JEE Advanced Previous Year Questions (2018 - 2023): Electrochemistry | Chemistry for JEE Main & Advanced PDF Download

2023

Q1: Plotting    against  for aqueous solutions of a monobasic weak acid (HX) resulted in a straight line with y-axis intercept of P and slope of S. The ratio P/S is     [JEE Advanced 2023 Paper 1]
[ = molar conductivity
  = limiting molar conductivity
c =  molar concentration
K_a = dissociation constant of HX]
(a)
(b)
(c)
(d)
Ans: (a)

2022

Q1: Consider the strong electrolytes Z_mX_n , U_m Y_P and V_m X_n . Limiting molar conductivity (Λ⁰) of U_m Y_p and V_m X_n are 250 and 440 S cm² mol⁻¹ , respectively. The value of  (m + n + p) is
Given:
λ⁰ is the limiting molar conductivity of ions
The plot of molar conductivity (Λ) of Z_m X_n vs c^1/2 is given below.      [JEE Advanced 2022 Paper 2]
Ans: 7
Given,

It is also given that


In the question, a graph of ( Λ ) of Z_mX_n Vs C^1/2 is given,

For electrolyte Z_mX_n and from given curve

so

Solving eqn (2) and eqn (3),

⇒ n = 3
Substituting the value of n in eqn (2), we get,

Now, substituting the value of m is eqn (1), we get

Hence, the required value of m + n + p is 7.

Q2: The reduction potential l (E⁰ , in V) of MnO₄⁻ (aq) / Mn (s) is __________.      [JEE Advanced 2022 Paper 1]
[Given:

Ans: 0.74 to 0.80

Adding (1), (2) and (3),

2021

Q1: At 298 K, the limiting molar conductivity of a weak monobasic acid is 4 × 10² S cm² mol⁻¹. At 298 K, for an aqueous solution of the acid the degree of dissociation is α and the molar conductivity is y × 10² S cm² mol⁻¹. At 298 K, upon 20 times dilution with water, the molar conductivity of the solution becomes 3y × 10² S cm² mol⁻¹.    [JEE Advanced 2021 Paper 2]
The value of α is __________.
Ans: 0.22
Degree of dissociation = α
Limiting molar conductivity,
Molar conductivity,
Molar conductivity of dilution,
Concentration before dilution = C
Concentration after dilution = C/20

Dissociation constant,
Putting Eq. (i),

Dissociation constant before dillution,

Dissociation constant after dilution,

Comparing Eqs. (ii) and (iii),


Putting in Eq. (i),

The value of α is 0.22.

Q2: At 298 K, the limiting molar conductivity of a weak monobasic acid is 4 × 10² S cm² mol⁻¹. At 298 K, for an aqueous solution of the acid the degree of dissociation is α and the molar conductivity is y × 10² S cm² mol⁻¹. At 298 K, upon 20 times dilution with water, the molar conductivity of the solution becomes 3y × 10² S cm² mol⁻¹.    [JEE Advanced 2021 Paper 2]
The value of y is __________.
Ans: 0.86
Degree of dissociation = α
Limiting molar conductivity,
Molar conductivity,
Molar conductivity of dilution,
Concentration before dilution = C
Concentration after dilution = C/20

Dissociation constant,
Putting Eq. (i),

Dissociation constant before dillution,

Dissociation constant after dilution,

Comparing Eqs. (ii) and (iii),

Q3: Some standard electrode potentials at 298 K are given below :
Pb²⁺ /Pb − 0.13 V
Ni²⁺ /Ni − 0.24 V
Cd²⁺ /Cd − 0.40 V
Fe²⁺ /Fe − 0.44 V
To a solution containing 0.001 M of X²⁺ and 0.1 M of Y²⁺, the metal rods X and Y are inserted (at 298 K) and connected by a conducting wire. This resulted in dissolution of X. The correct combination(s) of X and Y, respectively, is(are)
(Given : Gas constant, R = 8.314 JK⁻ mol⁻¹, Faraday constant, F = 96500 C mol⁻¹)   [JEE Advanced 2021 Paper 2]
(a) Cd and Ni
(b) Cd and Fe
(c) Ni and Pb
(d) Ni and Fe
Ans: (a, b, c)
Given,

Nernst equation,

E_cell should be positive for a reaction to be spontaneous.

Reaction is spontaneous.

Reaction is spontaneous.

Reaction is spontaneous.

Reaction is non-spontaneous.
Therefore, the correct combinations of X and Y are (a), (b) and (c).

2020

Q1: Consider a 70% efficient hydrogen-oxygen fuel cell working under standard conditions at 1 bar and 298 K. Its cell reaction is

The work derived from the cell on the consumption of 1.0 × 10⁻³ mole of H₂(g) is used to compress 1.00 mole of a monoatomic ideal gas in a thermally insulated container. What is the change in the temperature (in K) of the ideal gas?      
The standard reduction potentials for the two half-cells are given below :  [JEE Advanced 2020 Paper 1]

Ans: 13.32
Vessel is insulated, thus q = 0
For the given reaction :


= − 2 × 96500 × 1.23 J/mol
Therefore, work derived from this fuel cell using 70% efficiency and on consumption of 1.0 × 10⁻³ mol of H₂(g)
= 2 × 96500 × 1.23 × 0.7 × 1 × 10⁻³
= 166.17 J
This work done = change in internal energy (for monoatomic gas, C_v'm = 3R/2),

2019

Q1: Molar conductivityof aqueous solution of sodium stearate, which behaves as a strong electrolyte, is recorded at varying concentrations (C) of sodium stearate. Which one of the following plots provides the correct representation of micelle formation in the solution?
(critical micelle concentration (CMC) is marked with an arrow in the figures)    [JEE Advanced 2019 Paper 1]
(a)

(b)

(c)

(d)

Ans: (c)
At normal or low concentration, sodium stearate (CH3(CH2)16COO-Na+] behaves as strong electrolyte and for strong electrolyte, molar conductance  decreases with increase in concentration. Above particular concentration, sodium stearate forms aggregates known as micelles. The concentration is called as CMC. Since, number of ions decreases and hence also decreases.
Hence, option (c) is correct.

2018

Q1: Consider an electrochemical cell:

The value of ΔH^∘ for the cell reaction is twice that of ΔG^∘ at 300 K . If the emf of the cell is zero, the ΔS^∘ (in JK⁻¹ mol⁻¹) of the cell reaction per mole of B formed at 300 K is ___________.
(Given: ln ⁡ (2) = 0.7 , R (universal gas constant) = 8.3 JK⁻¹ mol⁻¹. H , S and G are enthalpy, entropy and Gibbs energy, respectively.)     [JEE Advanced 2018 Paper 2]
Ans: -11.62
At  300 K, following electrochemical cell operates:

The reactions at:
(i) Anode:

(ii) Cathode :

Multiplying equation (i) by 2,

The net electrochemical cell is written as

Given : Enthalpy change ( ΔH^∘) for cell reaction = 2 × Gibbs free energy than for cell reaction

According to the Nernst equation :

For a spontaneous reaction,





The change in entropy (ΔS^∘) per mol of B is

Q2: For the electrochemical cell,
 the standard emf of the cell is 2.70 V at 300 K. When the concentration of Mg²⁺ is changed to x M , the cell potential changes to 2.67 V at 300 K. The value of x is ___________.
(given, F/R = 11500 KV^-1, where F is the Faraday constant and R is the gas constant, In (10 = 2.30)    [JEE Advanced 2018 Paper 1]
Ans: 10
Equation of cell reaction according to the cell notation given, is
Given, E⁰_cell = 2.70 V, T = 300 K with [Mg²⁺(aq)] = 1 M and [Cu²⁺(aq)] = 1 M and n = 2
Further, E_cell = 2.67 V with [Cu²⁺(aq)] = 1 M and [Mg²⁺(aq)] = xM and F/R = 11500 KV⁻¹ where F = Faraday constant, R = gas constant
From the formula,

After putting the given values

or x = 10 (as given ln ⁡ (10) = 2.30)