KTVR KNOWLEDGE PARK FOR ENGINEERING AND TECHNOLOGY COIMBATORE-19.

QUESTION BANK UNIT I ELECTROCHEMISTRY

1)

Define electrochemical cell. An electrochemical cell is a device which produces electrical energy at the

expense of chemical energy. Example: Daniel cell. 2) What are the conditions for an electrochemical cell to act as a standard cell? (i) The standard cell must be capable of giving constant and reproducible emf. (ii) Change in emf with temperature should be negligibly small. 3) Distinguish between reversible and irreversible cell giving one example

for each. S.No 1. Reversible cell Cell reaction is reversed when external potential greater than 2. cell potential is applied. Eg;Zn|Zn2+|| Ag+|Ag Irreversible cell The cell reaction is not completely reversed. Eg;Zn| H2SO4 |Cu

4)

What is electrode potential? How is it developed? It is the measure of tendency of a metallic electrode to lose or gain

electrons when it is in contact with a solution of its own salt. It is developed when a metal is placed in a solution of its own salt. 5) Explain the term single electrode potential. It is the measure of tendency of a metallic electrode to lose or gain electrons when it is in contact with a solution of its own salt. 6) How will you increase the value of reduction potential of an electrode? (i) By increasing the concentration of metal ions in contact with metal electrode. (ii) By increasing the temperature. 7) How an emf of a cell is measured? Using a voltmeter or a potentiometer, the emf of a cell is measured. In the potentiometric measurement, the emf of the unknown cell is obtained using a standard Weston cell as Ex = Es X Where, Ex = emf of unknown cell. Es = emf of standard Weston cell. AD = balancing length when unknown cell is connected. ADl = balancing length when standard cell is connected.

8) Represent a galvanic cell according to IUPAC convention. Daniel cell Zn | ZnSO4 || CuSO4 | Cu (1M) (1M) (Or) Zn | Zn2+ || Cu2+ | Cu (1M) (1M)

9) Why is a salt bridge used in the construction of a cell? A wire cannot be used to connect two electrodes internally in an electrochemical cell, because it produces a voltage drop. Consequently, accurate electric cell measurements cannot be done. In order to make accurate electric measurements in solution, a salt bridge is employed. Salt bridge completes the electrical circuit thereby permitting the ions to move from one solution to another without direct contact (or mixing) of the two solutions. Moreover, salt bridge maintains the electrical neutrality of the solutions in the two half cells (electrodes). 10) What is the composition of the salt bridge used between two half cells? Salt bridge is usually an inverted U tube containing a paste of KCl or NH4NO3 and agar agar. 11) Can we use nickel spatula to stir a solution of copper sulphate? No, Since the reduction potential of Ni2+ || Ni electrode is less than (- 0.25 V) that of Cu2+ | Cu electrode (+ 0.34) . Nickel will otherwise displace copper from copper sulphate solution as below--

Ni (s) + Cu2+

(aq)

Cu (s) + Ni 2+ (aq)

12) How will you predict the spontaneity of any redox system using emf? If the standard emf of the cell calculated is positive value, the cell reaction is spontaneous. It is well known that for a reaction to be spontaneous, the standard free energy change (GO) of the reaction must be negative. Since in a cell GO = - nFEo J It is evident that if E0 is positive, GO would be negative. Hence, the cell reaction would be spontaneous. 13) Define a reference electrode. Give an example. An electrode whose electrode potential is accurately known or whose electrode potential has been arbitrarily fixed is known as reference electrode. Example: Calomel electrode. 14) What is an ion-selective electrode? An electrode that generates a potential in response to the presence of specific ions in a solution is known as an ion-selective electrode. 15) Zinc reacts with dil.H2SO4 to give hydrogen but Ag does not. Explain. Given that EO (Ag+ ,Ag) = 0.80V; EO (Zn2+ ,Zn) = -0.76V Since, zinc has negative reduction potential with respect to hydrogen electrode, it liberates hydrogen from dil.H2SO4. But silver has positive reduction potential with respect to hydrogen electrode; it does not liberate hydrogen from dil.H2SO4.

16) What is electromotive (electrochemical) series? When various metals are arranged in the order of the increasing value of standard reduction potential on the hydrogen scale at 298 K and 1M concentration of the electrolyte, then the arrangement is called electrochemical series. 17) Write Nernst equation for M Mn+ + neE = EO RT ln [ Mn+] V nF E = EO 2.303RT log [ Mn+] V nF E = EO 0.591 log [ Mn+] V nF 18) Suggest a method to determine the electrode potential of zinc. Zinc electrode is connected with saturated calomel electrode and the emf of the cell is determined potentiometrically. Here, zinc acts as anode (LHS) and calomel electrode acts as cathode (RHS). The cell is Zn | ZnSO4 || calomel electrode ECell = = EZn = ERight - ELeft Ecalomel EZn Ecalomel EZn

Ecalomel is 0.2422V ; hence EZn can be determined.

19) Write the construction of saturated calomel electrode. Calomel electrode consists of a glass tube containing mercury at the bottom over which mercurous chloride is placed. The remaining portion of the tube is filled with a saturated solution of KCI.The bottom of the tube is sealed with a platinum wire. The side tube is used for making electrical contact with a salt bridge. 20) Mention the applications of Nernst equation. i) Nernst equation can be used to calculate the electrode potential of the

unknown metal. ii) Corrosion behavior of metals can be predicted with the help of Nernst equation. 21) Why is SHE not generally used in PH measurement? 1) It is difficult to set up and transport. 2) It requires H2 gas at 1atm. Pressure continuously. 3) The solution poisons the surface of Pt electrode. 4) The electrode is readily affected by oxidizing agents many metal ions. 5) It cannot be used in redox system. 22) What are the advantages of conductometric titration? 1) Indicators are not required. 2) It gives more accurate end point. 3) Since the end point is detected graphically, no extra care is required near the end point. 4) It is used for analysis of solutions of all concentration, poisons, coloured solution and weak acid.

23) Why can glass electrode not be used for a solution of high alkalinity? At high alkalinity (PH above 12), cations of the solution affect the glass and make the glass electrode useless. 24) Give the significance of electrochemical series. i) Standard emf of the cell can be calculated. ii) Change in free energy and equilibrium constant can be calculated. iii) Behaviour of metals (corrosion)can be studied. iv) Spontaneity of the cell can be predicted. 25) Calculate the reduction potential of Cu2+ | Cu = 0.5M at 250C. E0 Cu2+ | Cu = 0.337V. Given: Concentration of [Cu2+] = 0.5M, E0 Cu2+ | Cu = 0.337V Nernst equation for reduction potential of Cu2+ is E = E0 + 0.0591 log [Cu2+] n E = 0.337 + 0.0591 log [0.5] 2 = 0.337 + 0.02955 (- 0.3010) = 0.337 0.0089 The reduction potential of Cu2+ | Cu = 0.328V