NCERT Exemplar: Electrochemistry- 2 - JEE PDF Download

SHORT ANSWER TYPE QUESTIONS

Q.28. Can absolute electrode potential of an electrode be measured?
Ans. No, only the difference in potential between two electrodes can be measured. This is due to the reason that oxidation or reduction cannot occur alone. So, when we measure electrode potential of any electrode we have to take a reference electrode.

Q.29. Can E°_Cell V or ∆_rG° for cell reaction ever be equal to zero?
Ans. No, otherwise the reaction becomes non-feasible.
The reaction is feasible only at E°_cell = positive or ∆_rG° = negative.
When E° = ∆_rG° = 0 the reaction reaches at equilibrium.

Q.30. Under what condition is E°_Cell = 0 or ∆_rG° = 0?
Ans. At the stage of chemical equilibrium in the cell we get E°_cell = 0.
E°_cell = 0
∆_rG° = -n F E°_cell
= −n x F x 0 = 0

Q.31. What does the negative sign in the expression E°_Zn²⁺_/Zn = − 0.76 V mean?
Ans. Greater the negative reactivity of standard reduction potential of metal greater is its reactivity. It means that Zn is more reactive than hydrogen. When zinc electrode will be connected to standard hydrogen electrode, Zn will get oxidised and H⁺ will get reduced.
Thus, zinc electrode will be the anode of the cell and hydrogen electrode will be the cathode of the cell.

Q.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.
Ans. Different masses of Cu and Ag will be deposited at cathode. According to Faraday's second law of electrolysis amount of different substances liberated by same quantity of electricity passes through electrolyte solution is directly proportional to their chemical equivalent weight.

Where, E₁ and E₂ have different values depending upon number of electrons required to reduce the metal ion. Thus, masses of Cu and Ag deposited will be different.

Q.33. Depict the galvanic cell in which the cell reaction is Cu + 2Ag⁺ → 2Ag + Cu²⁺
Ans. In a galvanic cell, oxidation half reaction is written on left hand side and reduction half reaction is on right hand side. Salt bridge is represented by parallel lines Cu | Cu²⁺ ||Ag⁺ | Ag.

Q.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^– oxidised at anode instead of water?
Ans. Under the condition of electrolysis of aqueous sodium chloride, oxidation of water at anode requires over potential. So, CI⁻ is oxidized at anode instead of water.
Possible oxidation half cell reactions occurring at anode are:
Cl^- (aq) →Cl₂ (g)+ e^- ;   E°_cell = 1.36V
2H₂ O(l) → O₂ (g) 4H⁺ (aq) 4e^- ; E°_cell = 1.23V
Species having lower E°_cell cell undergo oxidation first than the higher value but oxidation of H₂O to O₂ is kinetically so slow that it needs some overpotential.

Q.35. What is electrode potential?
Ans. The potential difference between the electrode and the electrolyte in an electrochemical cell is called electrode potential.

Q.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?
Ans. The cell drawn above represents electrochemical cell in which two different electrodes are fitted in their respective electrolytic solutions and cell drawn at bottom represents electrolytic cell. Cell representation can be represented as Zn | Zn²⁺ || Cu²⁺|Cu.
Zn is losing electrons which are going towards electrode (A) and copper is accepting electron from electrode B. Hence,
Charge on electrode A = positive (+)
Charge on electrode B = negative (−)

Q.37. Why is alternating current used for measuring resistance of an electrolytic solution?
Ans. Alternating current is used in electrolysis so that concentration of ions in the solution remains constant and exact value of resistance is measured. 

Q.38. A galvanic cell has electrical potential of 1.1V. If an opposing potential of 1.1V is applied to this cell, what will happen to the cell reaction and current flowing through the cell?
Ans. When an opposing potential of 1.1V is applied to a galvanic cell having electrical potential of 1.1V then cell reaction stops completely and no current will flow through the cell.

Q.39. How will the pH of brine (aq. NaCl solution) be affected when it is electrolysed?
Ans. The pH of the solution will rise as NaOH is formed in the electrolytic cell.
Chemical reactions occurring at cell when aqueous brine solution is electrolysed are as follows:





Q.40. Unlike dry cell, the mercury cell has a constant cell potential throughout its useful life. Why?
Ans. Life time of any cell depends upon ions present in cell. Ions are not involved in the overall cell reaction of mercury cell. Hence, mercury cell has a constant cell potential throughout its useful life.

Q.41. Solutions of two electrolytes ‘A’ and ‘B’ are diluted. The Λ_m of ‘B’ increases 1.5 times while that of A increases 25 times. Which of the two is a strong electrolyte? Justify your answer.
Ans. Strong electrolytes dissociate almost completely even on high concentration. Therefore, concentration of such solutions remains almost same on dilution. Electrolyte 'B' is stronger than 'A' because in 'B' the number of ions remains the same on dilution, but only interionic attraction decreases.
Therefore, ∧_m increases only 1.5 times, while in case of weak electrolyte on dilution, number of constituent ions increases.

Q.42. When acidulated water (dil.H₂SO₄ solution) is electrolysed, will the pH of the solution be affected? Justify your answer.
Ans. Since pH of solution depends upon concentration of H⁺present in solutions, pH of the solution will not be affected as [H⁺] remains constant.
At anode 2H₂O → O₂+ 4H⁺e^-
At cathode 4H⁺ + 4e⁻ → 2H₂

Q.43. In an aqueous solution how does specific conductivity of electrolytes change with addition of water?
Ans. Conductivity of solution due to total ions present in per unit volume of solution is known as specific conductivity. Specific conductivity decreases due to decrease in the number of ions per unit volume. We add water to aqueous solution, number of ions present in per unit volume decreases.

Q.44. Which reference electrode is used to measure the electrode potential of other electrodes?
Ans. Standard hydrogen electrode (SHE) is the reference electrode whose electrode potential is taken to be zero. The electrode potential of other electrodes are measured with respect to it.

Q.45. Consider a cell given below
Cu|Cu²⁺|| Cl^-|Cl₂,Pt
Write the reactions that occur at anode and cathode
Ans. Cell reaction represented in the question is composed of two half cell reactions. These reactions are as follows
At anode Cu → Cu²⁺ + 2e⁻
At cathode Cl₂ + 2e⁻ → 2Cl⁻
Copper is getting oxidised at anode and Cl₂ is getting reduced at cathode.

Q.46. Write the Nernst equation for the cell reaction in the Daniel cell. How will the E_Cell be affected when concentration of Zn²⁺ ions is increased?
Ans. Zn + Cu² → Zn²⁺ Cu


According to this equation
E°_cell is directly dependent on concentration of Cu²⁺ and inversely dependent upon concentration of Zn²⁺ ions.
E_cell decreases when concentration of Zn²⁺ ions is increased.

Q.47. What advantage do the fuel cells have over primary and secondary batteries?
Ans. Primary batteries contain a limited amount of reactants and are discharged when the reactants have been consumed. Secondary batteries can be recharged but it takes a long time. Fuel cell runs continuously as long as the reactants are supplied to it and products are removed continuously.

Q.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?
Ans. When a lead storage battery is discharged then the following cell reaction takes place
Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
Density of electrolyte depends upon number of constituent ions present in per unit volume of electrolyte solution in this case density of electrolyte decreases as water is formed and sulphuric acid is consumed as the product during discharge of the battery.

Q.49. Why on dilution the Λ_m of CH₃COOH increases drastically, while that of CH₃COONa increases gradually?
Ans. In the case of CH3COOH, which is a weak electrolyte, the number of ions increases on dilution due to an increase in degree of dissociation.
CH₃COOH + H₂O ⇌ CH₃COO^- + H₃O⁺
In case of strong electrolyte, the number of ions remains the same but the interionic attraction decreases.

MATCHING TYPE

Note : Match the items of Column I and Column II in the following questions. 
Q.50. Match the terms given in Column I with the units given in Column II.

Ans. (i) → c, (ii) → d, (iii) → a, (iv)→ b
Solution.

Q.51. Match the terms given in Column I with the items given in Column II.

Ans. (i) → d, (ii) → a, (iii)→ b, (iv) → c
Solution.
(i) Λ_m (molar conductivity) is the conductivity due to number of ions furnished by one mole of electrolyte. As dilution increases number of ions present in the solution also increases hence molar conductivity increases.
(ii) Eº_Cell of any atom/ion does not depend upon number of atoms/ions. Hence Eº_Cell of any atom/ion is an intensive properties.
(iii) κ represents specific conductivity which depends upon number of ions present in per unit volume.
(iv) ∆_rG_Cell is an extensive property as it depends upon number of particles (species)

Q.52. Match the items of Column I and Column II.

Ans. (i)→ d, (ii) → c, (iii)→ a, (iv)→ b
Solution.
(i) Chemical reaction occurring on lead storage battery can be represented as
At anode Pb(s) + SO₄²⁻(aq) → PbSO₄ (s) + 2e⁻
At cathode Pb(s) + SO₄²⁻(aq)
Thus, Pb is anode and PbO₂ is cathode.
(ii) Mercury cell does not include ions during their function hence produce steady current.
(iii) Fuel cell has maximum efficiency as they produce energy due to combustion reaction of fuel.
(iv) Rusting is prevented by corrosion.

Q.53. Match the items of Column I and Column II.

Ans. (i) → d, (ii) → c, (iii)→ b, (iv) → a
Solution.
 (i) Conductivity (k) =

(ii) Molar conductivity
(iii) Degree of dissociation
(iv) Charge Q =I x t
where, Q is the quantity of charge in coulomb when I ampere of current is passed through an electrolyte for t second.

Q.54. Match the items of Column I and Column II.

Ans. (i)→ d, (ii) → c, (iii) → a, e (iv) → b
Solution.
(i) The electrode reaction occurring at Lechlanche cell are
At anode Zn(s) → Zn² + 2e⁻
At cathode MnO₂NH₄⁺ e^-  → MnO (OH) NH₃
(ii) Ni-Cd cell is rechargeable. So, it has more life time.
(iii) Fuel cell produces energy due to combustion. So, fuel cell converts energy of combustion into electrical energy e.g., 2H₂ + O₂ → 2H₂O
(iv) Mercury cell does not involve any ion in solution and is used in hearing aids.

Q.55. Match the items of Column I and Column II on the basis of data given below:

Ans. (i) → c, (ii)  → a, (iii) → g, (iv) → e, (v)  → d, (vi)  → b, (vii)  → f
Solution.
(i) F₂ is a non-metal and best oxidising agent because SRP of F₂ is + 2.87 V.
(ii) Li is a metal and strongest reducing agent because SRP of Li is −3.05 V.
(iii) Au³⁺ is a metal ion which is an oxidising agent as SRP of Au³⁺ is +1.40V.
(iv) Br⁻ is an anion that can be oxidised by
Au³⁺ as Au³⁺ (E° = 1.40) is greater than
Br⁻ (E° = 1,09V).
(v) Au is an unreactive metal.
(vi) Li⁺ is a metal ion having least value of SRP (− 3.05 V), hence it is the weakest oxidising agent.
(vii) F⁻ is an anion which is the weakest reducing agent as F⁻ /F₂ has low oxidation potential (−2.87 V).

ASSERTION AND REASON TYPE

Note : In the following questions a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.

Q.56. Assertion : Cu is less reactive than hydrogen.
Reason : E°_Cu²⁺/Cu is negative.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (iii)
Solution.
Electrode potential of Cu²⁺ /Cu is + 0.34V and Electrode potential of 2H⁺ /H₂ is 0.00 V.
Hence, correct reason is due to positive value of Cu2⁺ /Cu it looses electron to H⁺ and get reduces, while H₂ gas evolves out.

Q.57. Assertion : E_Cell should have a positive value for the cell to function.
Reason : E_cathode < E_anode
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (iii)
Solution.
Feasibility of chemical reaction depends on Gibbs free energy which is related to E°cell 3S
ΔG^- = -nFE^- cell
When value of E°cell is positive then ΔG° becomes negative. Hence, reaction becomes feasible, Correct reason is E_cathode > E_anode.

Q.58. Assertion : Conductivity of all electrolytes decreases on dilution.
Reason : On dilution number of ions per unit volume decreases.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (i)
Solution.
Since, conductivity depends upon number of ions per unit volume. Therefore, the conductivity of all electrolytes decreases on dilution due to decrease in number of ions per unit volume.

Q.59. Assertion : Λ_m for weak electrolytes shows a sharp increase when the electrolytic solution is diluted.
Reason : For weak electrolytes degree of dissociation increases with dilution of solution.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (i)
Solution.
Molar conductivity of weak electrolytic solution increases on dilution, because as we add excess water to increase the dilution degree of dissociation increases which leads to increase in number of ions in the solution. Thus, Λ_m show a very sharp increase.

Q.60. Assertion : Mercury cell does not give steady potential.
Reason : In the cell reaction, ions are not involved in solution.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (v)
Solution.
Assertion is false but reason is true, correct assertion is mercury cell gives steady potential.
Reason is correct as ions are not involved in cell reaction.

Q.61. Assertion : Electrolysis of NaCl solution gives chlorine at anode instead of O₂.
Reason : Formation of oxygen at anode requires overvoltage.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (i)
Solution.
Electrolysis of NaCl is represented by following chemical reactions At cathode

At anode
2H₂O(aq) O₂(g) 4H⁺ (aq) 4e^-; E°_Cell 1.23V
E°cell for this reaction has lower value but formation of oxygen at anode requires over potential.

Q.62. Assertion : For measuring resistance of an ionic solution an AC source is used.
Reason : Concentration of ionic solution will change if DC source is used.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (i)
Solution.
Concentration of ionic solution changes on using DC current as a source of energy while on passing AC current concentration does not change. Hence, AC source is used for measuring resistance.

Q.63. Assertion : Current stops flowing when E_Cell = 0.
Reason : Equilibrium of the cell reaction is attained.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (i)
Solution.
Current stop flowing when E_cell = 0
As at E_cell = 0 reaction reaches the equilibrium.

Q.64. Assertion : E_Ag⁺/_Ag increases with increase in concentration of Ag⁺ ions.
Reason : E_Ag⁺/_Ag E has a positive value.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (ii)
Solution.
Both assertion and reason are correct but reason is not the correct explanation of assertion.

E = E° + 0.059 log {Ag⁺]
Thus E _Ag+/Ag increases with increase in concentration of Ag⁺

Q.65. Assertion : Copper sulphate can be stored in zinc vessel.
Reason : Zinc is less reactive than copper.
(i) Both assertion and reason are true and the reason is the correct explanation of assertion.
(ii) Both assertion and reason are true and the reason is not the correct explanation of assertion.
(iii) Assertion is true but the reason is false.
(iv) Both assertion and reason are false.
(v) Assertion is false but reason is true.
Ans. (iv)
Solution.
Copper sulphate can't be stored in zinc vessel as zinc is more reactive than copper due to negative value of standard reduction potential of Zn.

LONG ANSWER TYPE QUESTIONS

Q.66. Consider the Fig. 3.2 and answer the following questions.
(i) Cell ‘A’ has E_Cell = 2V and Cell ‘B’ has E_Cell = 1.1V 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 E_Cell = 0.5V and cell ‘B’ has E_Cell = 1.1V then what will be the reactions at anode and cathode?
Ans. (i) Cell 'B' will act as electrolytic cell due to its lesser value of emf.
The electrode reactions will be
At cathode Zn²⁺ + 2e⁻→ Zn
At anode Cu → Cu²⁺ + 2e⁻
(ii) If cell 'B' has higher emf, it acts as galvanic cell.
Now it will push electrons into cell ‘A’
In this case, the reactions will be
Zn →Zn²⁺ + 2e⁻ (At anode)
Cu²⁺ +2e⁻ → Cu (At cathode)

Q.67. Consider Fig. 3.2 and answer the questions (i) to (vi) given below.
(i) Redraw the diagram to show the direction of electron flow.
(ii) Is silver plate the anode or cathode?
(iii) What will happen if salt bridge is removed?
(iv) When will the cell stop functioning?
(v) How will concentration of Zn²⁺ ions and Ag⁺ ions be affected when the cell functions?
(vi) How will the concentration of Zn²⁺ ions and A^g+ ions be affected after the cell becomes ‘dead’?
Ans. (i) Electrons move from Zn to Ag as E' is more negative for Zn, so Zn undergoes oxidation and Ag⁺ undergoes reduction.
(ii) Ag is the cathode as it is the site of reduction where Ag⁺ takes electrons from medium and deposit at cathode.
(iii) Cell will stop functioning because cell potential drops to zero. At E = 0 reaction reaches equilibrium.
(iv) When E_cell = 0 because at this condition reaction reaches to equilibrium.
(v) Concentration of Zn²⁺ ions will increase and concentration of Ag⁺ ions will decrease because Zn is converted into Zn²⁺ and Ag⁺ is converted into Ag.
(vi) When E_cell = 0 equilibrium is reached and concentration of Zn²⁺ ions and Ag⁺ will not change.

Q.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?
Ans. If concentration of all reacting species is unity, then E_cell = E°cell and ΔG° = -nFE°cell where, Δ_rG° is standard Gibbs energy of the reaction
E°_cell = emf of the cell
nF = charge passed
If we want to obtain maximum work from a galvanic cell then charge has to be passed reversibly.
The reversibly work done by a galvanic cell is equal to decrease in its Gibbs energy.
Δ_rG = -nFE_cell
As E_cell is an intensive parameter but Δ_rG is an extensive thermodynamic property and the value depends on n.
For reaction, Zn (s) + Cu²⁺ (aq) → Zn²⁺ (aq)+ Cu (s)in a galvanic cell.
Δ_rG = -2FE_cell [Here, n = 2]