Electrochemistry — Class 12 MCQs with Answers

Electrochemistry — important questions & MCQs with answers (Class 12 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   In a galvanic cell, oxidation occurs at the:

   ACathodeBElectrolyteCSalt bridgeDAnode
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   Answer: Anode

   Explanation: The anode is where oxidation happens — electrons are lost there, e.g. Zn(s) → Zn²⁺(aq) + 2e⁻ — while the cathode is where reduction occurs; mixing the two up is the classic slip.

2. Q2 of 6Easy

   The SI unit of molar conductivity is:

   AS cm⁻¹BS m⁻¹CS m² mol⁻¹DΩ m
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   Answer: S m² mol⁻¹

   Explanation: Molar conductivity Λm = κ/c, and dividing S m⁻¹ by mol m⁻³ leaves S m² mol⁻¹ — don't confuse it with κ's own unit, S m⁻¹, the tempting distractor here.

3. Q3 of 6Medium

   The Nernst equation at 298 K for a cell reaction is:

   AE = E° × log QBE = E° + (0.059/n) log QCE = E° − (n/0.059) log QDE = E° − (0.059/n) log Q
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   Answer: E = E° − (0.059/n) log Q

   Explanation: The Nernst equation is E = E° − (0.059/n) log Q at 298 K, derived from E = E° − (RT/nF) ln Q; note the minus sign and that n sits in the denominator, not multiplying log Q.

4. Q4 of 6Medium

   Assertion (A): A solution of silver nitrate cannot be stored in a copper vessel. Reason (R): Silver is a better conductor of electricity than copper.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: E°(Ag⁺/Ag) = +0.80 V is higher than E°(Cu²⁺/Cu) = +0.34 V, so copper reduces Ag⁺ ions (Cu + 2Ag⁺ → Cu²⁺ + 2Ag) and the vessel would dissolve. R is also true, but how well a metal conducts electricity has nothing to do with whether copper reduces Ag⁺ ions, so R does not explain A.

5. Q5 of 6Hard

   Conductivity of saturated AgCl solution at 298 K is 1.382 × 10⁻⁶ S cm⁻¹ (water κ negligible). Given Λ°m(AgCl) = 138.2 S cm² mol⁻¹, the solubility (mol L⁻¹) of AgCl is:

   A1.0 × 10⁻⁶B1.0 × 10⁻⁴C1.0 × 10⁻⁵D1.0 × 10⁻³
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   Answer: 1.0 × 10⁻⁵

   Explanation: c = 1000κ/Λ°m = 1000 × 1.382 × 10⁻⁶ / 138.2 = 1.0 × 10⁻⁵ mol L⁻¹ — for a sparingly soluble salt like AgCl this dissolved-ion concentration equals the molar solubility directly, since it fully dissociates.

6. Q6 of 6Hard

   Assertion (A): For the cell Mg(s)|Mg²⁺(0.130 M)||Ag⁺(0.0001 M)|Ag(s), with E°cell = 3.17 V, the cell potential at 298 K is about 2.96 V. Reason (R): For the cell reaction Mg(s) + 2Ag⁺(aq) → Mg²⁺(aq) + 2Ag(s), the number of electrons transferred, n, is 1.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is true but R is false

   Explanation: A is true: Ecell = 3.17 − (0.059/2) log[0.130/(0.0001)²] = 3.17 − 0.0295 × log(1.3 × 10⁷) = 3.17 − 0.21 = 2.96 V. R is false: one Mg atom gives up two electrons, which are taken up by two Ag⁺ ions, so n = 2 for this reaction.

Electrochemistry notes: key concepts (Class 12 Chemistry)

Electrochemistry connects redox chemistry to electricity: galvanic and electrolytic cells, standard electrode potentials, the Nernst equation for non-standard conditions, the link ΔG° = −nFE°cell to thermodynamics and equilibrium, conductance and Kohlrausch's law, plus Faraday's laws of electrolysis and common batteries and fuel cells.

Galvanic (voltaic) cell

Converts chemical energy to electrical energy via a spontaneous redox reaction. Oxidation at the anode (negative), reduction at the cathode (positive).

Electrolytic cell

Uses external electricity to drive a non-spontaneous reaction; here the anode is positive and the cathode negative — the reverse of a galvanic cell's sign convention.

Standard electrode potential

Potential of an electrode against the standard hydrogen electrode (SHE, E° = 0) at 1 M, 298 K, 1 bar. More positive E° means a stronger oxidising agent.

Cell EMF

E°cell = E°cathode − E°anode using reduction potentials. A positive E°cell means the cell reaction is spontaneous (negative ΔG).

Nernst equation

Gives potential at non-standard concentrations: E = E° − (0.059/n) log Q at 298 K. At equilibrium E = 0 and Q = K.

EMF and thermodynamics

ΔG° = −nFE°cell and ΔG° = −2.303RT log K, so E°cell fixes the equilibrium constant K of the cell reaction.

Conductivity vs molar conductivity

Conductivity κ falls on dilution; molar conductivity Λm = κ×1000/c RISES on dilution as ions move apart and interference decreases.

Kohlrausch's law

Limiting molar conductivity is the sum of independent ionic contributions: Λ°m = ν₊λ°₊ + ν₋λ°₋. Lets us find Λ°m of weak electrolytes indirectly.

Faraday's laws of electrolysis

Mass deposited ∝ charge passed (w = ZIt); one faraday (96500 C) deposits one equivalent, so 3 F are needed per mole of Al from Al³⁺.

Primary vs secondary cells

Primary cells (dry/Leclanché cell) are non-rechargeable; secondary cells (lead-acid, Ni-Cd, Li-ion) are rechargeable by reversing the reaction.

Fuel cells

Convert the energy of fuel combustion (e.g. H₂–O₂) directly and continuously into electricity with high efficiency and no polluting by-products.

Corrosion

An electrochemical process where a metal is oxidised; rusting of iron forms hydrated Fe₂O₃ and is prevented by galvanising, painting or cathodic protection.