Engineering Chemistry

Electrochemistry Viva Questions

Electrochemistry Viva Questions

Electrochemistry Viva Questions, Viva Questions on Electrochemistry, Short answer questions on Electrochemistry, Engineering Chemistry Viva Questions, Engineering Viva Questions, Electrochemistry Viva Questions

Polymer Viva Questions

Short Answer Questions

Q.1. How can you test whether the given electrolyte is a strong electrolyte or a weak electrolyte?

Ans. If the aqueous solution of the electrolyte conducts electricity to a large extent, it is a strong electrolyte and if to a small extent, it is a weak electrolyte

Q.2. Define specific conductivity and mention its units.

Ans. The specific conductance of a solution is defined as the conductance of 1 cm3 of the solution of the electrolyte. Its unit is ohm−1 cm−1 (or Ω−1 cm−1) (or S cm−1).

Q.3. What is the effect of temperature on the electrical conduction of (i) metallic conductor (ii) electrolytic conductor?

Ans. With the increase of temperature, the electrical conduction of metals decreases whereas that of electrolytes increases.

Q.4. Define molar conductance and give its units.

Ans. It is defined as the conductance of all the ions produced by dissolving 1 mole of the electrolyte in V cm3 of the solution. Its unit is ohm−1 cm2 mol−1 (or S cm2 mol−1).

Q.5. Why do electrochemical cells stop working after some time?

Ans. Electrochemical cells produce electrical energy at the cost of chemical energy as a spontaneous redox reaction takes place in them. When the redox reaction is completed, the cell stops working.

Q.6. Give the relationship between molar conductivity and specific conductivity.

Ans.

\Lambda _{m}=K\times \frac{1000}{C}

Λm = Molar conductivity

K = Specific conductivity

C = Molar concentration

Q.7. What is the relationship between specific conductance and equivalent conductance?

Ans. \Lambda _{m}=K\times V

Λeq = Equivalent conductivity

K = Specific conductance

V = Volume of the solution containing 1g eq of the substance

Q.8. Give the relationship between equivalent and molar conductance.

Ans.

\frac{\Lambda _{m}}{\Lambda _{eq}}=\frac{Normality}{Molarity}

Q.9. What is a cell constant? Give its units.

Ans. it is the ratio of the distance between the parallel plates of the cell and the area of electrolyte.

Cell constant = l/a.

Its unit is cm−1.

Q.10. How is cell constant calculated from conductance values.

Ans. cell constant=\frac{specific conductance}{observed conductance}

Q.11. Why voltmeter cannot be used for the precise measurement of EMF of the galvanic cell.

Ans. Because a part of the cell current is drawn by the voltmeter itself, thereby giving lower value of EMF than the actual one.

Q.12. What flows in the internal circuit of galvanic cells?

Ans. Ions flow in the internal circuits of galvanic cells.

Q.13. What is the EMF of the cell when the cell reaction attains equilibrium?

Ans. At equilibrium, EMF of the cell is zero.

Q.14. Can we use a copper vessel to store 1 M AgNO3 solution, given that E°Cu2+/Cu = +0.34 V, E°Ag+/Ag = +0.80 V.

Ans. As the reduction potential of Ag+/ Ag electrode is higher than that of Cu2+/ Cu electrode. So Cu metal is incapable of displacing silver from silver nitrate solution. Thus, we can use a copper vessel to store 1 M AgNO3 solution.

Q.15. Why electrode potential of zinc is assigned a negative value; whereas that of copper a positive value.

Ans. Because Zn electrode is anodic w.r.t. S.H.E and Cu electrode is cathodic w.r.t. S.H.E.

Q.16. What is the relationship between the standard EMF of the cell and the equilibrium constant of the cell reaction at 298K?

Ans. E_{cell}^{o}=\frac{0.0591}{n}logKc

Where E_{cell}^{o} = standard EMF of the cell

Kc = Equilibrium constant

n = number of electrons involved in reactions

Q.17. What is the electrolyte used in a dry cell?

Ans. A paste of NH4Cl, MnO2 and C is used in dry cell.

Q.18. Why the blue colour of the solution gradually fades when CuSO4 solution is electrolyzed using platinum electrodes.

Ans. The blue color is due to the presence of Cu2+ ions. During electrolysis these ions are converted into metallic copper hence the blue color fades.

Q.19. Define Kohlrausch’s law?

Ans. At infinite dilution, equivalent conductance of an electrolyte is equal to the sum of ionic conductances of cations and anions at infinite dilution.

Q.20. Why D.C current is not used while determining the resistance of an electrolyte?

or

Why only A.C is used and not D.C source in conductometric estimation?

Ans. If conductometric estimation is carried out by using D.C then the products of electrolysis collect at the electrodes and set up a back emf, Which apparently increases the resistance of the electrolyte. So, such estimations are carried out by using AC and detecting the flow of current.

Q.21. What is the basic reason that a lead storage battery can be recharged?

Ans. PbSO4 is deposited on the electrodes. So, the electrode reactions can be reversed.

Q.22. Out of zinc and tin which are protects iron better even after cracks and why?

Ans. Zinc protects better because oxidation potential of zinc is greater but that of tin is less than that of iron.

Q.23. Why does the equivalent conductivity of a weak electrolyte increase with dilution?

Ans. Because degree of dissociation of weak electrolyte increases with dilution. As a consequence, the total number of ions present per gram equivalent also increases. Hence, equivalent conductivity of weak electrolyte increases with dilution.

Q.24. Why equivalent conductivity at infinite dilution for a weak electrolytes solution cannot be determined experimentally.

Ans. Equivalent conductivity for a weak electrolyte increases steadily with dilution, and the curve between Λeq and \sqrt{C} is not a straight line. The curve does not meet the equivalent conductivity axis. So, the value of Λ cannot be obtained by extrapolation. Hence Λ for a weak electrolyte is determined indirectly by using kohlrausch’s law.

Q.25. Why, with dilution, equivalent conductance increases but specific conductance decreases.

Ans. With increase in dilution, two things happen, namely degree of dissociation increases, but the total volume increases. The number of ions per cm3 decreases, because the effect of increased volume in decreasing the number of ions per cm3 outweighs the minor increase in conductance with increase of dilution. On the other hand, equivalent conductance increases, because the total number of ions per gram equivalent increases with dilution, due to increased degree of ionization.

Q.26. Why a dry cell becomes dead after a long time even if it has not been used?

Ans. This is because the acidic NH4Cl corrodes the zinc container even if it has not been used.

Q.27. What is the role of ZnCl2 in dry cell?

Ans. ZnCl2 combine with NH3 produces to form the complex salt [Zn(NH3)2Cl2] as otherwise the pressure developed due to NH3 would crack the seal of the cell.

Q.28. Which types of cells are rechargeable?

Ans. Those cells are rechargeable in which the products formed during discharge are deposited on the electrodes and these can be decomposed to give the original substances when electrical energy is supplies.

Q.29. The standard reduction potential values of three metallic cations X, Y, Z are 0.52, -3.03, -1.18 V respectively. What will be the order of reducing power of the corresponding metals?

Ans. The standard oxidation potential (equal and opposite in sign of standard reduction potential) of the metals X, Y, Z will be -0.52, 3.03, 1.18 V respectively. Higher the oxidation potential, more easily metal is oxidized and here greater is the reducing power. Hence, the reducing power will be in the order Y > Z > X.

Q.30. Why a cell stop working after some time?

Ans. With time, concentration of the electrolytic solutions change. Hence, their electrode potentials change. When the electrode potentials of two half cells become equal, the cell stops working.

Q.31. Write any two advantage of H2 – O2 fuel cell over ordinary cell.

Ans.

  • They do not cause pollution.
  • They have high efficiency of 60 – 70%.

Q.32. Write the Nearst’s equation for the electrode reaction:

M^{n+}(aq)+ne^{-}\rightarrow M(s)

Ans.

E_{M^{n+}/M}=E_{M^{n+}/M}^{o}+\frac{2.303RT}{nF}log[M^{n+}(aq)]

Q.33. Define standard hydrogen electrode.

Ans. Standard hydrogen electrode (SHE) or Normal hydrogen electrode (NHE) is a reference electrode that is obtained by dipping platinum foil in 1M HCl solution through which hydrogen gas is passed at 298 K under 1 atm pressure. Its electrode potential is zero.

Q.34. Glass electrode is preferred to quinhydrone electrode in measuring pH of a solution, Give reason.

Ans. Glass electrode is simple, not easily oxidized, and attains equilibrium rapidly. It can safely be used up to pH of 10. On the other hand, quinhydrone electrodes can be used up to pH of 8 only. It cannot be used in solutions containing a redox system. Hence glass electrode is preferred over a quinhydrone electrode in pH measurement of a solution.

Q.35. Alkaline dry cells are considered better than Lechanche cells why?

Ans. Alkaline dry cell lasts longer because zinc electrode does not corrode easily.

Q.36. What is the purpose of MnO2 in the dry cells?

Ans. It acts as an oxidizing agent in dry cells.

Q.37. Define fuel cell.

Ans. The fuel cell is a device that converts the energy of a fuel directly into electrical energy.

Q.38. What is a Lead-acid accumulator?

Ans. A secondary cell consisting of lead electrodes, the positive one covered with PbO2, dipping into H2SO4 solution. Its emf is about 2V.

Q.39. Write major applications of lithium-ion cells.

Ans. Lithium-ion cells are used in cell phones, laptops, electrical equipment, etc.

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