Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi , Ph. : ,

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1 C 1 LCTROCHMISTRY 3 Galvanic Cell : Q What are galvanic cells? xplain with exaple Solution : A galvanic cell is an electrocheical cell that converts the cheical energy of a spontaneous redox reaction into electrical energy In this device the Gibbs energy of the spontaneous redox reaction is converted into electrical work which ay be used for running a otor or other electrical gadgets like heater, fan, geyser, etc For eg, Daniell cell discussed earlier is one such cell in which the following redox reaction occurs Zn(s) + Cu + (aq) Zn + (aq) + Cu(s) This reaction is a cobination of two half reactions whose addition gives the overall cell reaction : (i) Cu + + e Cu(s) (ii) Zn(s) Zn + + e (reduction half reaction) (oxidation half reaction) These reactions occur in two different portions of the Daniell cell The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode These two portions of the cell are also called half-cells or redox couples The copper electrode ay be called the reduction half cell and the zinc electrode, the oxidation half-cell Q Define electrode potential? Solution : At each electrode-electrolyte interface there is a tendency of etal ions fro the solution to deposit on the etal electrode trying to ake is positively charged At the sae tie, etal atos of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to ake it negatively charged At equilibriu, there is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode ay be positively or negatively charged with respect to the solution A potential difference develops between the electrode and the electrolyte which is called electrode potential Q What is Cell Potential? Solution : The potential difference between the two electrodes of a galvanic cell is called the cell potential and is easured in volts The cell potentials is the difference between the electrode potentials (reduction potentials) of the cathode and anode It is called the cell electrootive force (ef) of the cell when no current is drawn through the cell cell = right left Q xplain standard hydrogen electrode with neat diagra Solution : A half-cell called standard hydrogen electrode represented by Pt(s) H (g) H + (aq), is assigned a zero potential at all teperatures corresponding to the reaction H+(aq) + e ½ H (g) The standard hydrogen electrode consists of a platinu electrode coated with platinu black The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it The concentration of both the reduced and oxidised fors of hydrogen is aintained at unity instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

2 C This iplies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one olar Q Why platinu or gold are used as electrodes Solution : Metals like platinu or gold are used as inert electrodes They do not participate in the reaction but provide their surface for oxidation or reduction reactions and for the conduction of electrons For exaple, Pt is used in the following half-cells : Hydrogen electrode : Pt(s) H (g) H + (aq) With half-cell reaction : H + (aq) + e ½ H (g) Broine electrode : Pt(s) Br (aq) Br (aq) Q How would you deterine the standard electrode potential of the syste Mg + Mg? Solution : A cell consisting of Mg MgSO 4 (1 M) as one electrode (by dipping a agnesiu wire in 1 M MgSO 4 solution) and standard hydrogen electrode Pt, H (1 at) H + (1 M) as the second electrode is set up and ef of the cell and the direction of deflection in volteter and noted The direction of deflection shows that electrons flow fro agnesiu electrode to hydrogen electrode, ie, oxidation takes place on agnesiu electrode and reduction on hydrogen electrode The cell ay be represented as follows : Mg Mg + (1 M) H + (1 M) H, (1 at), Pt or cell H /1/ H Mg / Mg Put Hence, H /1/ H Mg / Mg cell Q Can you store copper sulphate solution in a zinc pot? Solution : Zn Cu 76V, / Zn / Cu 34V To check whether zinc reacts with CuSO 4 solution, we shall see whether the following reaction takes place or not : Zn(s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu(s) The cell ay be represented as : Zn Zn + Cu + Cu cell Cu / Cu Zn 34V ( 76 V) 11 V / Zn As is positive, the reaction takes place and we cannot store copper sulphate solution in zinc pot cell Q Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

3 ions under suitable conditions Solution : Oxidation of ferrous ions eans the following reaction should occur : Fe + Fe 3+ + e ; ox = 77 V C 3 Only those substances can oxidise Fe + to Fe 3+ which are stronger oxidising agents and have positive reduction potentials greater than 77 V so that ef of the cell reaction is positive leents lying below Fe 3+ /Fe + in the electrocheical series, eg, Br, Cl and F will satisfy this requireent 33 Nernst quation : Q xplain the Nernst equation Solution : Nernst showed that for the electrode reaction : Mn + (aq) + ne M(s), the electrode potential at any concentration easured with respect to standard hydrogen electrode can be represented by : n (M / M) but concentration of solid M is taken as unity (M / M) n (M / M) RT 1 ln nf n [M n standard electrode potentia, R is gas constant (8314 JK 1 ol 1 ), F is Faraday constant (96487 C ol 1 ), T is teperature in kelvin and [M n+ ] is the concentration of the species, M n+ Q Write the Nernst equation for the following cell reaction Ni(s) Ni + (aq) Ag + (aq) Ag (s) Solution : The cell reaction is Ni(s) + Ag + (aq) Ni + (aq) + Ag(s) The Nernst equation can be written as (cell) RT [Ni ] cell ln F [Ag ] Q Represent the cell in which the following reaction takes place Mg(s) + Ag + (1 M) Mg + (13 M) + Ag(s) Calculate its (cell) if = 317 V [NCRT Solved xaple 31] (cell) Solution : 96 V Q What is the condition for equilibriu in a galvanic cell reaction What is the relation between K c and cell 33RT Solution : (cell) =, ( cell) log K C nf Q Calculate the equilibriu constant of the reaction : Cu(s) + Ag + (aq) Cu + (aq) + Ag(s) (cell) = 46 V [NCRT Solved xaple 3] Solution : K C = Q How is the electrical work is deterined in a galvanic cell? Solution : lectrical work done in one second is equal to electrical potential ultiplied by total charge passed If we want to obtain axiu work fro a galvanic cell then charge has to be passed reversibly The reversible work done by a galvanic cell is equal to decrease in its Gibbs energy and therefore, if the ef of the cell is and nf is the aount of charge passed and r G is the Gibbs energy of the reaction, then r G = nf (cell) Q Aong cell, r G which is intensive and extensive property Solution : It ay be reebered that (cell) is an intensive paraeter but r G is an extensive therodynaic property and the value depends on n Q What is relation between r G and K C Solution : r G = RT ln K Q The standard electrode potential for Daniell cell is 11V Calculate the standard Gibbs energy for the ] instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

4 C 4 reaction : Zn(s) + Cu + (aq) Zn + (aq) + Cu(s) [NCRT Solved xaple 33] Solution : 17 kj ol 1 Q Calculate the potential of hydrogen electrode in contact with a solution whose ph is 1 [NCRT Solved xaple 34] Solution : 591 V Q Calculate the ef of the cell in which the following reaction takes place : Ni(s) + Ag + ( M) Ni + (16 M) + Ag(s) Given that (cell) = 15 V Solution : 91 V Q The cell in which the following reaction occurs : Fe 3+ (aq) + I (aq) Fe + (aq) + I (s) has = 36 V at 98 K Calculate the standard Gibbs energy and cell the equilibriu constant of the cell reaction Solution : r G = 4555 kj ol 1, K C = Conductance of lectrolytic Solutions : Q Give the relation between electrical resistance of any object with length l and area of cross-section A of object Solution : The electrical resistance of any object is directly proportional to its length l and inversely proportional to its area of cross section, A That is, l R A l or R A The constant of proportionality, (Greek, rho), is called resistivity (specific resistance) Its SI units are oh etre () Q Define resistivity Solution : The resistivity for a substance is its resistance when it is one etre long and its area of cross section is one Q Define conductance and give its SI units Solution : The inverse of resistance, R, is called conductance, G, and we have the relation : G 1 R A l A k l The SI unit of conductance is sieens, represented by the sybol S and is equal to oh 1 (also known as ho) or 1 Q Define conductivity along with its unit Solution : The inverse of resistivity, called conductivity (specific conductance) is represented by the sybol, k (Greek, kappa) The SI units of conductivity are S 1 Conductivity of a aterial in S 1 is its conductance when it is 1 long and its area of cross section is 1 Q What are conductors, seiconductors, insulators and superconductors Give exaples also Solution : Metals and their alloys have very large conductivity and are known as conductors Certain non-etals like carbon-black, graphite and soe organic polyers are also electronically conducting Substances like glass, ceraics, etc, having very low conductivity are known as insulators Substances like silicon, doped silicon and galliu arsenide having conductivity between conductors and insulators are called seiconductors and are iportant electronic aterials Certain aterials called superconductors by definition have zero resistivity or infinite conductivity arlier, only etals and their alloys at very low teperatures ( to 15 K) were known to behave as superconductors, but nowadays a nuber of ceraic aterials and ixed oxides are also known to show superconductivity at teperatures as high as 15 K Q What is electrical conductance and on what factors it depends? instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

5 C 5 Solution : lectrical conductance through etals is called etallic or electronic conductance and is due to the oveent of electrons The electronic conductance depends on (i) the nature and structure of the etal (ii) the nuber of valence electrons per ato (iii) teperature (it decreases with increase of teperature) Q What is electrolytic or ionic conductance and on what factors it depends? Solution : ven very pure water has sall aounts of hydrogen and hydroxyl ions (~1 7 M) which lend it very low conductivity ( S 1 ) When electrolytes are dissolved in water, they furnish their own ions in the solution hence its conductivity also increases The conductance of electricity by ions present in the solution is called electrolytic or ionic conductance The conductivity of electrolytic (ionic) solutions depends on : (i) the nature of the electrolyte added (ii) size of the ions produced and their solvation (iii) the nature of the solvent and its viscosity (iv) concentration of the electrolyte (v) teperature (it increases with the increase of teperature) Q Conductivity of different ions having different charge is different or sae in sae solvent Solution : The conductivity of solutions of different electrolytes in the sae solvent and at a given teperature differs due to charge and size of the ions in which they dissociate, the concentration of ions or ease with which the ions ove under a potential gradient Q What is the relation between olar conductivity with conductivity (k) and C (ol 3 ) what are units of Solution : Molar conductivity = k C In the above equation, if k is expressed in S 1 and the concentration, c in ol 3 then the units of in S ol 1 are Q Resistance of a conductivity cell filled with 1 ol L 1 KCl solution is 1 If the resistance of the sae cell when filled with ol L 1 KCl solution is 5, calculate the conductivity and olar conductivity of ol L 1 KCl solution The conductivity of 1 ol L 1 KCl solution is 19 S/ [NCRT Solved xaple 34] Solution : 14 S c ol 1 Q The electrical resistance of a colun of 5 ol L 1 NaOH solution of diaeter 1 c and length 5 c is oh Calculate its resistivity, conductivity and olar conductivity [NCRT Solved xaple 35] Solution : c, 1148 S c 1, 96 S c ol 1 Q What is effect of dilution on conductivity of weak electrolyte and strong electrolyte? Solution : Conductivity always decreases with decrease in concentration both, for weak and strong electrolytes This can be explained by the fact that the nuber of ions per unit volue that carry the current in a solution decreases on dilution Q Define olar conductivity Solution : Molar conductivity of a solution at a given concentration is the conductance of the volue V of solution containing one ole of electrolyte kept between two electrodes with area of cross section A and distance of unit length Therefore, ka k l Since l = 1 and A = V (volue containing 1 ole of electrolyte) kv Q What is the effect of increase of concentration on olar conductivity? instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

6 C 6 Solution : Molar conductivity increases with decrease in concentration For strong electrolytes, increases slowly with dilution and can be represented by the equation : Ac It can be seen that if we plot against c ½, we obtain a straight line with intercept equal to and slope equal to A The value of the constant A for a given solvent and teperature depends on the type of electrolyte ie, the charges on the cation and anion produced on the dissociation of the electrolyte in the solution Q Show graphically variation of olar conductivity with concentration for weak electrolyte and strong electrolyte ½ Solution : Q What is relation between, and C (concentration) for strong electrolyte? Solution : Ac ½ Q The olar conductivity of KCl solutions at different concentrations at 98 K are given below : c/ol L 1 /S c ol Show that a plot between and c ½ is a straight line Deterine the values of and A for KCl [NCRT Solved xaple 36] Solution : = 15 S c ol 1, A = slope = 8746 S c ol 1 /(ol/l 1 ) ½ Q Define Kohlrausch law of independent igration of ions Solution : The law states that liiting olar conductivity of an electrolyte can be represented as the su of the individual contributions of the anion and cation of the electrolyte Thus, if and Cl are liiting Na olar conductivity of the sodiu and chloride ions respectively, then the liiting olar conductivity for (NaCl) Na sodiu chloride is given by the equation : Q What is effect of dilution on Solution : Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in with dilution is due to increase in the degree of dissociation and consequently the nuber of ions in total volue of solution that contains 1 ol of electrolyte Q Why cannot be calculated for weak electrolyte graphically Cl instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

7 C 7 Solution : In such cases increases steeply on dilution, especially near lower concentrations Therefore, cannot be obtained by extrapolation of to zero concentration At infinite dilution (ie, concentration c zero) electrolyte dissociates copletely ( = 1), but at such low concentration the conductivity of the solution is so low that it cannot be easured accurately Therefore for weak electrolytes is obtained by using Kohlrausch law of independent igration of ions Q What is the relation between, c and for weak electrolyte Also write expression of k a for acetic acid Solution : At any concentration c, if is the degree of dissociation then it can be approxiated to the ratio of olar conductivity at the concentration c to liiting olar conductivity, Thus we have : But we know that for a weak electrolyte like acetic acid (Class XI, Unit 7), K a c c c (1 ) 1 Q Calculate for CaCl and MgSO ( 119, 763, 16, 16 4 ) [NCRT Solved xaple 38] Solution : 716 S c ol 1, 66 S c ol 1 Ca Q for NaCl, HCl and NaAc are 164, 454, 459 and 91 S c ol 1 respectively Calculate for HAc [NCRT Solved xaple 38] Solution : 395 S c ol 1 Q The conductivity of 18 ol L 1 acetic acid is S c 1 Calculate its dissociation constant if for acetic acid is 395 S c ol 1 [NCRT Solved xaple 39] Solution : 4815 S c 3 ol 1, 133, ol L 1 Q Why does the conductivity of a solution decrease with dilution? Solution : Conductivity of a solution is the conductance of ions present in a unit volue of the solution On dilution, the nuber of ions per unit volue decreases Hence, the conductivity decreases Q Suggest a way to deterine the value of water Solution : (H O) = (HCl) + (NaOH) (NaCl) Q The olar conductivity of 5 ol L 1 ethanoic acid is 461 S c ol 1 Calculate its degree of dissociation and dissociation constant Given (H + ) = 3496 S c ol 1 and (HCOO ) = 546 S c ol 1 Solution : lectrolytic Cells and lectrolysis : Q What are electrolytic cells xplain with exaple? Solution : In an electrolytic cell external source of voltage is used to bring about a cheical reaction One of the siplest electrolytic cell consists of two copper strips dipping in an aqueous solution of copper sulphate If a DC voltage is applied to the two electrodes, then Cu + ions discharge at the cathode (negatively charged) and the following reaction takes place : Cu + (aq) + e Cu(s) Copper etal is deposited on the cathode At the anode, copper is converted into Cu + ions by the reaction : Cu(s) Cu + (s) + e Thus copper is dissolved (oxidised) at anode and deposited (reduced) at cathode This is the basis for an industrial process in which ipure copper is converted into copper of high purity Q Define Faraday s laws of electrolysis Cl Mg SO 4 c instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

8 C 8 Solution : (i) First Law : The aount of cheical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or elt) (ii) Second Law : The aount of different substances liberated by the sae quantity of electricity passing through the electrolytic solution are proportional to their cheical equivalent weights (Atoic Mass of Metal Nuber of electrons required to reduce the cation) Q A solution of CuSO 4 is electrolysed for 1 inutes with a current of 15 aperes What is the ass of copper deposited at the cathode? Solution : 938 g Q Define Faraday Solution : We, know that charge on one electron is equal to C Therefore, the charge on one ole of electrons is equal to : N A C = ol C = C ol 1 This quantity of electricity is called Faraday and is represented by the sybol F For approxiate calculations we use 1F 965 C ol 1 Q If a current of 5 apere flows through a etallic wire for hours, then how any electrons would flow through the wire? Solution : 46 1 electrons Q Suggest a list of etals that are extracted electrolytically Solution : Sodiu, Calciu and Aluiniu Q Consider the reaction : Cr O H + + 6e Cr H O What is the quantity of electricity in coulobs needed to reduce 1 ol of Cr O 7? Solution : 579 C 36 Batteries : Q Write the cheistry of recharging the lead storage battery, highlighting all the aterials that are involved during recharging Solution : A lead storage battery coprises anode of lead, cathode of a grid of lead packed with lead dioxide (PbO ) and 38% solution of sulphuric acid as electrode The following reactions take place in lead storage battery when it is working On charging the battery, the reverse reaction takes place, ie, PbSO 4 deposited on the electrodes is converted back into Pb and PbO and H SO 4 gets regenerated Q Suggest two aterials other than hydrogen that can be used as fuels in fuel cells Solution : Methane, carbon onoxide and ethanol Q How does fuel cell operate? Solution : Cheical energy of fuel is converted into electrical energy Q Which type of etal can be used in cathodic protection of iron against rusting? Solution : A etal which is ore reactive (higher on the electrocheical series) than iron is used Q How does cathodic protection of iron operate? Solution : More reactive etal acts as anode ie, loses electrons Thus, it prevents the oxidation of iron Q Why does an alkaline ediu inhibit the rusting of iron? Solution : Rusting takes place in a presence of H + Alkali neutralises hydrogen ions Q Nae any two etals which can be used for cathodic protection of iron Solution : Zinc and agnesiu Q What is the function of platinised platinu in standard hydrogen electrode? instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

9 Solution : 1 It acts as inert etal for electrical contact With platinised platinu, H gas gets absorbed on the surface Q What is priary cells? C 9 Solution : Cells which are not rechargeable are priary cells ie, the products cannot be converted into reactants Q Write the reactions taking place at positive and negative terinals in lead acid accualator Solution :At anode : Pb(s) + SO 4 (aq) PbSO 4 (s) + e At cathode : Q What are secondary cells? PbO (s) + SO 4 (aq) + 4H + (aq) + e PbSO 4 (s) + H O(l) Solution : Cells which are rechargeable are called secondary cells ie, products can be converted into reactants Q Write electrode reactions taking place in Ni Cd cell? Is it priary or secondary cell? Solution :At anode : Cd + OH CdO + H O + e It is secondary cell At cathode : Ni(OH) 3 + e Ni(OH) + OH Q Define corrosion Solution : It is a process in which the etal reacts with substances present in air to for copounds on the surface For exaple, Fe O 3 xh O is fored at the surface of Fe Q What is galvanisation? Solution : Coating zinc over iron is called galvanisation Q What are fuel cells? Write the electrode reactions of a fuel cell which uses the reaction of hydrogen with oxygen Solution : Those cells in which cheical energy of fuel is converted into electrical energy are called fuel cells Q What are fuel cells? How do they reseble and differ fro galvanic cells? Solution : Fuel cells are those cells in which cheical energy of fuel is converted into electrical energy In both cells, cheical energy is converted into electrical energy The efficiency of fuel cell is higher than that of glavanic cell Fuel cells are non-polluting Q Write the cell reactions which occur in lead storage battery (i) when the battery is in use and (ii) when the battery is on charging Solution : (i) When the battery is in use Pb(s) + PbO (s) + H SO 4 (aq) PbSO 4 (s) + H O (l) At anode : Pb(s) + SO 4 (aq) PbSO 4 (s) + e At cathode : (ii) When the battery is on charging PbO (s) + SO 4 (aq) + 4H + (aq) + e PbSO 4 (s) + H O (l) PbSO 4 (s) + H O(l) Pb(s) + PbO (s) + H SO 4 (aq) Q Describe the coposition of anode and cathode in a ercury cell Write the electrode reactions for the cell Solution : It consists of zinc ercury aalga as anode, a paste of HgO and carbon as cathode The electrolyte is paste of KOH and ZnO lectrode reactions are given as under : Q State two advantages of H O fuel cell over ordinary cells instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

10 Solution : (i) (ii) It does not create efficiency It has uch higher efficiency C 1 Q What is corrosion? Describe the role of zinc in cathodic protenction of iron Can we use tin in place of zinc for this purpose? Give reason Solution : Corrosion is a process in which etal reacts with substances present in atospheric to for copounds at its surface Role of Zinc Zn acts as anode It loses electrons in preference to iron because it is ore reactive than Fe Zn Zn + + e If any Fe + ions are fored, they gain electrons to for iron back Fe + + e Fe Tin cannot be used to protect iron because it is less reactive than Fe Q Draw a neat diagra of H O fuel cell (i) Indicate anode and cathode of the cell (ii) What is the overall reaction taking place in the cell? NCRT XRCIS 31 Arrange the following etals in the order in which they displace each other fro the solution of their salts Al, Cu, Fe, Mg and Zn 3 Given the standard electrode potentials, K + /K = 93 V, Ag + /Ag = 8 V, Hg + /Hg = 79 V, Mg + /Mg = 37 V, Cr 3+ /Cr = 74 V Arrange these etals in their increasing order of reducing power 33 Depict the galvanic cell in which the reaction : Zn(s) + Ag + (aq) Zn + (aq) + Ag(s) takes place Further, show : (i) Which of the electrodes is negatively charged? (ii) (iii) The carriers of the current in the cell Individual reaction at each electrode 34 Calculate the standard cell potentials of galvanic cell in which the following reactions takes place : (i) (ii) given Cr Cr(s) + 3Cd + (aq) Cr 3+ (aq) + 3Cd Fe + (aq) + Ag + (aq) Fe 3+ (aq) + Ag(s) Cd Ag,Ag 3 74V, 4 V, 8 V, 3,Cr,Cd 77 V Calculate the G and equilibriu constant of the reactions instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : , Fe,Fe

11 C Write the Nerst equation and ef of the following cells at 98 K : (i) Mg (s) Mg + (1 M) Cu + (1 M) Cu (s) (ii) Fe(s) Fe + (1 M) H + (1 M) H (g) (1 bar) Pt (s) (iii) Sn(s) Sn + (5 M) H + ( M) H (g) (1 bar) Pt (s) (iv) Pt(s) Br (l) Br (1 M) H + (3 M) H (g) (1bar) Pt(s) Given : Mg / Mg Sn / Sn 37 V, 14 V, Cu,Cu Br,Br 34 V, 18 V Fe,Fe 44 V, 36 In the button cells widely used in watches and other devices, the following reaction takes place : Zn(s) + Ag O(s) + H O (l) Zn + (aq) + Ag(s) + OH (aq) Deterine and G for the reaction Zn/ Zn Ag / Ag 76V, 8V 37 Define conductivity and olar conductivity for solution for an electrolyte Discuss their variation with concentration 38 The conductivity of M solution of KCl at 98 K is 48 S c 1 Calculate its olar conductivity 39 The resistance of a conductivity cell containing 1 M KCl solution at 98 K is 15 What is the cell constant if conductivity of 1 M KCl solution at 98 K is S c 1 31 The conductivity of sodiu chloride at 98 K has been deterined at different concentrations and the result are given below : Concentration/M /S Calculate for all concentrations and draw a plot between and C ½ Find the value of 311 Conductivity of 41 M acetic acid is S c 1 Calculate its olar conductivity If for acetic acid is 395 S c ol 1, what is its dissociation constant? 31 How uch charge is required for the following reductions : (i) 1 ol of Al 3+ to Al? (ii) 1 ol of Cu + to Cu? (iii) 1 ol of MnO 4 to Mn +? 313 How uch electricity in ters of Faraday is required to produce (i) g of Ca fro olten CaCl? (ii) 4 g of Al fro olten Al O 3? [At Wt Ca = 4, Al = 7] 314 How uch electricity is required in coulob for the oxidation of (a) 1 ol of H O to O? (b) 1 ol of FeO to Fe O 3? 315 A solution of Ni(NO 3 ) is electrolysed between platinu electrodes using a current of 5 apere for inutes What ass of Ni is deposited at the cathode? 316 Three electrolytic cells A, B,C containing solutions of ZnSO 4, AgNO 3 and CuSO 4, respectively are connected in series A steady current of 15 aperes was passed through the until 145 g of silver deposited at the cathode of cell B How long did the current flow? What ass of copper and zinc were deposited? [At wt Cu = 635, Zn = 653, Ag = 18] instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

12 C Using the standard electrode potentials, predict if the reaction between the following is feasible : (a) Fe 3+ (aq) and I (aq) (b) Ag + (aq) and Cu(s) (c) Fe 3+ (aq) and Br (aq) (d) Ag (s) and Fe 3+ (aq) (e) Br (aq) and Fe + (aq) Given standard electrode potentials : / I, I 1 / Br, Br 1 Fe, Fe 541V, 34V 3 Cu, Cu 19V, 8V 77 V Ag, Ag 318 Predict the products of electrolysis in each of the following : (i) An aqueous solution of AgNO 3 with silver electrodes (ii) An aqueous solution of AgNO 3 with platinu electrodes (iii) A dilute solution of H SO 4 with platinu electrodes (iv) An aqueous solution of CuCl with platinu electrodes 31 Mg, Al, Zn, Fe, Cu, Ag 3 Ag < Hg < Cr < Mg < K , 895 kj ol 1, 3 35 (i) 68 V (ii) 53 V (iii) 78 V (iv) 188 V V, 318 kj ol S c ol c (i) 3F (ii) F (iii) 5F 313 (i) 1 F (ii) 444 F 314 (i) F (ii) 1 F g of Ni s, 46 g Cu, 438 g Zn ANSWRS instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,

13 C 13 ADDITIONAL QUSTIONS AND PROBLMS Q xplain why electrolysis of aqueous solution of NaCl gives H at cathode and Cl at anode Write overall reaction [Given Na 1 / Na 71V; Cl / Cl 136 V and O (g) + H + (aq) + e H O(l); = 13 V] Q Calculate the cell ef and G for the cell reaction at 5 C for the cell Zn(s) Zn + (1 M) Cd + (1 M) Cd (s) values at 5 C : / Zn 76V and Cd / Cd 43 V Zn ; F = 96, 5 C, R = 8314 JK 1 ol 1 Q Calculate the standard electrode potential of Ni + Ni electrode if the cell potential of the cell Ni Ni + (1 M) Cu + (1 M) Cu is 59 V at 5 C [Given Cu / Cu 34V ] Q Blue colour of copper sulphate solution is slowly discharged (disappears) when an iron rod is dipped into it xplain this by calculating G with the help of the following data : [ Cu / Cu 34V; Fe / Fe 44V and 1 F = 965 C ol 1 ] Q (a) Calculate the electrode potential of silver electrode dipped in a 1 M solution of silver nitrate at 98 K assuing silver nitrate to be copletely dissociated The standard electrode potential of Ag + /Ag is 8 V at 98 K (b) At what concentration of silver ions will this electrode have a potential of zero volt? Q The ef of the cell Zn Zn + (1 M) Cd + (M 1 ) Cd has been found to be 335 V at 98 K Calculate the value of M 1 Given : / Zn 76V, Cd / Cd 4V Zn Q Why does the olar conductance increase on diluting the solution of a weak electrolyte? lectrolytic conductivity of 3 M solution of KCl at 98 K is 37 1 S c 1 Calculate its olar conductivity Q (a) State the factors that influence the value of cell potential of the following cell : Mg (s) Mg + (aq) Ag + (aq) Ag (s) (b) Write Nernst equation to calculate the cell potential of the above cell Q (i) Account for the following : (a)alkaline ediu inhibits the rusting of iron (b) Iron does not rust even if the zinc coating is broken in a galvanised iron pipe (ii) Cu + + e Cu, = +34 V Ag + + e Ag, = +8 V (a) Construct a galvanic cell using the above data (b) For what concentration of Ag + ions will the ef of the cell be at 5 C, if the concentration of Cu + is 1 M? instein Classes, Unit No 1, 13, Vardhan Ring Road Plaza, Vikas Puri xtn, Outer Ring Road New Delhi 11 18, Ph : ,