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CE – 1 ELECTROCHEMISTRY C1 Conductance : The resistance of any conductor varies directly as its length (l) and inversely as its cross-sectional area (a). R l a R l . a where is a constant depending upon the nature of the material and is called specific resistance of resistivity of the material. Specific resistance is the resistance of one centrimetre cube of a material. 1 The reciprocal of the specific resistance, is called specific conductance or conductivity.. Conductivity is the conductance of one cm cube of a material. taking l 1 1 K (specific conductance), C (conductance) K = C × a .... R l is called cell constant, l = distance between two electrodes , a = cross-sectional area of electrodes a C2A Equivalent Conductance : It is defined as the conducting power of all the ions present in one gram equivalent of an electrolyte in a given solution. At concentration N (in gm equivalent L–1) equivalent conductance is denoted by eq (equivalent conductance at concentration c) c = k 1000 ; units: ohm–1 cm2, eq–1 N Equivalent conductance increases with dilution. When the solution is infinitely diluted the equivalent conductance is denoted as . The can be determined by extrapolation method, in which graph between c and C is extended to zero concentration. C2B Molar Conductance : It is defined as the conducting power of all the ions present in one mol of an electrolyte in a given solution. m k 1000 unit : ohm–1 cm2 mol–1 M Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 2 eq and m both increases with decrease in concentration. C2C Kohlrausch’s Law : At infinite dilution, when ionisation is complete, each ion makes its contribution towards equivalent conductance of the electrolyte and at infinite dilution equivalent conductance is given by the sum of the equivalent conductances of the contributing ions. m = xa + yb Thus for AxBy, where, a is the molar conductance of A(say cation) and b that of B(say anion) at infinite dilution. (degree of dissociation of weak electrolyte) cm m Practice Problems : 1. Molar conductance at infinite dilution of BaCl2, H2SO4 and HCl aq. solution are x1, x2 and x3 respectively. Molar conductance of BaSO4 solution is : (a) x1 + x2 – x3 (b) x1 – x2 – x3 (c) x1 + x2 – 2x3 (d) x1 – 2x2 + x3 [Answers : (1) c] C3A Electrical Conductors : All substances which can allow the flow of electricity are known as electrical conductors. Mainly we have two types of electrical conductors which are given below : 1. Electronic Conductor : They are those conductors in which the flow of electricity is due to the movement of loosely bonded electrons in their own standard state. In this case, the movement of matter does not take place during the flow of electricity. For example : (a) 2. all metals in their elemental state (b) graphite and (c) alloys Electrolytical Conductors : In this case of electrolytic conductors, the flow of electricity is due to the movement of ions i.e., here actual transport of matter takes place. C3B Electrolytes : Electrolytes may be pure substances (e.g., salts, acids or bases) in their fused states or more commonly they are aqueous solutions of these compounds or they are sometimes pure liquid. There are two types of electrolytes : (a) Strong Electrolyte : The compounds which are 100% ionised at any dilution, are treated as strong electrolytes, for e.g., HClO4, HI, HBr, HCl, H2SO4, HNO3, NaOH, KOH, NaCl etc. (b) Weak Electrolyte : The compounds which are less or feebly ionised (at lower dilution) are treated as weak electrolytes. All weak acids, weak bases or the salts having less ionic character are treated as weak electrolytes e.g. CH3COOH, , H2CO3, Mg(OH)2, Zn(OH)2 etc. C4A Electrolysis : Electrolysis is a process which involves a chemical change at the electrodes when electricity is passes through an electrolyte. “Electrolysis is a process which involves a chemical change at the electrodes when electricity is passed through an electrolyte”. It was found experimentally that the positive ions from the electrolyte are attracted on the negative electrode (cathode) and get reduced. Similarly, the negative ions are attracted on the positive electrode (anode) and are oxidised. Hence due to electrolysis, the species under consideration gets decomposed at cathode and anode. Faraday has established a relationship between the amount of electricity passes through the electrolyte and the amount of chemical change occurring at an electrode. This relationship is known as Faraday’s law of electrolysis. Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 3 C4B Faraday’s Laws of Electrolysis : First Law of Electrolysis : The amount of chemical change produced is proportional to the quantity of electric charge passing through an electrolysis cell. Suppose after passing Q coulombs of electricity (amount of electricity), W amount of a substance has appeared (or disappeared). Thus WQ W = ZQ (Where Z is a constant, which is known as electrochemical equivalent) * Also W = Z i t..... (Q = i t, where i is current passed for t seconds) Q = nF..... (F is a faraday constant i.e., it is charged carried by one mole of electron, where n is no.of mole of electrons transfer takes place). Second Law of Electrolysis : When same amount of amount of electricity is passed through different electrolytes, the weight of the substance appeared or disappeared by any electrode is always directly proportional to the equivalent weight of the substance appeared or disappeated. According to the second law of electrolysis W E, in general, W1 W2 E1 E2 C4C Relation between Electrochemical Equivalent and Equivalent Weight : By the use of above two laws EZ Thus, E = FZ.... (where F is Faraday’s constant, E is equivalent weight and Z is electrochemical equivalent). If we combine first and second law of electrolysis then following expressions takes place : n eq W Q W It , (n ) = (neq)reduced or mol n.factor eq oxidised E F E F Practice Problems : 1. The density of Cu is 8.94 g cm–3. The quantity of electricity needed to plate an area 10 cm 10 cm to a thickness of 10–2 cm using CuSO4 solution is (a) 2. (a) 3. 13586 C (b) 27172 C (c) 40758 C (d) 20348 C The same amount of electricity was passed through two cells containing molten Al2O3 and molten NaCl. If 1.8 g of Al were liberated in one cell, the amount of Na liberated in the other cell is 4.6 g (b) 2.3 g (c) 6.4 g (d) 3.2 g Silver is removed electrolytically from 200 mL of a 0.1 N solution of AgNO3 by a current of 0.1 ampere. How long will it take to remove half of the silver from the solution (At. wt. of Ag = 108 g) (a) 10 sec Einstein Classes, (b) 16 sec (c) 100 sec (d) 9650 sec Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 4 4. Consider the following electrolysis (1) CuSO4 (2) Fe2(SO4)3 (3) AlCl3 (4) AgNO3 The quantity of electricity needed to electrolyse completely 1 M solutions of these electrolytes will be 5. (a) 2F, 6F, 3F and 1F respectively (b) 6F, 2F, 3F and 1F respectively (c) 2F, 6F, 1F and 3F respectively (d) 6F, 2F, 1F and 3F respectively Assume that during electrolysis of AgNO3, only H2O is electrolysed and O2 is formed as : 2H2O 4H+ + O2 + 4e— O2 formed at N.T.P. due to passage of 2 amperes of current for 965 s : (a) 0.112 L (b) 0.224 L (c) 11.2 L (d) 22.4 L [Answers : (1) b (2) a (3) d (4) a (5) a] C5A Electrochemical Cell : A devide that converts chemical energy into electrical energy. The cell is based on the principle of indirect redox reactions, i.e, the oxidation and reduction reactions takes place in different container. The electrochemical cells (galvanic or voltaic) consists of two half cells connected with each other by means of an electric wire to allow an indirect redox reaction. Although the solutions of two half cells are in different containers, but for the continuous flow of electricity, transfer of ions from one solution to another solution are always allowed. Oxidation at anode and is a –ve electrode, reduction at cathode and is a +ve electrode. For e.g., Zn-Cu galvanic cell is represented as follows : C5B Electrode Potential : It is the potential difference established between the electrode and its electrolyte. It is of two types i.e., Reduction Potential and Oxidation Potential. Reduction Potential is tandency of an electrode to receive electrons for the deposition of the solvated positive ions from its own solution. Whereas oxidation potential is the tandency of an electron to get oxidised in its own solution. C5C E.M.F. (Electromotive Force) or Cell Potential of a Cell : The difference between the reduction potentials of two half cells constituting the Galvanic cell is known as its cell potential (or e.m.f.) i.e., Ecell(e.m.f.) = (Reduction Potential)Cathode – (Reduction Potential)Anode or, ECell = ECathode – EAnode For the spontaneous flow of electricity from the Galvanic cell, the e.m.f. of the cell must be positive. C5D Concentration Effect in Voltaic Cell – Nernst Equation Nerst equation gives a quantitative relationship between the concentration of ions and electrode potential. For a general electrode reaction : Mn+ + L(s) Ln+ + M(s). Emf(E) = E0 – 2.303 Einstein Classes, RT [Ln ] log nF [M n ] Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 5 Practice Problems : 1. 2. By how much will the potential of half-cell Cu2+ | Cu change if the solution is diluted to 100 times at 298 K ? (a) Increase by 59 mV (b) Decreases by 59 mV (c) Increase by 29.5 mV (d) Decreases by 29.5 mV Which of the following changes will increase the EMF of the cell : Co(s) | CoCl2(M1) || HCl (M2) | Pt (H2, g) 3. (a) increase in the volume of CoCl2 solution from 100 mL to 200 mL (b) increase M2 from 0.01 M to 0.50 M (c) increase the pressure of the H2(g) from 1.00 to 2.00 atm (d) increase M1 from 0.01 M to 0.50 M The measured voltage of the following cell is 0.9 V at 250C. Pt, H2(1 atm) | H+(aq) || Ag+ (1.0 M) | Ag If E 0 0.80V . The pH of the aqueous solution of H+ ion is Ag / Ag (a) 1.69 (b) 2.50 (c) 5.20 (d) 9.69 [Answers : (1) b (2) b (3) a] C6 Application of Nernst Equation 1. Electrical work : G = –nFE (in a given state), G0 = –nFE0 (in a standard state), G0 = –RT In K aq G10 + G20 = G30 (when different no. of element are involved, if equal no of electrons are involved E10 + E20 = E30. 2. At equilibrium E0 3. E = 0 and Q = K (equilibrium constant) RT 0.059 ln K log10 K nF n For standard hydrogen electrods (SHE) E0(SHE) = 0.00 V. SHE Pt | H2(g) (1 atm) | HCl aq (1M) SHE, colomel-electrode and silver, silver-chloride electrodes are used as reference half cells. EH 4. 2 / H 0.059 p H Concentration cells Zn | Zn2+ (C1) || Zn2+ (C2) | Zn E cell C 0.059 log 2 ........[C > C ] 1 2 n C1 Pt (H2) (P1) | HCl | Pt (H2) (P2) Ecell = 0.059 log P1 ......[P1 > P2] P2 Practice Problems : 1. E0 for the cell Zn | Zn2+(aq) || Cu2+ (aq) | Cu is 1.10 V at 250C. The equilibrium constant for the reaction Zn + Cu2+ (aq) (a) 10–37 Einstein Classes, Cu + Zn2+ (aq) is of the order of (b) 1037 (c) 10+18 (d) 1017 Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 6 2. Pt (H 2 ) | H (1M) | Pt (H 2 ) (where p1 and p2 are pressures) cell reaction will be spontaneous if : p1 3. p2 (a) p1 = p2 (c) p2 > p1 (b) p1 > p2 (d) 2+ 2+ p1 = p2 = 1atm + The standard reduction potential of Cu | Cu and Cu | Cu are 0.337 and 0.153 V respectively. The standard electrode potential of Cu+ | Cu half cell is (a) 0.184 V (b) 0.827 V (c) 0.521 V (d) 0.490 V [Answers : (1) b (2) b (3) c] C7A Batteries Any battery (actually it may have one or more than one cell connected in series) or cell that we use as a source of electrical energy is basically a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy. 1. Primary Batteries : In primary batteries, the reaction occurs only once and battery then becomes dead after use over a period of time and cannot be reused again, for e.g., : (a) Dry cell : Which is used commonly in our transistors and clocks. The cell consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered magnese dioxide and carbon. The space between the electrodes is filled by a moist paste of NH4Cl and ZnCl2. The electrode reactions are complex, but they can be written approximately as follows : Anode : Zn(s) Zn2+ + 2e– Cathode : MnO2 + NH4+ + e– MnO(OH) + NH3 Ammonia produced in the reaction forms complex with Zn2+ to give [Zn(NH3)4]2+. The cell has a potential of nearly 1.5 V. (b) Mercury cell : Suitable for the low current devides like hearing aids and camera etc. consists of zinc-mercury amalgam as anode and a paste of HgO and carbon as the cathode. The electrolyte is a paste of KOH and ZnO. The electrode reactions for the cell are given below : Anode : Zn(Hg) + 2OH– ZnO(s) + H2O + 2e– Cathode : HgO + H2O + 2e– Hg(l) + 2OH– 2. Secondary Batteries : A secondary cell after use can be rechanged by passing current through it in opposite direction so that it can be used again. A good secondary cell can undergo a large number of discharging and charging cycles, for e.g. : (a) Lead storage battery : commonly used in automobiles and invertors. It consists of a lead anode and a grid of lead packed with lead dioxide (PbO2) as cathode. A 38% solution of sulphuric acid is used as an electrolyte. The cell reactions when the battery is in use are given below : Anode : Pb(s) + SO42–(aq) PbSO4(s) + 2e– Cathode : PbO2(s) + SO42–(aq) + 4H+(aq) + 2e– PbSO4(s) + 2H2O (l) i.e., overall cell reaction consisting of cathode and anode reaction is : Pb(s) + PbO2(s) + 2H2SO4(aq) 2PbSO4(s) + 2H2O(l) On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode is converted into Pb and PbO2, respectively. (b) Nickel-cadmium cell : Which has longer life than the lead storage cell but more expensive to manufacture. We shall not go into detains of working of the cell and the electrode reactions during charging and discharging. The overall reaction during discharge is : Cd(s) + 2Ni(OH)3(s) CdO (s) + 2Ni(OH)2 (s) + H2O(l) 3. Fuel Cells : Production of electricity by thermal plants is not a very efficient method and is a major source of pollution. In such plants, the chemical energy (heat of combustion) of fossil fuels (coal, gas or oil) is first used for converting water into high pressure steam. This is then used to run a turbine to produce electricity. Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 7 It is now possible to make such cells in which reactants are fed continuously to the electrodes and products are removed continuously from the electrolyte compartment. Galvanic cell that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol etc. directly into electrical energy are called fuel cells. One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water. The cell was used for providing electrical power in the Apollo space programme. The water vapours produced during the reaction were condensed and added to the drinking water supply for the astronauts. Catode : O2(g) + 2H2O(l) + 4e– 4OH–(aq) Anode : 2H2 + 4OH–(aq) 4H2O(l) + 4e– Overall reaction being : 2H2(g) + O2(g) 2 H2O(l) C7B Corrosion Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal. The rusting of iron, tarnishing of silver, development of green coating on copper and bronze are some of the examples of corrosion. In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as an anode and we can write the reaction : Anode : 2 Fe(s) 2 Fe2+ + 4e– E0(Fe2+, Fe) = –0.44 V Cathode : O2(g) + 4H+(aq) + 4e– 2 H2O (l) [E0 = 1.23 V] The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe2O3 . xH2O) and with further production of hydrogen ions. Prevention of corrosion : one of the simplest method of preventing corrosion is to prevent the surface of the metallic object to come incontact with atmosphere. This can be done by covering the surface by paint or by some chemicals (e.g. bisphenol). Other simple method is to cover the surface by other metals (Sn, Zn etc.) that are inert or react to save the object. An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn etc.) which corrodes itself but saves the object. Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 8 SINGLE CORRECT CHOICE TYPE 1. 2. 3. The density of Cu is 8.94 g cm–3. The quantity of electricity needed to plate an area 10 cm 10 cm to a thickness of 10–2 cm using CuSO4 solution is (a) 13586 C (b) 27172 C (c) 40758 C (d) 20348 C A gas X at 1 atm is bubbled through a solution containing a mixture of 1 M Y— and 1 M Z— at 250C. If the reduction potential of Z > Y > X, then The Zn electrode had twice the surface of the Cu electrode. (c) The [Zn2+] was larger than the [Cu2+] (d) The volume of the Zn2+ solution was larger than the volume of the Cu2+ solution. (b) Y will oxidize Z and not X (c) Y will oxidize both X and Z pressures) cell reaction will be spontaneous if : (d) Y will reduce both X and Z (a) p1 = p2 (b) p1 > p2 (c) p2 > p1 (d) p1 = p2 = 1atm 0 cell The E 7. 2+ + 2+ 3+ (a) 50 (b) 10 (c) 1050 (d) 105 8. Which of the following changes will increase the EMF of the cell : (a) increase in the volume of CoCl2 solution from 100 mL to 200 mL (b) increase M2 from 0.01 M to 0.50 M (c) increase the pressure of the H2(g) from 1.00 to 2.00 atm (d) increase M1 from 0.01 M to 0.50 M Cu + Zn2+ (aq) is of the order (a) 10–37 (b) 1037 (c) 10+18 (d) 1017 9. Using the standard potential values given below, decide which of the statements I, II, III, IV are correct. Choose the right answer from (a), (b), (c) and (d) — 0 Fe + 2e = Fe, E = 0.44 V Cu2+ + 2e— = Cu, E0 = +0.34 V Ag+ + e— = Ag, E0 = +0.80 V I. Copper can displace iron from FeSO4 solution II. Iron can displace copper from CuSO4 solution III. Silver can displace copper from CuSO4 solution IV. Iron can displace silver from AgNO3 solution (a) I and II (b) II and III (c) II and IV (d) I and IV For the cell Zn(s) | Zn2+ || Cu2+ | Cu(s), the standard cell voltage, E0cell is 1.10 V. When a cell using these reagents was prepared in the lab, the measured cell voltage was 0.98 V. One possible explanation for the observed voltage is : Einstein Classes, p2 Co(s) | CoCl2(M1) || HCl (M2) | Pt (H2, g) E0 for the cell Zn | Zn2+(aq) || Cu2+ (aq) | Cu is 1.10 V at 250C. The equilibrium constant for the reaction 2+ Pt (H 2 ) | H (1M) | Pt (H 2 ) (where p1 and p2 aree p1 in which the reaction : Zn + Cu2+ (aq) of 6. (b) Y will oxidize X and not Z MnO + Fe + H Mn + Fe + H2O occurs is 0.59 V at 250C. The equilibrium constant for the reaction is approximately of the order of 5. There were 2.00 mol of Zn2+ but only 1.00 mol of Cu2+. (a) — 4 4. (a) 10. 11. 12. Cost of electricity for the production of x L H2 at NTP at cathode is Rs. x, then cost of electricity for the production of x L O2 gas at NTP at anode will be (assume 1 mol of electrons as one unit of electricity) : (a) 2x (b) 4x (c) 16x (d) 32x Molar conductance at infinite dilution of BaCl2, H 2SO 4 and HCl aq. solution are x 1, x 2 and x 3 respectively. Molar conductance of BaSO4 solution is : (a) x1 + x2 – x3 (b) x1 – x2 – x3 (c) x1 + x2 – 2x3 (d) x1 – 2x2 + x3 The same amount of electricity was passed through two cells containing molten Al2O3 and molten NaCl. If 1.8 g of Al were liberated in one cell, the amount of Na liberated in the other cell is (a) 4.6 g (b) 2.3 g (c) 6.4 g (d) 3.2 g Salts of A (atomic weight 7), B (atomic weight 27) and C (atomic weight 48) were electrolysed under identical conditions using the same quantity of electricity. It was found that when 2.1 g of A was deposited, the weights of B and C deposited were 2.7 g and 7.2 g. The valencies of A, B and C are respectively Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 9 13. 14. (a) 3, 1 and 2 (b) 1, 3 and 2 (c) 3, 1 and 3 (d) 2, 3 and 2 20. Silver is removed electrolytically from 200 mL of a 0.1 N solution of AgNO 3 by a current of 0.1 ampere. How long will it take to remove half of the silver from the solution (At. wt. of Ag = 108 g) (a) 10 sec (b) 16 sec (c) 100 sec (d) 9650 sec 21. The cell reaction for the given cell is spontaneous if : The value of the standard potentials for reduction reactions of A+/A, B2+/B, C2+/C and D2+/D at 250 are 0.80, 0.34, –0.76 and –1.66 volts respectively. The correct sequence in which these metals will be deposited on the cathode is (a) A, B, C, D (b) D, C, B, A (c) A, C, B, D (d) D, B, C, A The standard reduction potential for half-reaction for four different elements A, B, C and D are PtCl2|Cl—(1M) || Cl— (1M) | PtCl2 P1 P2 15. 16. 17. (a) P1 > P2 (b) P1 < P2 (c) P1 = P2 (d) P2 = 1 atm B2 + 2e 2B— E0 = +1.36 V C2 + 2e— 2C— E0 = +1.06 V D2 + 2e— 2D— E0 = +0.53 V (a) 2.89 × 105 (b) 2.89 (a) would be A and D respectively (c) 1.89 × 102 (d) 5.89 × 109 (b) would be D and A respectively (c) would be B and C respectively (d) cannot be ascertained from the given data as the species being subjected to oxidation or reduction have not been indicated The molar conductance of KCl, KNO3 and AgNO3 are 149.9, 145.0, and 133.4 ohm–1 cm2 mol–1. (all at 25 0C). The molar conductance of AgCl at this temperature is (a) 144.4 (b) 120.8 (c) 138.3 (d) 178.2 22. Given the following half cell reaction and corresponding reduction potentials : The reaction ½ H2(g) + AgBr(s) H+(aq) + Br— (aq) + Ag(s) occurs in the galvanic cell (a) Ag | AgBr(s) | KBr (aq) || AgNO3 (aq) | Ag (i) A + e — A— E0 = – 0.24 V (b) Pt | H2(g) | HBr (aq) || AgNO3 (aq) | Ag (ii) B— + e— B2— E0 = 1.25 V (c) Pt | H2(g) | HBr (aq) | AgBr(s) | Ag (iii) C— + 2e— C3— E0 = 0.68 V (d) Pt | H2(g) | KBr (aq) || AgBr (s) | Ag E + 4e— E4— E0 = 0.38 V 23. The largest potential resulted by the combination of two half cells is In a electrolytic cell is which of the following statement is correct : (a) oxidation occurs at cathode (a) E0 = 2.50 V (b) E0 = 1.50 V (b) reduction occurs at anode (c) E0 = 0.50 V (d) E0 = 1.20 V (c) anode acts as a negative terminal (d) flow of electron takes place from anode to cathode Consider the following electrolysis (1) CuSO4 (2) Fe2(SO4)3 (3) AlCl3 (4) AgNO3 24. The quantity of electricity needed to electrolyse completely 1 M solutions of these electrolytes will be 19. –E0 = +2.85 V The strongest oxidising and reducing agents among these The charge in coulomb on 1 g ion of N–3 is (iv) 18. A2 + 2e— 2A— (a) 2F, 6F, 3F and 1F respectively (b) 6F, 2F, 3F and 1F respectively (c) 2F, 6F, 1F and 3F respectively (d) 6F, 2F, 1F and 3F respectively 25. The standard oxidation potentials of the electrodes Ag|Ag+, Sn|Sn2+, Ca|Ca2+, Pb|Pb2+ are –0.8, 0.1.36, 2.866 and 10.126 V respectively. The most powerful oxidising agent among these metals is (a) Pb (b) Ca (c) Sn (d) Ag By how much will the potential of half-cell Cu2+ | Cu change if the solution is diluted to 100 times at 298 K ? A solution of sodium sulphate in water is electrolysed using inert electrodes. The products at the cathode and anode are respectively (a) Increase by 59 mV (b) Decreases by 59 mV (c) Increase by 29.5 mV (a) H2, O2 (b) O2, H2 (d) Decreases by 29.5 mV (c) O2, Na (d) O2, SO2 Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 10 26. Given that E 0Fe 3 |Fe and E 0Fe 2 |Fe are –0.36 V and – 0.439 V, respectively. The value of 31. E 0Fe3 ,Fe 2 |Pt would be 27. (a) (–0.36 – 0.439)V (b) [3(–0.36) + 2(–0.439)]V (c) (–0.36 + 0.439)V (b) [3(–0.36) – 2(–0.439)]V For the half cell 32. A hydrogen electrode placed in a buffer solution of CH3COONa and acetic in the ratio’s x : y and y : x has electrode potential values E1 volt and E2 volt respectively at 2500C. The pKa values of acetic acid is (E1 and E2 are oxidation potential) (a) E1 E 2 0.118 (c) E1 E 2 0.118 (b) E 2 E1 0 0.118 (d) E1 E 2 0 0.118 The measured voltage of the following cell is 0.9 V at 250C. Pt, H2(1 atm) | H+(aq) || Ag+ (1.0 M) | Ag If E 0 0.80V . The pH of the aqueous Ag / Ag + 2H+ + 2e—, E0 = 1.30 V solution of H+ ion is At pH = 2, electrodes potential is : (a) 1.36 V (c) 28. 1.42 V + (b) 1.30 V (d) 1.20 V + Ag | Ag (1M) || Ag (2M) | Ag 1 L solution 1 L solution 0.5 F of electricity in the LHS (anode) and 1F of electricity in the RHS (cathode) is first passed making them independent electrolytic cells at 298 K. EMF of the cell after electrolysis will be : 29. 30. 33. 34. (a) 1.69 (b) 2.50 (c) 5.20 (d) 9.69 A 100 watt. 110 volt incandescent lamp in series with an electrolytic cell containing cadmium sulphate solution. The mass of cadmium will be deposited by the current flowing for 10 hours is (At. wt. Cd = 112.4). (a) 16.02 (b) 19.06 (c) 20.22 (d) 25.22 The solubility product of AgI from the following data (a) increased (b) decreased E 0Ag / Ag 0.80V and E 0I |AgI|Ag 0.15V (c) no change (a) 8.9 × 10–17 (b) 6.9 × 10–14 (d) time is also required (c) 2.3 × 10–10 (d) 5.2 × 10–9 During electrolysis of acidified water, O2 gas is formed at the anode. To produce O2 gas at the anode at the rate of 0.224 c.c. per second at STP, current passed is (a) 0.224 A (b) 2.24 A (c) 9.65 A (d) 3.86 A 100 mL of a buffer of 1M NH3(aq) and 1 M NH4+ (aq) are placed in two voltaic cells separately. A current of 1.5 A is passed through both cells for 20 minutes. If electrolysis of water only takes place : 2H2O + O2 + 4e— 4OH— (RHS) + — 2H2O 4H + O2 + 4e (LHS) then pH of the : 35. 36. The standard reduction potential of Cu2+ | Cu and Cu2+ | Cu+ are 0.337 and 0.153 V respectively. The standard electrode potential of Cu+ | Cu half cell is (a) 0.184 V (b) 0.827 V (c) 0.521 V (d) 0.490 V Zn + Cu2+ (aq) Cu + Zn2+ (aq). Reaction quotient is Q [ Zn 2 ] . Variation of Ecell with log [Cu 2 ] Q is of the type with OA = 1.10 V. Ecell will be 1.1591 V when : (a) LHS half-cell will increase (a) [Cu2+]/[Zn2+] = 0.01 (b) RHS half-cell will increase (b) [Zn2+]/[Cu2+] = 0.01 (c) both half-cells will increase (c) [Zn2+]/[Cu2+] = 0.1 (d) both half-cells will decrease (d) [Zn2+]/[Cu2+] = 1 Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 11 41. 42. 37. Assume that during electrolysis of AgNO3, only H2O is electrolysed and O2 is formed as : 2H2O 4H+ + O2 + 4e— 38. 39. 108 g 59.0 g 52.0 g 22.4 L (c) 108 g 108 g 108 g (d) 108 g 117.5 g 166 g ANSWERS (SINGLE CORRECT CHOICE TYPE) Chromium plating can involve the electrolysis of an electrolyte of an acidified mixture of chromic acid and chromium sulphate. If during electrolysis the article being plated increases in mass by 2.6 g and 0.6 dm3 of oxygen are evolved at an inert anode, the oxidation state of chromium ions being discharged must be (a) –1 (b) Zero (c) +1 (d) +2 Equal quantities of electricity are passed through three voltmeters containing FeSO4, Fe2(SO4)3 and Fe(NO3)3 consider the following statements in this regard. 1. The amount of iron deposited in FeSO4 and Fe2(SO4)3 is equal 2. The amount of iron deposited in Fe(NO3)3 is two thirds of the amount of iron deposited in FeSO4. 3. The amount of iron deposited in Fe2(SO4)3 and Fe(NO3)3 is equal to these statements (a) 1 alone is correct (b) 1 and 2 are correct (c) 2 and 3 are correct (d) 3 alone is correct Einstein Classes, One faraday of current was passed through the electrolysis cells placed in series containing solutions of Ag+, Ni++ and Cr+++ respectively. The amount of Ag, Ni and Cr, atomic masses 108, 59, and 52 g mol–1 respectively; deposited will be (b) (assuming Cr = 52 and 1 mole of gas at room temperature and pressure occupies a volume of 24 dm3) 40. 4.8 L 0.224 L How much will the reduction potential of a hydrogen electrode change when its solution initially at pH = 0 is neutralised to pH = 7 Decrease by 0.41 V (d) 17.5 g (d) (d) 2.6 L 29.5 g 11.2 L Increase by 0.41 V (c) 108 g (c) (c) 2.4 L (a) (b) Decrease by 0.059 V (b) Cr 0.112 L (b) 1.2 L Ni (a) Increase by 0.059 V (a) Ag O2 formed at N.T.P. due to passage of 2 amperes of current for 965 s : (a) In an electrolysis of an aqueous solution containing sodium ions, 2.4 L of oxygen at STP was liberated at anode. The volume of hydrogen at STP liberated at cathode would be 1. 2. b a 22. 23. c d 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. c b d c b b a c a b d b a c b a a a a 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. d b d c c d b a a b a c b a d d c d a Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 12 EXCERCISE BASED ON NEW PATTERN COMPREHENSION TYPE Comprehension-3 Comprehension-1 For the reaction MnO4– + 8H+ + 5Fe2+ Mn2+ + 4H2O + 5Fe3+, it is given that E0(MnO4–, Mn2+, H+|Pt) = 1.51 V and E0(Fe3+, Fe2+|Pt) = 0.77 V. Tollens reagent (ammonical solution of silver nitrate) is used to test aldehydes. The following data are available. 1. Ag+ + e– Ag : E10 = 0.80 V C6H12O7 + 2H+ + 2e– C6H12O6 + H2O; [Ag(NH3)2]+ + e– Ag(s) + 2NH3 The value of log K 0 eq 7. (a) increasing [Mn2+] E20 = 0.05 V (b) increasing [Fe3+] (c) decreasing [MnO4–] E30 = 0.373 V (d) decreasing pH of the solution 8. for the reaction C6H12O6 + 2Ag+ + H2O = C6H12O7 + 2H+ + Ag is 2. 3. (a) 12.7 (b) 25.4 (c) 29.27 (d) 58.54 The use of NH3 makes the pH of solution equal to 11. This causes 5. (a) –0.059 V (b) –0.0178 V (c) 0.059 V (d) 0.0178 V – (a) decrease in the value of E2 (b) increase in the value of E2 (a) –0.018 V (b) 0.0036 V (c) increase in the value of E1 (c) 0.018 V (d) –0.0036 V (d) increase in the value of E10 Comprehension-4 Ammonia is used in this reaction rather than any other base. This is due to the fact that The Edison storage cell is represented as Fe(x) | FeO(s) | KOH (aq) | Ni2O3(s) | Ni(s) (a) [Ag(NH3)2]+ is a weaker oxidizing agent than Ag+ (b) ammonia prevents the decomposition of gluconic acid Ni2O3(s) + H2O(l) + 2e— E0 = +0.40 V (c) silver precipitates gluconic acid as its silver salt FeO(s) + H2O(l) + 2e— E0 = –0.87 V (d) the standard reduction potential of [Ag(NH3)2]+ is changes. The half-cell reaction are 10. 2NiO(s) + 2OH— Fe(s) + 2OH— The cell reaction is Comprehension-2 (a) Ni2O3(s) + Fe(s) A saturated solution of silver bromide is made 10–7 M in silver nitrate. Given : (b) 2NiO(s) + FeO(s) Ksp0(AgBr) = 3.0 × 10–13, m(Ag+) = 6 × 10–3 S m2 mol–1, (c) Ni2O3(s) + Fe(s) Ni(s) + Fe2+ (d) Ni2O3(s) + Fe(s) NiO + Fe2+ Calculate the specific conductance of AgNO3 11. What is the cell e.m.f. ? How does it depend on the concentration of KOH ? 14 × 10–7 (b) Ecell = 1.27 V independent of concentration of KOH (b) 13 × 10–7 (c) (d) 70 × 10 –7 Ecell = –1.27 V dependent of concentration of KOH (d) (b) 13 × 10 –7 Ecell = 1.27 V dependent of concentration of KOH (d) 70 × 10–7 (b) 13 × 10–7 (c) 14 × 10–3 (d) The specific conductor of AgBr is (c) 14 × 10 –3 Specific conductance of solution is –7 (a) 158 × 10 (c) –3 14 × 10 Einstein Classes, Ni2O3(s) + Fe(s) Ecell = 0.27 V independent of concentration of KOH 5 × 10–7 5 × 10–7 2NiO(s) + FeO(s) (a) (a) (a) 6. 9. Reducing [Fe 3+] to 0.50 M keeping all other concentrations at unity, the emf of the cell will be changed by Reducing [MnO4 ] to 0.50 M keeping all other concentrations at unity, the change in emf of the cell will be changed by m(NO3–) = 7 × 10–3 S m2 mol–1, m(Br–) = 8 × 10–3 S m2 mol–1, and K(water) = 7.5 × 10–6 S m–1. 4. The cell emf could be increased above the standard emf by Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 13 12. The maximum amount of electrical energy that can be obtained from one mole of Ni2O3 (a) 24.52 kJ (b) 2.452 kJ (c) 0.2452 kJ (d) 245.11 kJ (A) Comprehension-5 The standard reduction potential for the following half-cell is 0.78 V – 3 + (B) — NO (aq) + 2H (aq) + e NO2(g) + H2O 13. 14. 15. (A) (B) (C) (D) (E) (A) (B) (C) (D) The reduction potential in 8 M H+ is (a) 0.887 V (b) –0.887 V (c) 0.32 V (d) –0.78 V The reduction potential of the half-cell in a neutral solution. Assume all the other species to be at unit concentration. (a) 0 (b) 0.887 (c) 0.0474 V (d) –0.0474 V How much will the reduction potential of the above mentioned half cell will change when its solution initially at pH = 0 is neutralized to pH = 7 while keeping the other ions concentration to be 1 molar. (a) decreases by 0.41 V (b) increases by 0.826 V (c) decreases by 0.826 V (d) increases by 0.41 V MATRIX-MATCH TYPE Matching-1 Column - A Galvanic cell Cathode of an electrolytic cell Electrode potential Current, I Faraday constant Matching-2 Column-A Faraday’s required to reduce Cr2O7– to Cr3+ By passing one Faraday of electricity Zn | Zn2+ (C1 = 0.05) || Zn2+ (C2 = 0.5) | Zn E0OCl–/Cl– = .94 and E0Cl–/Cl2 = –1.36 v. The E0OCl–/Cl2 will be Einstein Classes, Column - B (P) Ecell = ER – EL (Q) reduction potentials (R) Q/t (S) Symbol : F (T) negatively charged (C) (D) 1. 2. Column-B (P) 1/3 mol of Al deposited (Q) 6F (R) –0.42 V (S) 0.0295 V (T) 3F 3. Matching-3 Column-A Column-B The equivalent (P) 189 ohm–1 cm2 conductivity of 1M mol–1 H2SO4 solution whose conductivity is 26 × 10–2 ohm–1 cm–1 is Given that : (Q) 130 ohm–1 cm2 m [Al2(SO4)3] = eq–1 –1 2 –1 858 ohm cm mol m(SO42–) = 160 ohm–1 cm2 mol–1 then m (Al3+) is Given that : (R) 1.66 cm–1 c m (NH4OH) = 9.33 ohm–1 cm2 m(NH4OH) = 238.3 ohm–1 cm2 mol–1 the degree of dissociation of NH4OH is The conductivity of (S) 5% N/10 KCl solution at 200C is 0.0212 ohm–1 cm–1 and the resistance to the cell containing this solution at 200C is 55 ohm. The cell constant is (T) 3.92 % MULTIPLE CORRECT CHOICE TYPE Rusting of iron can be prevented (a) by electroplating the metal with silver (b) by electroplating the metal with Gold (c) by electropting the metal with Zn (d) by connecting the iron material with Mg For the cell : Tl|Tl+(0.0001M)||Cu2+(0.1 M)|Cu Ecell = 0.83 at 298 k the cell potential can be increased by (a) increasing [Cu2+] (b) increasing (Tl+) (c) decreasing [Cu2+] (d) decreasing [Tl+] When net cell reaction is spontaneous which of the following are correct (a) E0 cell is negative (b) Ecell > 0 (c) Ecell = E0cell (d) G < 0 Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 14 4. 5. 6. 7. 0 cell In which case Ecell – E = zero (a) Cu|Cu2+ (0.01M) || Ag+(0.1M) | Ag (b) Pt(H2)|PH = 1 || Zn2+ (0.01M) | Zn (c) Pt(H2)|PH = 1||Zu2+(1M)|Zn (d) Pt(H2)|H+(0.01M) || Zn2+ (0.01M) | Zn By passing IF of electricity (a) 1 mol of Zn deposited (b) 12 g of Mg deposited (c) 1/3 mol of Al deposited (d) 11.2 L of O2 at N.T.P. evolved KI solution containing starch turns blue on additionof Cl2. Which of the following statements helps to explain this : (a) Reduction potential of Cl2 > I2 (b) E0OX of Cl2 > E0OH of I2 (c) the product of Cl2 and starch is blue in colour (d) the product of I2 and starch is blue in colour The chemical reactions taken place in the process of rusting of iron are listed below, pick up the correct chemical reactions (a) 2Fe(s) + 4H++ O2 2Fe2+aq + 2H2O (b) 4Fe2+aq + O2(g) + 4H2O 2Fe2O3 + 8H+ (c) Fe2O3 xH2O Fe2O3 + x H2O (d) 3Fe + 4H2O Fe2O3 + FeO + 4H2 8. 9. 10. Which of the following are not reference electrode (a) Normal hydrogen electrode (b) Calomel electrode (c) Silver-Silver chloride electrode (d) Platinium electrode For a galvanic cell, (a) anode is a negative terminal and cathode is a positive terminal (b) oxidation takes place at anode and reduction at cathode. (c) electrons in the external wire move from anode to cathode. (d) Ecell = ER – EL The emf of the cell Pt|H2(g)||HCl(c1)||HCl(c2)|H2(g)|Pt can be increased by (a) decreasing c2 (b) decreasing c1 (c) increasing c1 (d) increasing c2 (Answers) EXCERCISE BASED ON NEW PATTERN COMPREHENSION TYPE 1. b 2. a 3. a 4. b 5. d 6. a 7. d 8. d 9. d 10. a 11. b 12. d 13. a 14. d 15. c MATRIX-MATCH TYPE 1. [A-P; B-T; C-P; D-R; E-S] 2. [A-Q; B-P; C-S; D-R] 3. [A-Q, B-P, C-T, D-R] 4. b, c 6. MULTIPLE CORRECT CHOICE TYPE 1. c, d 2. a, d 3. b, d 7. a, b,c 8. a, c 9. a, b, c, d Einstein Classes, a, b 5. 10. b, d a, d Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 15 INITIAL STEP EXERCISE (SUBJECTIVE) 1. 2. 3. 4. A current of 1.50 A flows through a cell containing aqueous NiSO4 for 40 min. What mass of Ni is deposited on the cathode and what volume at (STP) of oxygen will be evolved at the anode ? (At. wt. of Ni = 58.9). In an electrolysis experiment electric current was passed for 5 hrs through two cells connected in series. The first cell contains gold salt and second contain CuSO 4 solution. 9.83 g of gold was deposited in the first cell. If the oxidation number of Au is +3, find the amount of copper deposited in the second cell. Also calculate the magnitude of the current in amperes. (At wt. of Au = 197 and At. wt. of Cu = 63.5) An electric current is passed through two electrolytic cells connected in series, one containing a solution of silver nitrate and the other solution of sulphuric acid. What volume of oxygen measured at 250C and 750 mm of Hg would be liberated from H2SO4 if (i) 1 mole and (ii) 8 × 1022 ions of Ag+ are deposited from the silver nitrate solution ? Calculate the e.m.f. of the following cell and predict whether the given cell representation is correct or wrong. If wrong, write the correct representation and correct cell reaction 8. The emf of the cell Zn | ZnCl2(0.05 mol dm–3) | Ag+ | AgCl(s) | Ag is 1.015 V at 298 K, the silver electrode being positive, while the temperature coefficient of its emf is –0.00492 VK–1. Write down the equation for the reaction occuring when the cell is allowed to discharge and calculate the changes in (a) free energy (G), and (b) heat content (H) and (c) entropy (S) accompanying this reaction, at 298 K. 9. The standard reduction potential of Cu2+/Cu and Ag +/Ag electrodes are 0.337 and 0.799 volt respectively. Construct a galvanic cell using these electrodes so that its standard emf is positive. For what concentration of Ag+ will the emf of the cell at 25 0C be zero, if the concentration of Cu 2+ is 0.10 M ? 10. The electrolysis of a metal salt solution was carried out by passing a current of 4 amp for 45 min. It resulted in deposition of 2.977 of a metal. If atomic mass of the metal is 106.4 g mol–1, calculate the charge on the metal cation. 11. 40 ml of 0.126 M NiSO4 solution is electrolysed by a current of 0.05 amp for 40 min. Cu(s) | Cu2+ (aq) || Zn2+(aq) | Zn(s) (a) Write equation for the reactions occurring at each electrode (b) How many coulombs of electricity are passed through the electrolyte (c) How many grams of product is deposited at cathode (d) How long the same current will have to be passed to remove completely the metal ions from the solution. Given : E 0 Cu 2 / Cu = +0.34 V and E 0 Zn 2 / Zn 0.76 V 5. Calculate emf of the cell Pt(H2) | CH3COOH (0.1 M) || NH4OH (0.01 M) | (H2) Pt K a for CH 3 COOH = 1.8 × 10 —5 and K b for NH4OH = 1.8 × 10–5 6. 7. Consider the reaction 2Ag+ + Cd 2Ag + Cd2+. The standard electrode potentials for Ag+/Ag and Cd2+/Cd couples 0.80 V and 0.40 V respectively. (i) What is the standard potential for this reaction ? (ii) For the electrochemical cell in which the above reaction takes place which electrode is negative electrode ? (iii) Will the total emf of the given reaction be more negative or positive, if the concentration of Cd2+ ions is 0.1 M rather than 1.0 M ? A zinc rod is placed in 0.1 M solution of ZnSO4 at 250C. Assuming that the salt is dissociated to the extent of 95% at this dilution, calculate the potential of the electrode at this temperature. Given that E 0 Zn 2 / Zn 0.76 V Einstein Classes, 12. Silver is electrodeposited on a metallic vessel of surface area 800 cm2 by passing a current of 0.2 ampere for 3 hours. Calculate the thickness of the silver deposited. Given the density of silver as 10.47 g/cc. 13. Calculate standard electrode potential of Ni+2/Ni electrode if the cell potential of a cell Ni | Ni+2 (0.01 M) || Cu 2+ (0.1 M) | Cu is 0.59 V. Given that E 0Cu 2 / Cu 0.34V 14. Calculate the maximum possible electrical work that can be obtained from the following cell under standard conditions at 250C. Zn | Zn+2(aq) || Cu2+ (aq) | Cu At 250C, E 0Zn 2 / Zn 0.76V and E 0Cu 2 / Cu 0.34V Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 16 15. Calculate the E.M.F. of the cell 2+ + 23. Calculate pH of the following half cell, Pt H2 | H2SO4. The oxidation electrode potential is + 0.3 V. 24. (a) –3 Mg | Mg (0.2 M) || Ag (1 × 10 M) | Ag E 0Mg 2 / Mg 2.37 V, E 0Ag / Ag 0.80 V What will be the effect on EMF if concentrations of Mg2+ ion is decreased to 0.1 M 16. The measured voltage of the following cell is 0.9 V at 250C. + volts (b) + Pt, H2(1 atm) | H (aq) || Ag (1.0 M) | Ag If E 0 0.80V . Calculate the pH of the Ag / Ag aqueous solution of H+. 17. How many grams of silver could be plated out on a serving tray by electrolysis of a solution containing silver in + 1 oxidation state for a period of 8.0 hours at a current of 8.46 Amperes ? What is the area of the tray if the thickness of the silver plating is 0.00254 cm ? Density of silver is 10.5 g/cm3. 18. A 100 watt. 110 volt incandescent lamp in series with an electrolytic cell containing cadmium sulphate solution. What mass of cadmium will be deposited by the current flowing for 10 hours ? (At. wt. Cd = 112.4). 19. A current of 10.0 A is passed through 1.0 L of 1.0 M HCl solution for 1.0 h. Calculate the pH of the solution at the end of the experiment. What is the volume of total gas evolved at STP ? 20. Consider the cell Zn | Zn2+ (aq. 1.0 M) || Cu2+ (aq 1.0 M) | Cu 21. Write down the cell reaction (ii) Calculate the emf of the cell (iii) Is the cell reaction spontaneous or not ? A current was passed through a series of cells containing AgNO3, CuSO4 and H2SO4 solutions for a period of 25 minutes. If the weight of silver deposited was 0.5394 g, what would be (i) the weight of copper and (ii) the volume of H 2 at N.T.P. liberated by the current ? Also calculate the magnitude (strength) of current assuming that it remained constant. [At. wt. Ag = 108, Cu = 63.5]. 26. A solution of a salt of a metal of atomic weight 112 was electrolysed for 150 minutes with a current of 0.15 amperes. The weight of metal deposited was 0.783 mg. Find the equivalent weight and valency of the metal in the salt. 27. A current deposits 10–2 kg of Cu in 3.96 × 103 minute from a solution of Cu++ ions. What is the strength of current in amperes ? How many grams of Cu will the same current deposit from a solution of cuprous ions ? 28. A 1.5276 g sample of CdCl 2 was converted to metallic cadmium by an electrolytic process. 0.9367 g of cadmium was obtained. What is the atomic mass of cadmium from this experiment if the atomic mass of chlorine is taken as 35.453 g mol—1. 29. Calculate heat of reaction inside the cell Zn(s) + 2AgCl(s) Given the following cell ZnCl2(0.555 M) + 2Ag(s) Al | Al3+ (0.1 M) || Fe2+ (0.2 M) | Fe Given that E = 1.015 V at 00 C and E 0Al3 / Al 1.66V and E 0Fe 2 / Fe 0.44V . E 4 1 4.02 10 VK T P Calculate the maximum work that can be obtained by the cell. 22. At what concentration of copper ions will this electrode have a potential of zeo volt ? 25. The standard reduction potentials are 0.350 V for Cu2+ (aq) + 2e— Cu(s) and – 0.763 V for Zn2+ (aq) + 2e— Zn(s) (i) Calculate the electrode potential at a copper electrode dipped in a 0.1 M solution of copper sulphate at 298 K; assuming CuSO4 to be completely dissociated. The standard electrode potential of Cu2+ | Cu system is + 0.34 at 298 K. 30. (i) Determine the equilibrium constant of the following reaction at 298 K 2Fe3+ + Sn2+ 0 E 0Fe 2 / Fe 2 0.771V , E Sn 0.150V 4 / Sn 2 Einstein Classes, Cl2(g) + SO2(g) + 2H2O(l) 2Cl—(aq) + 3H+(aq) + HSO4— (aq) proceeds readily and rapidly in aqueous acid solution. Write the half cell reactions and construct the cell. 2Fe2+ + Sn4+ Also predict whether Sn2+ ions can reduce Fe3+ ions to Fe2+ quantitatively or not The chemical reaction : (ii) If the fully charged cell initially held 1.0 M of Cl2, for how many days could it sustain a current of 0.05 A assuming that the cell becomes inoperative when 90% of the initial Cl2 has been consumed ? Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 17 31. Zinc granules are added in excess to a 500 ml of 1.0 M nickel nitr ate solut ion at 250 C until the equilibrium is reached. If the standard reduction potential of Zn2+ | Zn and Ni2+ | Ni are –0.75 V and –0.24 V respectively, find out the concentration of Ni2+ in soluble at equilibrium. 32. (a) The standard reduction potential of Cu++ / Cu and Ag+ / Ag electrodes are 0.337 and 0.799 volt respectively. Construct a galvanic cell using these electrodes so that its standard e.m.f. if positive. For what concentration of Ag+ will the e.m.f. of the cell, at 250C, be zero if the concentration of Cu++ is 0.01 M ? (b) 33. Calculate the quantity of electricity that would be required to reduce 12.3 g of nitrobenzene to aniline, if the current efficiency for the process is 50 per cent. If the potential drop across the cell is 3.0 volts, how much energy will be consumed ? A cell, Ag | Ag+ || Cu2+ | Cu, initially contains 1 M Ag+ and 1 M Cu2+ ions. Calculate the change in the cell potential after the passage of 9.65 A of current for 1 h. FINAL STEP EXERCISE (SUBJECTIVE) 1. 2. 3. 4. 19 g fused SnCl 2 was electrolysed using inert electrode 0.119 g Sn was deposited at cathode. If nothing was given out during electrolysis, calculate the ratio of weight of SnCl2 and SnCl4 in fused state after electrolysis. (At. wt. of Sn = 119) For the following galvanic cell, calculate the e.m.f. at 25 0 C. Assign the correct polarity for the spontaneous reaction to take place Ag | AgCl(s), KCl (0.2 M) || KBr (0.001 M), AgBr(s) | Ag The solubility product of AgCl and AgBr are 2.8 × 1010 and 3.3 × 1013 respectively. Calculate the e.m.f. of the following cell at 250C H2(1 atm) | 0.5 M HCOOH || 1 M CH3COOH | H2 (1 atm) The K a for HCOOH and CH 3 COOH are 1.77 × 10–4 and 1.80 × 10–5 respectively. The following Galvanic cell Zn | Zn++ (1 M, 100 mL) || Cu++ (1 M, 100 mL) | Cu was operated as an electrolytic cell as; Cu as anode and Zn as cathode. 0.48 A current was passed for 10 hours and then the cell was allowed to function as galvanic cell. Calculate e.m.f. of the cell at 250C, assuming that only electrode reactions occuring were those involving Cu/Cu++ and Zn/Zn++. 6. 7. reaction, 2H2O + 2e— H2 + 2OH— is – 0.8277 V. Calculate the equilibrium constant for the reaction 2H2O 8. 9. 10. An aqueous solution of NaCl on electrolysis gives H2(g), Cl2(g) and NaOH according to the reaction : 2Cl–(aq) + 2H2O = 2OH— (aq) + H2(g) + Cl2(g). A direct current of 25 amperes with a current effeciency of 62% is passed through 20 litres of NaCl solution (20% by weight). Write down the reactions taking place at the anode and the cathode. How long will it take to produce 1Kg of Cl2 ? What will be the molarity of the solution with respect to hydroxide ion ? (Assume no loss due to evaporation) Einstein Classes, H3O+ + OH— at 250C. Find the solubility product of a saturated solution of Ag2CrO4 in water at 298 K if the emf of the cell Ag/Ag+ (satd. Ag2CrO4 soln.) // Ag+ (0.1 M)/Ag is 0.164 V at 298 K. The standard reduction potential for Cu2+ / Cu is +0.34 V. Calculate the reduction potential at pH = 14 for the above couple. Ksp of Cu(OH)2 is 1.0 × 10–19. An excess of liquid mercury is added to an acidified solution of 1.0 × 10–3 M Fe3+. It is found that 5% of Fe3+ remains at equilibrium at 250C. 0 Calculate E Hg 2 / Hg , assuming that the only reaction that occurs is 2Hg + 2Fe3+ Hg22+ + 2Fe2+ E 0Cu / Cu 0.34V, E 0Zn / Zn 0.76V, Zn 65.5 g mol 1 . 5. An acidic solution of Cu2+ salt containing 0.4 g of Cu 2+ is electrolysed until all the copper is deposited. The electrolysis is continued for seven more minutes with the volume of solution kept at 100 ml and the current at 1.2 amp. Calculate the volume of gases evolved at NTP during the entire electrolysis. (At. wt. Cu = 63.6) The standard reduction potential at 250C of the [ Given E 0 Fe3 / Fe 2 0.77V] 11. 12. The standard reduction potential of the Ag+/Ag electrode at 298 K is 0.799 V. Given that for AgI, Ksp = 8.7 × 10 –17, evaluate the potential of the Ag+/Ag electrode in a saturated solution of AgI. Also calculate the standard reduction potential of the I—/AgI/Ag electrode. The E.M.F. of the following cell at 298 K is 1.0495V. Pt/H2(1 atm) |LiOH (0.01 mol dm–3)| |LiCl (0.01) mol/dms)| AgCl (s) / Ag. Determine the ionic product of water. Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111 CE – 18 ANSWERS SUBJECTIVE (INITIAL STEP EXERCISE) 1. 208.7 mL 2. 0.8026 amp. 4. 6. 6. E0cell = – 1.10 V Ecell = 0.4575 V (i) E0cell = 0.40 V (iii) Ecell = 0.4295 V 7. E Zn / Zn 2 = – .79 volt 9. 11. [Ag+] = 1.47 × 109M (b) 120 c (c) 3. (i) (ii) 6.2 litre 0.823 litre (ii) Cd(s) | Cd2+ is negative electrone 8. (a) (b) (c) n=4 (d) 10. .036 gm 0 Ni 2 / Ni – 195.895 kJ mol–1 H298k = – 167.7 kJ mol–1 S298 = – 95.0 Jk–1 mol–1 5 hr 21 min. 12. 2.89 × 10–4 cm 14. 16. 18. 20. 212.3 kJ 15. 3.103 V, Ecell will increase to 3.02 V 1.69 17. 1.02 × 104 cm2 19.06 gm 19. 0.20, 8360.4 mL (i) Zn + Cu2+ Zn2+ + Cu (ii) 1.113 V (iii) spontaneous 705.801 kJ 22. 1.035 × 1021, yes 5.08 24. 0.3105 V, 2.95 × 10–12 M 0.1589 gm, 56 mL, 0.31218 amperes 26. 55.97, 2 0.131 A, 20 gm 28. 110.88 gm mol–1 –2.171 × 102 kJ Pt, SO2|H2SO4|| HCl | Cl2, Pt, 40.2 days 31. [Ni2+] = 5.6 × 10–18 M (a) [Ag+] = 1.477 × 10–9 M (b) 115800 coulomb, 347.4 kJ 0.01 V 21. 23. 25. 27. 29. 30. 32. 33. 13. E = .22 V ANSWERS SUBJECTIVE (FINAL STEP EXERCISE) 1. 2. 3. 5. 6. 8. 10. SnCl2 : SnCl4 = 18.26 : 0.26 (s) Ag | AgBr(s), kBr || KCl, AgCl(s) | Ag(s), Ecell = –0.337 V –.0204 V 4. 1.137 – – 2Cl Cl2 + 2e (Anode) 2e– + 2H2O 2OH– + H2 (cathode), 48.7 hr, 1.408 mol/lit 158.15 mL 7. 9.35 × 10–15 2.287 × 10–12 9. –0.22 V 0.792 V 11. E Ag / Ag 0.325 V E I / AgI / Ag 0.1485 V 12. 10–14 Einstein Classes, Unit No. 102, 103, Vardhman Ring Road Plaza, Vikas Puri Extn., Outer Ring Road New Delhi – 110 018, Ph. : 9312629035, 8527112111