CBSE Class 12 Chemistry HOTs Electrochemistry

ASSERTION REASON TYPE OF QUESTIONS
The question given below consist of an assertion and a reason use the following key to choose appropriate answer
a) Both assertion and reason are correct and reason is the correct explanation of the assertion
b) Both assertion and reason are correct and reason is not the correct explanation of the assertion
c) Assertion is correct but reason is incorrect
d) Assertion is wrong Reason is correct.

Question. Assertion: The electrode potential of SHE is zero
Reason: In SHE HCl 1M and H₂ gas at one bar pressure is taken
Answer. B

Question. Assertion: H⁺ ion cannot oxidize copper
Reason: Reduction potential of Cu²⁺ / Cu is greater than H+/H
Answer. A

Question. Assertion: The reduction potential of F-/F is highest among all electrodes
Reason: Fluorine is the strongest oxidising agent
Answer. B

Question. Assertion: Electronic conduction decreases with temperature
Reason: The flow of electrons hindered on increasing the temperature
Answer. A

Question. Assertion: conductivity decreases with increasing dilution
Reason: No of ions increases per unit volume increases with dilution.
Answer. C

Question. Assertion: Electrolytic conduction increases with temperature
Reason: On increasing the temperature mobility of ion increases
Answer. A

Question. Assertion: Molar conductivity of electrolytes decreases with dilution
Reason: For weak electrolytes degree of dissociation increases with dilution.
Answer. D

Question. Assertion: It is difficult to measure the conductivity of ionic solutions
Reason: Electrolytes conduct electricity and undergoes chemical change.
Answer. A

Question. Assertion: Molar conductivity of strong electrolytes increases with dilution
Reason: On dilution inter ionic interaction increases.
Answer. C

Question. Assertion: Molar conductivity of acetic acid increases sharply with dilution
Reason: Degree of dissociation of acetic acid decreases with dilution
Answer. C

Question. Assertion:An external potential of 1.1V is passed through Danielcell, no current flow through it.
Reason: Standard emf of galvanic cell is 1.1 volt.
Answer. B

Question. Assertion: Out of Li and K Potassium is the strongest reducing agent
Reason: Reduction potential of K is greater than that of Li
Answer. D

Question. Assertion: Electrolytic conduction of electrolytes depends on size of ions
Reason: Larger the size of ion lesser will be the mobility of ions.
Answer. A

Question. Assertion: Limiting molar conductivity of weak electrolytes can be obtained graphically
Reason: Limiting molar conductivity of weak electrolytes increases with dilution
Answer. B

Question. Assertion: Electrolysis of aqueous NaCl produces Oxygen at the anode
Reason: Oxidation potential of Oxygen is lower than chlorine.
Answer. D

Question. Define the following terms : (i) Molar conductivity (Λ_m) (ii) Secondary batteries
Answer. Molar conductivity of a solution at a given concentration is the conductance of the volume ‘V’ of a solution containing one mole of electrolyte kept between two electrodes with area of cross section ‘A’ and distance of unit length. It is represented by Λm. Unit of Λm is S cm² mol^-1
Λ_m = 𝑘c x 1000/Molarity Secondary batteries are those batteries which can be recharged by passing an electric current through them and can be used again and again are called secondary batteries.

Question. Calculate the time to deposit 1.27 g of copper at cathode when a current of 2A was passed through the solution of CuSO4. (Molar mass of Cu = 63.5 g mol-1,1 F = 96500 C mol^-1)
Answer. CuSO₄ → Cu+ + SO₄^2-
Cu²⁺ + 2e^– → Cu
63.5 gram of copper is deposited = 2 × 96500 C
1.27 gram of Cu is deposited =( 2×96500/63.5) × 1.27
= 3860
Q = I × t
3860 = 2 x t
t = 3860/2 = 1930 seconds

Question. Write the name of the cell which is generally used in transistors. Write the reactions taking place at the anode and the cathode of this cell.
Answer. Leclanche cell is used in transistors.
Reaction at Anode: Zn(s) → Zn²⁺ + 2e^–
At Cathode: MnO₂ + NH₄⁺ + e^– → MnO(OH) + NH₃

1. Read the given passage and answer the questions 1 to 5 that follow:
"Car battery is the most important type of secondary cell having a lead anode and a grid of Lead packed with PbO2 as cathode. It is also called lead storage battery. It contains 40% solution of sulphuric acid (Density = 1.294 gmL^-1) as electrolyte. The battery holds 3.5 L of the acid. During the discharge of the battery, the density of H₂SO₄ falls to 1.139 gmL^-1 (20% H₂SO₄ by mass)"

Question. Write the reaction taking place at the cathode when the battery is in use.
Answer. Cathode reaction is
PbO₂ + SO₂ + 4H⁺ + 2e^- → 2PbSO₄ + 2H₂O

Question. How much electricity in terms of Faraday is required to carry out the reduction of one mole of PbO₂
Answer. 2F

Question. What is the molarity of sulphuric acid before discharge?
Answer. Molarity = (% ×10 ×d) ÷ (Molarity of H₂SO₄) = (40 x 10 x 1.294) ÷ 98 = 5.28 mol L^-1

Question. Why is lead storage battery considered a secondary cell?
Answer. It can be recharged again and again.

Question. Write the products of electrolysis when dilute sulphuric acid is electrolysed using platinum electrodes.
Answer. H₂ at cathode and O₂ at anode.

Question. A steady current of 2 amperes was passed through two electrolytic cells X and Y connected in series containing electrolytes FeSO₄ and ZnSO₄ until 2.8g of Fe deposited at the cathode of cell X. How long did the current flow ? Calculate the mass of Zn deposited at the cathode of cell Y. (Molar mass : Fe = 56 g mol^–1 Zn = 65.3 g mol^–1, 1F = 96500 C mol^–1)
Answer. (a) 𝑚 = 𝑧 𝐼 𝑡
2.8 g = (56 ×2 ×𝑡)/2 ×96500
t= 4825 s 0r 80.417 min
𝑚1 / 𝑚2 = 𝐸1 /𝐸2
2.8/ 𝑚𝑍𝑛 = (56 / 2) X( 2 / 65.3)
mZn = 3.265 g 

Question. (a) Out of the following pairs, predict with reason which pair will allow greater conduction of electricity :
(i) Silver wire at 30^oC or silver wire at 60^oC.
(ii) 0·1M CH₃COOH solution or 1M CH₃COOH solution.
(iii) KCl solution at 20⁰C or KCl solution at 50^oC.
(b)Give two points of differences between electrochemical and electrolytic cells. (3+2)
Answer. (a) (i) Silver wire at 300C because as temperature decreases, resistance decreases so conduction increases.
(ii) 0.1 M CH₃COOH, because on dilution degree of ionization increases hence conduction increases.
(iii) KCl solution at 500C, because at high temperature mobility of ions increases and hence conductance increases
(b) Electrochemical cell
(1) Anode -ve Cathode +ve
(2) Convert chemical Energy to electrical energy Electrolytic cell
(1) Anode +ve ,Cathode -ve
(2) Convert electrical Energy to chemical energy (or any other correct differences)

Question. a) Write the Nernst equation for the following cell reaction: Zn_(s)+Cu²⁺_(aq) ➔ Zn²⁺_(aq)+Cu_(s)
b) How will the E cell be affected when concentration of
(i)Cu ²⁺ ions is increased and (ii) Zn ²⁺ ions is increased ?
Answer. a) Zn_(s)|Zn²⁺(aq)|| Cu²⁺_(aq)|Cu_(s)
b) (i) E_(cell) increases (ii) E_(cell) decreases

Question. (a) The standard Gibbs energy (ΔrG0 ) for the following cell reaction is -300 kJ mol^-1:
Zn(s) + 2Ag + (aq) ➔ Zn ²⁺ (aq) + 2Ag(s) 
Calculate E0 cell for the reaction. (Given: lF = 96500 moI^-1 )
Answer. Δ_rG⁰ =-nFE⁰_(cell)
-300x1000j/mol=-2x96500xE⁰_(cell)
E⁰_(cell)=300000/2x96500=1.55v

Question. Calculat λ0m for MgCl₂ if λ0 values for Mg²⁺ ion and Cl - ion are 106 S cm2mol-1 and 76.3 S cm2mol--1respectively.
Answer. Λ⁰m MgCl₂ = λ0Mg2++ λ⁰2Cl^-
= 106 Scm²mol^—1+76.3x2 Scm2mol^—1
= 258.6 Scm²mol