CBSE Class 12 Chemistry Electrochemistry Question Bank

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Unit 3 Electrochemistry Chemistry Worksheet for Class 12

Class 12 Chemistry students should refer to the following printable worksheet in Pdf in Class 12. This test paper with questions and solutions for Class 12 Chemistry will be very useful for tests and exams and help you to score better marks

Class 12 Chemistry Unit 3 Electrochemistry Worksheet Pdf

Question. Which of the following is a secondary cell?
(a) Leclanche cell
(b) Lead storage battery
(c) Concentration cell
(d) All of these
Answer. B

Question. If limiting molar conductivity of Ca 2+ and Cl are 119.0 and 76.3 S cm2 mol-1, then the value of limiting molar conductivity of CaCl2 will be
(a) 195.3 S cm2 mol-1
(b) 271.6 S cm2 mol-1
(c) 43.3 S cm2 mol-1
(d) 314.3 S cm2 mol-1.
Answer. B

Question. The reaction, 3ClO (aq) → ClO3 (aq) + 2Cl (aq) is an example of
(a) Oxidation reaction
(b) Reduction reaction
(c) Disproportionation reaction
(d) Decomposition reaction
Answer. C

Question. Without losing its concentration ZnCl2 solution cannot be kept in contact with
(a) Au
(b) Al
(c) Pb
(d) Ag
Answer. B 


SHORT ANSWER TYPE QUESTIONS

Question. The chemistry of corrosion of iron is essentially an electrochemical phenomenon. Explain the reactions occurring during the corrosion of iron in the atmosphere.
Answer. The redox reaction involves
At anode : Fe(S) → Fe2+ (aq) + 2e
At cathode : H2O + CO2 ⇌ H2CO3 (Carbonic acid)
H2CO3 ⇌2H+ + CO22-
H+ + e → H
4H + O2 → 2H2O
Then net resultant Redox reaction is
2Fe(s) + O2 (g) + 4H+ → 2Fe2+ + 2H2O

Question. Write the reactions taking place at cathode and anode in lead storage battery when the battery is in use. What happens on charging the battery?
Answer. At Anode: Pb + SO4-2 → PbSO4 + 2e
at Cathode: PbO2 + SO4-2 + 4H+ + 2e → PbSO4 + 2H2O
On charging the battery, the reaction is reversed and PbSO4 on anode and cathode is converted into Pb and PbO2 respectively.

Question. State Kohlrausch law of independent migration of ions. Why does the conductivity of a solution decrease with dilution?
Answer. The limiting molar conductivity of an electrolyte is the sum of the limiting ionic conductivities of the cation and the anion each multiplied with the number of ions present in one formula unit of the electrolyte Λ°m for AxBy = xλ°+ + yλ°–
For acetic acid Λ° (CH3COOH) = λ°CH3COO– + λ°H+
Λ°(CH3COOH) = Λ° (CH3COOK) + Λ° (HCl) – Λ° (KCl)

Question. Using the E° values of A and B, predict which one is better for coating the surface of iron [E°(Fe2+/Fe) = – 0.44V] to prevent corrosion and why?
Given: E°(A2+|A) = -2.37 V and E°(B2+|B) = – 0.14 V 
Answer. ‘A’ will prevent iron from rusting. So, we can coat the iron surface with metal A because it has more negative value. 

 

1. Read the passage given below and answer the following questions:

The potential difference between the two electrodes of a galvanic cell is called the cell potential and is measured in volts. The cell potential is the difference between the electrode potentials (reduction potentials) of the cathode and anode. It is called the cell electromotive force (emf) of the cell when no current is drawn through the cell. It is now an accepted convention that we keep the anode on the left and the cathode on the right while representing the galvanic cell. A galvanic cell is generally represented by putting a vertical line between metal and electrolyte solution and putting a double vertical line between the two electrolytes connected by a salt bridge.
In a galvanic cell, the following cell reaction occurs:
Zn(s) + 2Ag+(aq) → Zn2+(aq) + 2Ag(s)
cell = +1.56 V

Question. What is the direction of the flow of electrons?
(a) First from silver to zinc, then the direction reverses
(b) Silver to zinc
(c) First from zinc to silver, then the direction reverses
(d) Zinc to silver
Answer. D

Question. How will concentration of Zn2+ ions and Ag+ ions be affected when the cell functions?
(a) Concentration of both Zn and Ag+ ions increase
(b) Concentration of Zn2+ increases and Ag+ ions decreases
(c) Concentration of Zn2+ decreases and Ag+ ions increases
(d) Concentration of both Zn2+ and Ag+ ions decreases
Answer. B

Question. Name the cell which is generally used in inverters?
(a) Mercury cell
(b) Leclanche cell
(c) Lead storage battery
(d) Lithium ion battery
Answer. C

Question. Which cell used in hearing aids?
(a) Mercury cell
(b) Leclanche cell
(c) Dry cell
(d) Nickel Cadmium cell
Answer. A

Question. The standard electrode potential for Daniell cell is 1.1V. Calculate the standard Gibbs energy for the reaction:
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s).
(a) 215.36 kJ mol-1
(b) -212. 27 kJ mol-1
(b) 212.27 kJ mol-1
(d) -218 kJ mol-1
Answer. B

Question. Write the name of the cell which is generally used in hearing aids. Write the reactions taking place at the anode and the cathode of this cell. 
Answer. Mercury cell, Anode : Zn(Hg)+2OH- →ZnO(s)+H2O+2e-
Cathode : HgO+H2O+2e→Hg(l) + 2OH-

Question. (a) The cell in which the following reaction occurs :2 Fe3+(aq) + 2I- (aq) ⎯→ 2Fe2+(aq)+I2(s) has E0cell = 0·236 V at 298 K. Calculate the standard Gibbs energy of the cell reaction. (Given : 1 F = 96,500 C mol−1 ) (2)
(b) How many electrons flow through a metallic wire if a
current of 0·5 A is passed for 2 hours ?
(Given : 1 F = 96,500 C mol−1) (3)
Answer. (a)ΔG0 = -nFE0 cell
n= 2 ,ΔG 0 = - 2 x 96500 C /mol x 0.236 V
= - 45548 J/mol
= -45.548 kJ/mol
(b) Q= I t = 0.5 x 2 x 60 x 60
= 3600 C
96500 C = 6.023 x 1023 electrons
so 3600 C = 2.25 x 1022 electrons

Question. A current of 1.50 A was passed through an electrolytic cell containing AgNO3 solution with inert electrodes. The weight of silver deposited was 1.50 g. How long did the current flow ? (Molar mass of Ag = 108 g mol–1, 1F = 96500 C mol–1). 
Answer. Ag+ + 1e→Ag
96500C depositing 108 g of Ag
Quantity of electricity required for depositing
1.5g of Ag= 96500x1.5/108 =1340.27C
Q=Ixt so t=Q/I t=1340.27/1.5=893.5s.

Question.The conductivity of a 0.01 M solution of acetic acid at 298 K is 1.65 × 10–4 S cm–1 . Calculate molar conductivity (Λm ) of the solution. 
Answer. Λm =1000k/c, Λm=1000x1.65x10-4/ 10-2 =16.5Scm2mol-1

Question. Consider the following reaction : Cu(s) + 2Ag+ (aq) → 2Ag(s) + Cu2+(aq)
(i) Depict the galvanic cell in which the given reaction takes place.
(ii) Give the direction of flow of current. (iii) Write the half-cell reactions taking place at cathode and anode.
Answer. (i) Cu(s)|Cu2+(aq)||Ag+(aq)|Ag(s)
(ii)Silver electrode to Copper electrode
(iii) Anode, Cu→ Cu2+ +2e
Cathode, Ag+ +1e→Ag

Question. Eo cell for the given redox reaction is 2.71 V
Answer. Mg(s) + Cu 2+ (0.01 M) –––→ Mg2+ (0.001 M) + Cu(s)
(a)Calculate Ecell for the reaction.(b) Write the direction of flow of current when an external opposite potential applied is
(i) less than 2.71 V and (ii) greater than 2.71 V (3+2)
Answer. (a) E cell = E0 cell - 0.059 logK c
                                         n
= Eo cell - 0.059 log(10-3)
2 (10-2)
= 2.71+ 0.0295
Ecell = 2.7395 V
(b) i)Cu to Mg / Cathode to anode / Same direction
ii)Mg to Cu / Anode to cathode / Opposite direction

QUESTION BANK
 
Electrochemistry
 
1. Define the term – Electrode potential? 
 
2. Define the term – standard electrode potential? 
 
3. Single electrode potential cannot be determined. Why? 
 
4. What is SHE? What is its electrode potential? 
 
5. What does the positive value of standard electrode potential indicate? 
 
6. What would happen if Nickel spatula is used to stir a solution of CuSO4?
E0Cu2+ / Cu = 0.34 V, E0 Ni2+ / Ni = -0.25V?
 
7. Write Nernst equation for a Daniel cell? 
 
8. What do you understand by the term- conductivity? 
 
9. How is molar conductance related to conductivity of an electrolyte? 
 
10. Write an expression relating cell constant and conductivity? 
 
11. What do you understand by strong and weak electrolytes? 
 
12. State kohlrausch’s Law? 
 
13. How many faradays are needed to reduce 3g mole of Cu2+ to Cu metal? 
 
14. Give the reaction taking place in lead storage battery when it is on charging? 
 
15. What are fuel cells? 
 
16. What is the electrolyte used in a dry cell?
 
17. What is an electrochemical series? How does it predict the feasibility of acertain redox reaction?
 
18. How is standard electrode potential of a cell related to :-
i) Equilibrium constant?
ii) Gibbs free energy change.
 
19. The conductivity of an aqueous solution of NaCl in a cell is 92 Ω-1 cm-1 theresistance offered by this cell is 247.8Ω. Calculate the cell constant?
 
20. The molar conductivity of 0.1M CH3COOH solution is 4.6cm2 mol-1.What is the conductivity and resistivity of the solution?
 
21. The conductivity of metals decreases while that of electrolytes increaseswith increases in temperature. Why?
 
22. The measured resistance of a cell containing 7.5 x 10-3 M solution of KCl at 250C was 1005 Ω calculate,
(a) Specific conductance and
(b) Molar conductance of the solution. Cell Constant = 1.25 cm-1
 
23. How is Limiting molar conductivity related to
i) degree of ionization and   ii) dissociation constant
 
24. State Faraday’s Laws of electrolysis? 
 
25. How many gram of chlorine can be produced by the electrolysis of molten NaCl with a current of 1 amp for 15 min?
 
26. How many electrons flow when a current of 5 amps is passed through a solution for 193 sec. Given F = 96500 C. NA=6.002 × 1023 mol-1?
 
27. Silver is deposited on a metallic vessel by passing a current of 0.2 amps for 3 hrs.  Calculate the weight of silver deposited.
(At mass of silver = 108 amu, F = 96500 C?
 
28. Name the cell used for low current devices like hearing aids, watches etc. Also give thehalf cell reactions for such a cell?
 
29. Rusting of iron is quicker in saline water than in ordinary water .Explain? 
 
30. Enlist the factors affecting corrosion? 
 
31. What do you mean by primary and secondary battery? 
 
32. Explain construction and working of standard Hydrogen electrode? 

33. What is the cell potential for the cell at 250C
Cr / Cr3+(0.1 M) Fe2+ (0.01M) /Fe
E0Cr3+/Cr = -0.74V ;E0 Fe2+/Fe = - 0.44V.

34. Calculate ΔG0 for the reaction at 250
Zn (s)/ Zn2+ (0.0004M) Cd2+ (0.2m) 1 Cd (s)
E0Zn2+/Zn = -0.763V , E0Cd2+/Cd = -0.403V , F=96500 C Mol-1 , R = 8.314 J/K.

35. Calculate Equilibrium constant K for the reaction at 298K
Zn (s) + Cu2+ (aq) →Zn2+/aq) +Cu
E0Zn2+Zn = -0.076V ; E0Cu2+Cu = +0.34V.

36. For what concentration of Ag+ (aq) will the emf of the given cell be zero at 250C ,if the concentration of Cu2+ (aq) is 0.1 M ?
Cu (s) / Cu2+ (0.1M)// Ag+ (aq) / Ag(s)
E0 Ag+ /Ag = +0.80V; E0 Cu2+ /Cu = 0.34 V

37. Calculate the standard free energy change for the cell- reaction.
Fe2+ (aq) + Ag+ (s) →Fe3+ (aq) + Ag(s)
How is it related to the equilibrium constant of the reaction?
E0Fe3+/Fe2+= + 0.77V , E0Ag+/Ag = +0.08V F= 96500 C/mol.

38. Predict the products of electrolyzing of the following
(a) A dil. Solution of H2SO4 with Pt. electrode
(b) An aqueous solution of AgNO3 with silver electrode

 

Unit 3 Electrochemistry CBSE Class 12 Chemistry Worksheet

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