( ) leads to overall reaction( ) in the cell and that silver electrode acts as a cathode and copperelectrode acts as an anode.

9 The cell can be represented as:Cu(s)|Cu2+(aq)||Ag+(aq)|Ag(s)and we have Ecell = Eright Eleft = EAg+ Ag ECu2+ Cu( )The potential of individual half-cell cannot be measured. We canmeasure only the difference between the two half-cell potentials thatgives the emf of the cell. If we arbitrarily choose the potential of oneelectrode (half-cell) then that of the other can be determined with respectto this. According to convention, a half-cellcalled standard hydrogen electrode ( )represented by Pt(s) H2(g) H+(aq), is assigneda zero potential at all temperaturescorresponding to the reaction H+ (aq) + e 12H2(g)The standard hydrogen electrode consistsof a platinum electrode coated with platinumblack. The electrode is dipped in an acidicsolution and pure hydrogen gas is bubbledthrough it. The concentration of both thereduced and oxidised forms of hydrogen ismaintained at unity (Fig.)

10 This impliesthat the pressure of hydrogen gas is one barand the concentration of hydrogen ion in thesolution is one ElectrodePotentialFig. :Standard Hydrogen Electrode (SHE).2015-16(20/01/2015)67 ElectrochemistryAt 298 K the emf of the cell, standard hydrogen electrode secondhalf-cell constructed by taking standard hydrogen electrode as anode(reference half-cell) and the other half-cell as cathode, gives the reductionpotential of the other half-cell. If the concentrations of the oxidisedand the reduced forms of the species in the right hand half-cell areunity, then the cell potential is equal to standard electrode potential,E R of the given = E R E LAs E L for standard hydrogen electrode is = E R 0 = E RThe measured emf of the cell:Pt(s) H2(g, 1 bar) H+ (aq, 1 M) Cu2+ (aq, 1 M) Cuis V and it is also the value for the standard electrode potentialof the half-cell corresponding to the reaction:Cu2+ (aq, 1M) + 2 e Cu(s)Similarly, the measured emf of the cell:Pt(s) H2(g, 1 bar) H+ (aq, 1 M) Zn2+ (aq, 1M) Znis V corresponding to the standard electrode potential of thehalf-cell reaction:Zn2+ (aq, 1 M) + 2e Zn(s)The positive value of the standard electrode potential in the firstcase indicates that Cu2+ ions get reduced more easily than H+ ions.