NCERT Exemplar Problems Class 12 Chemistry Chapter 3 Electrochemistry
Multiple Choice Questions
Single Correct Answer Type
Question 1. Which cell will measure standard electrode potential of copper electrode?
Solution: (c) When copper electrode is connected to standard hydrogen electrode, it acts as cathode and its standard electrode potential can be measured.
To calculate the standard electrode potential of the given cell it is coupled with the standard hydrogen electrode in which pressure of hydrogen gas is one bar and the cone, of H+ ion in the solution is one molar and also the concentrations of the oxidized and the reduced forms of the species in the right hand half-cell are unity.
Question 2. Electrode potential for Mg electrode varies according to the equation
Question 3. Which of the following statements is correct?
Solution: (c)Ecell is an intensive property as it does not depend upon mass of species (number of particles) but ArG of the cell reaction is an extensive property because this depends upon mass of species (number of particles).
Question 4. The difference between the electrode potentials of two electrodes when no
current is drawn through the cell is called
(a) cell potential (b) cell emf
(c) potential difference (d) cell voltage
Solution: (b) EMF is the difference between the electrode potentials of two electrodes • when no current is drawn through the cell.
Question 5. Which of the following statements is not correct about an inert electrode in a cell?
(a) It does not participate in the cell reaction.
(b) It provides surface either for oxidation or for reduction reaction.
(c) It provides surface for conduction of electrons.
(d) It provides surface for redox reaction.
Solution: (d) Inert electrode does not participate in redox reaction and acts only as source or sink for electrons. It provides surface either for oxidation or for reduction reaction.
Question 6. An electrochemical cell can behave like an electrolytic cell when
Solution: (c) If an external opposite potential is applied on the galvanic cell and increased reaction continues to take place till the opposing voltage reaches the value 1.1 V.
At this stage no current flow through the cell and if there is any further increase in the external potential then reaction starts functioning in opposite direction.
Hence, this works as an electrolytic cell.
Question 7. Which of the following statement about solutions of electrolytes is not correct?
(a) Conductivity of solution depends upon size of ions.
(b) Conductivity depends upon viscosity of solution.
(c) Conductivity does not depend upon salvation of ions present in solution.
(d) Conductivity of solution increases with temperature.
Solution: (c) Conductivity depends upon salvation of ions present in the solution. The greater the salvation of ions, the lesser is the conductivity.
Question 8.Using given below find strongest reduction agent.
Solution: (b) A negative value of standard reduction potential for Cr3+ to Cr means that the redox couple is a stronger reducing agent.
Question 9. Use the data given in Q. 8 and find out which of the following is the strongest oxidizing agent.
Solution: (c) The higher the positive value of standard reduction potential of metal ion, the higher will be its oxidizing capacity.
has value equal to 1.51 V hence it is the strongest oxidizing agent.
Question 10. Using the data given in Q. 8, find out in which option the order of reducing power is correct.
Solution: (b) The lower the reduction potential, the higher is the reducing power.
The order of reducing power is
Question 11. Use the data given in Q. 8 and find out the most stable ion in its reduced form.
Solution: (d) Mn+2 is most stale in its reduced form due to highest E° value.
Question 12. Use the data given in Q.8 and find out the most stable oxidized species.
Question 13. The quantity of charge required to obtain one mole of aluminium from
Al2O3 is (a) IF (b) 6F (c) 3F (d) 2F
Question 14. The cell constant of a conductivity cell
(a) changes with change of electrolyte
(b) changes with change, of concentration of electrolyte
(c) changes with temperature of electrolyte
(d) remains constant for a cell
Solution: (d) The cell constant of a conductivity cell (a) remains constant for a cell.
Question 15. While charging the lead storage battery
(a)PbSO4 anode is reduced to Pb
(b)PbSO4 cathode is reduced to Pb
(c)PbSO4 cathode is oxidized to Pb
(d)PbSO4 anode is oxidized to Pb02
Solution: (a) While charging the lead storage battery the reaction occurring on cell is reversed and PbSO4(s) on anode and cathode is converted into Pb and Pb02 respectively.
Hence, option (a) is the correct choice The electrode reactions are as follows:
Question 17. In the electrolysis of aqueous sodium chloride solution, which of the half-cell reaction will occur at anode?
Solution: (d) During electrolysis of aqueous
(ii)The reaction at anode with lower value of E° should be preferred, but oxidation of 02 is kinetically slow process and needs high voltage thus reaction (i) takes place.
More than One Correct Answer Type
Question 18. The positive value of the standard electrode potential of Cu+2/Cu indicates
(a) this redox couple is a stronger reduction agent than the H/H2 couple
(b) this redox couple is a stronger oxidizing agent than H+/H2
(c) Cu can displace H2 from acid
(d) Cu cannot displace H2 from acid
Solution: (b, d) The Lesser the E° value of redox couple, the higher the reducing power ’.
Since, 2 H+/H2 has lesser SRP than Cu+2/Cu redox couple. Therefore,
(i) This redox couple is a stronger oxidizing agent than H+/H2
(ii) Cu cannot displace H2 from acid.
Hence, (b) and (d) are correct.
Question 19. E°ell for some half-cell reactions are given below. On the basis of these marks the correct answer will be
(a) In dilute sulphuric acid solution, hydrogen will be reduced at cathode.
(b) In concentrated sulphuric acid solution, water will be oxidized at anode.
(c) In dilute sulphuric acid solution, water will be oxidized at anode.
(d) In dilute sulphuric acid solution, SO2-ion will be oxidized to tetrathionate ion at anode.
Solution: (a, c) In dilute sulphuric acid solution, hydrogen will be reduced at cathode.
while in concentrated solution of sulphuric acid, SO-2 ions are oxidized to tetrathionate (SO2) ions.
Question 20. E°en = 1.1 V for Daniell cell. Which of the following expressions are correct description of state of equilibrium in this cell?
Question 21. Conductivity of an electrolytic solution depends on
(b) concentration of electrolyte
(d) distance between the electrodes
Solution: (a, b) Conductivity of electrolyte solution is due to presence of mobile ions in the solution. This type of conductance is known as ionic conductance. Conductivity of these type of solutions depend upon
(i) the nature of electrolyte added
(ii) size of the ion produced and their solvation
(iii) concentration of electrolyte
(iv) nature of solvent and its viscosity
While power of source or distance between electrodes has no effect on conductivity of electrolyte solution.
Hence, options (a) and (b) are the correct choices.
However, the sum of molar conductivities of constituent ions gives the molar conductivity of water but here NH4OH is a weak electrolyte of which complete decomposition is not possible.
Question 23. What will happen during the electrolysis of aqueous solution of CuSO4 by using platinum electrodes?
(a) Copper will deposit at cathode.
(b) Copper will deposit at anode.
(c) Oxygen will be released at anode.
(d) Copper will dissolve at anode.
The reaction with lower value of E° will be preferred at anode, hence 02 is released at anode.
Question 24. What will happen during the electrolysis of aqueous solution of CuSO4 in the presence of Cu electrodes?
(a) Copper will deposit at cathode.
(b) Copper will dissolve at anode.
(c) Oxygen will be released at anode.
(d) Copper will deposit at anode.
Solution: (a, b) Electrolysis of CuSO4 can be represented by two half-cell reactions these occurring at cathode and anode respectively as
Here, Cu will deposit at cathode while copper will dissolved at anode. Hence, options (a) and (b) are the correct choices.
Question 25. Conductivity K, is equal to
Question.26. Molar conductivity of ionic solution depends on .
(b) distance between electrodes
(c) concentration of electrolytes in solution
(d) surface area of electrodes
Solution: (a, c) Molar conductivity of ionic solution depends on temperature and concentration of electrolytes in solution.
Question 27. For the given cell, Mg|Mg2+||Cu2+ || Cu
(a) Mg is cathode
(b) Cu is cathode
(c) The cell reaction Mg + Cu2++ —»Mg2+ + Cu
(d) Cu is the oxidizing agent
Solution: (b, c) Left side of cell reaction represents oxidation half-cell i.e., oxidation of Mg and right side of cell represents reduction half-cell reactions i.e., reduction of copper.
(ii) Cu is reduced and reduction occurs at cathode.
(iii) Mg is oxidized and oxidation occurs at anode.
(iv) Whole cell reaction can be written as
Hence, options (b) and (c) both are correct choices.
Short Answer Type Questions
Question 28. Can absolute electrode potential of an electrode be measured?
Solution: No, absolute electrode potential of an electrode cannot be measured.
Question 29. Can
for cell reaction ever be equal to zero?
Question 30. Under what condition is
Solution: At equilibrium i.e., when the cell is completely discharged,
Question 31. What does the negative sign in the expression
mean? Zn /Zn
Question 32. Aqueous copper sulphate solution and aqueous silver nitrate solution are electrolysed by 1 ampere current for 10 minutes in separate electrolytic cells. Will the mass of copper and silver deposited on the cathode be same or different? Explain your answer.
Mass of silver will be different because the equivalent mass of Ag is different.
Question 33. Depict the galvanic cell in which the cell reaction is Cu + 2Ag+ > 2Ag+Cu2
Solution: Cu | Cu2+ (aq, 1M) || Ag+ (aq, 1M) | Ag
Question 34. Value of standard electrode potential for the oxidation of Cl ions is more positive than that of water, even then in the electrolysis of aqueous sodium chloride, why is Cl- oxidized at anode instead of water?
Solution: Under the conditions of electrolysis of aqueous sodium chloride, oxidation of water at anode requires over potential and therefore, Cu is oxidised instead of water.
Question 35. What is electrode potential?
Solution: The electrical potential difference set up between the metal and its solution is called electrode potential.
Question 36. Consider the following diagram in which an electrochemical cell is coupled to an electrolytic cell. What will be the polarity of electrodes ‘A’ and ‘B’ in the electrolytic cell?
Solution: ‘A’ will have -ve polarity and ‘B’ will have +ve polarity.
Question 37. Why is alternating current used for measuring resistance of an electrolytic solution?
Solution: The alternating current is used to prevent electrolysis so that the concentration of ion in the solution remains constant.
Question 38. A galvanic cell has electrical potential of 1.1 V. If an opposing potential of 1.1 V is applied to this cell, what will happen to the cell reaction and current flowing through the cell?
Solution: When the opposing potential becomes equal to electrical potential, the cell reaction stops and no current flows through the cell. Thus, there is no chemical reaction.
Question 39. How will the pH of brine (aq. NaCl solution) be affected when it is electrolysed?
Solution: Aqueous solution of brine contains Na+, CP, H+ and H–. Electrode process are given as follows:
Remaining solution will contain NaOH, which is base, therefore, pH will increase.
Question 40. Unlike dry cell, the mercury cell has a constant cell potential throughout its useful life. Why?
Solution: Electrolyte is not consumed in the cell process of mercury cell hence it will deliver the current at constant potential throughout its life.
Question 41. Solutions of two electrolytes ‘A’ and ‘B’ are diluted. The Am of ‘B’ increases 1.5 times while that of A increases 25 times. Which of the two is a strong electrolyte? Justify your answer.
Solution: Electrolyte ‘B’ will be strong electrolyte because it is completely ionised and on dilution the molar conductance will increase to the small extent due to increase in speed of ions only.
Question 42. When acidulated water (dil. H2SO4 solution) is electrolysed, will the pH of the solution be affected? Justify your answer.
Solution: The pH of solution will not be affected because there is no change in the concentration of H+ions.
Question 43. In an aqueous solution, how does specific conductivity of electrolytes change with addition of water?
Solution: On the addition of water, number of ions per unit volume decreases and therefore conductivity decreases.
Question 44. Which reference electrode is used to measure the electrode potential of other electrodes?
Solution: The standard hydrogen electrode is used as a reference electrode whose electrode potential is taken to be. zero. The electrode potential of other electrodes is measured with respect to it.
Question 45. Consider a cell given below:
Cu |Cu2+ || Cl |Cl2, Pt
Write the reactions that occur at anode and cathode.
Solution: The given cell is:
Question 46. Write the Nemst equation for the cell reaction in the Daniell cell. How will the Ecell be affected when concentration of Zn2+ ions is increased?
Solution: Daniell cell
Zn(s) | Zn2+ || Cu2+ | Cu(s)
Above equation shows that the cell potential will decrease with increase in the concentration of Zn2+ ion.
Question 47. What advantage do the fuel cells have over primary and secondary batteries?
Solution: Primary batteries contain a limited amount of reactants and are discharged when the reactants have been consumed. Secondary batteries can be recharged but take a long time to recharge. Fuel cell runs continuously as long as the reactants are supplied to it and products are removed continuously.
Question 48. Write the cell reaction of a lead storage battery when it is discharged. How does the density of the electrolyte change when the battery is discharged?
Density of electrolyte decreases because water is formed and sulphuric acid consumed as the product during discharge of the battery.
Question 49. Why on dilution the Am of CH3COOH increases drastically, while that of
CH4COONa increases gradually?
Solution: In the case of CH3COOH, which is a weak electrolyte, the number of ions increase on dilution due to an increase in degree of dissociation.
In the case of strong electrolyte such as CH3COONa, the number of ions remains the same but the interionic attraction decreases.
Matching Column Type Questions
Question 50. Match the terms given in Column 1 with the units given in Column II.
Solution: (i —» c), (ii —> d), (iii —> a), (iv —> b)
Question 51. Match the terms given in Column I with the items given in Column II.
Solution: (i -> d), (ii -> a), (iii -> b), (iv -> c)
Question 52. Match the items of Column I and Column II.
Solution: (i —> d), (ii -> c) (iii -> a), (iv -> b)
Question 53.Match the items of Column I and Column II.
Solution:(i -> d), (ii -> c), (iii -> b), (iv -> a)
Question 54.Match the items of Column I and Column II.
Solution: (i -> d), (ii -> c), (iii —> a, e), (iv -> b)
Question 55. Match the items of Column I and Column II on the basis of data given below:
Assertion and Reason Type Questions
In the following questions, a statement of Assertion (A) followed by a statement of Reason (R) is given. Choose the correct option out of the following choices:
(a) Both Assertion and Reason are true and the Reason is the correct explanation for Assertion.
(b) Both Assertion and Reason are true and the Reason is not correct explanation for Assertion
(c) Assertion is true but the Reason is false.
(d) Both Assertion and Reason are false.
(e) Assertion is false but Reason is true.
Question 56. Assertion (A): Cu is less reactive than hydrogen.
Reason (R): E°u2+/Cu is negative.
Solution: (c) Cu is less reactive than hydrogen because E°u2+ Cu is positive.
Question 57. Assertion (A): Ecell should have a positive value for the cell to function. Reason (R). Ecadlode Eanode
Solution: (c) Ecell = Ecathode – Eanode. To have positive value of Ecell, Ecathode > Eanode.
Question 58. Assertion (A): Conductivity of all electrolytes decreases on dilution.
Reason (R): On dilution number of ions per unit volume decreases.
Solution: (a) Conductivity depends on number of ions per unit volume which decreases on dilution of electrolytes.
Question 59. Assertion (A): Lm for weak electrolytes shows a sharp increase when the electrolytic solution on dilution of solution.
Reason (R): For weak electrolytes dissociate partially in concentrated solution. On dilution, their degree of dissociation increases hence their Am increases sharply.
Solution: (a) Weak electrolytes dissociate partially in concentrated solution. On dilution, their degree of dissociation increases hence their Am increases sharply.
Question 60. Assertion (A): Mercury cell does not give steady potential.
Reason (R): In the cell reaction, ions are not involved in solution.
Solution: (e) Mercury cell gives a steady potential because in the cell reaction ions are not involved in the solution.
Question 61. Assertion (A): Electrolysis of NaCl solution gives chlorine at anode instead of 02.
Reason (R): Formation of oxygen at anode requires overvoltage.
Solution: (a) Formation of oxygen has lower value of E° than formation of chlorine even then it is not formed because it requires overvoltage.
Question 62. Assertion (A): For measuring resistance of an ionic solution an AC source is used.
Reason (R): Concentration of ionic solution will change if DC source is used.
Solution: (a) Alternating current is used in the measurement of resistance of electrolyte solution because concentration changes with DC current due to electrolysis.
Question 63. Assertion (A): Current stops flowing when Ecell = 0.
Reason (R): Equilibrium of the cell reaction is attained.
Solution: (a) At equilibrium Ecell = 0 and no current flows.
Question 64. Assertion (A): E , increases with increase in concentration of Ag+ ions.
Ag / Ag+
Reason (R): E + has a positive value.
On increasing [Ag+],EA + will increase and it has a positive value.
Question 65. Assertion (A): Copper sulphate can be stored in zinc vessel.
Reason (R): Zinc is less reactive than copper.
Solution: (d) Zinc will get dissolved in CuS04 solution since zinc is more reactive than copper.
Long Answer Type Questions
Question 66. Consider the following figure and answer the following questions.
(i) Cell ‘A’ has Ecell = 2 V and cell ‘B’ has Ecen = 1.1 V which of the two cells ‘A’ or ‘B’ will act as an electrolytic cell. Which electrode reactions will occur in this cell?
(ii) If cell ‘A’ has Ecen = 0.5 V and cell ‘B’ has Ecen = 1.1 V, what will be the reactions at anode and cathode?
Solution: (i) Cell ‘B’ will act as electrolytic cell because potential of ‘B’ is less than
that of ‘A’. Electrode process in the cell ‘B’ may be given as
Question 67. Consider the figure given below and answer the questions (i) to (vi) that follow.
(i) Redraw the diagram to show the direction of electron flow.
(ii) Is silver plate anode or cathode?
(iii)What will happen if salt bridge is removed?
(iv)When will the cell stop functioning?
(v)How will concentration of Zn+2 cell functions?
(vi)How will the concentration of Zn+2 ions and Ag+ ions be affected after the cell becomes ‘dead’?
Solution:(i)The cell is:
Zn(s) | Zn+2 || Ag+ | Ag
(ii)Electron will flow from zinc anode to silver cathode in external circuit. Silver will act as cathode, since its standard reduction potential is greater than that of zinc.
(iii)Potential will drop to zero if salt bridge is suddenly removed.
(iv)Cell will stop functioning when it is discharged i.e., when cell potential is zero.
(v)Nemst equation for the cell is: 0.059,
Cell potential will decrease with increase in concentration of [Zn+2] while it will increase with the concentration of [Ag+].
(vi) When cell is dead or discharged, E will be zero and the cell will be at equilibrium. Then, concentration of Zn+2 and Ag+ will not change.
Question 68. What is the relationship between Gibbs free energy of the cell reaction in a galvanic cell and the emf of the cell? When will the maximum work be obtained from a galvanic cell?
Solution: The required relations are: