JEE Chemistry Electrochemistry MCQs Set 11

Answer: (b) 1.8 g

Explanation:
1. 4.6 g Na = 0.2 mol = 0.2 F.
2. The same charge deposits 0.2/3 mol of Al = 0.0667 × 27 = 1.8 g.

Teacher's Note:
1. The same charge deposits the same number of equivalents.
2. Equivalent weight of Al is 27/3 = 9.

Answer: (b) 15.9 g

Explanation:
1. 0.5 g H₂ is 0.5 equivalent (equivalent weight of hydrogen is 1).
2. The same current deposits 0.5 equivalent of copper = 0.5 × 31.75 = 15.9 g.

Teacher's Note:
1. Same charge means the same number of equivalents.
2. Equivalent weight of Cu is 63.5/2.

Answer: (c) 0.112 L

Explanation:
1. 2H₂O → 2H₂ + O₂, so the volume of O₂ is half that of H₂.
2. V(O₂) = 0.224 / 2 = 0.112 L.

Teacher's Note:
1. Each O₂ needs 4 electrons and each H₂ needs 2.
2. Gas volumes are proportional to moles under identical conditions.

Answer: (a) 4.6 Ω⁻¹ cm² eq⁻¹

Explanation:
1. κ = cell constant / R = 1.15 / 250 = 4.6 × 10⁻³ Ω⁻¹ cm⁻¹.
2. Λeq = κ × 1000 / N = 4.6 × 10⁻³ × 1000 / 1 = 4.6 Ω⁻¹ cm² eq⁻¹.

Teacher's Note:
1. Specific conductance = cell constant / resistance.
2. For a 1 N solution, Λeq = 1000 κ.

Answer: (b) 0.672 cm⁻¹

Explanation:
1. Cell constant = κ × R.
2. = 0.012 × 56 = 0.672 cm⁻¹.

Teacher's Note:
1. Cell constant has the unit cm⁻¹ (length/area).
2. It is found by calibrating with a standard KCl solution.

Answer: (d) [x₁ + x₂ − 2x₃]/2

Explanation:
1. Λ°m(BaSO₄) = Λ°m(BaCl₂) + Λ°m(H₂SO₄) − 2Λ°m(HCl) = x₁ + x₂ − 2x₃.
2. BaSO₄ has valence factor 2, so Λ°eq = (x₁ + x₂ − 2x₃)/2.

Teacher's Note:
1. Two HCl are needed to cancel the two Cl⁻.
2. Divide the molar value by 2 to get the equivalent value.

Answer: (c) 392

Explanation:
1. α = √(Ka/C) = √(25 × 10⁻⁶ / 0.01) = 0.05.
2. Λ° = Λ/α = 19.6 / 0.05 = 392 S cm² eq⁻¹.

Teacher's Note:
1. For a weak electrolyte Ka is approximately C α².
2. Then use α = Λ/Λ°.

Answer: (d) [Cr(NH₃)₆]Cl₃

Explanation:
1. Conductivity depends on the number of free ions.
2. [Cr(NH₃)₆]Cl₃ gives 4 ions, [Cr(NH₃)₅Cl]Cl₂ gives 3, [Cr(NH₃)₄Cl₂]Cl gives 2 and [Cr(NH₃)₃Cl₃] gives none.

Teacher's Note:
1. Only ions outside the coordination sphere ionise in water.
2. More free ions mean higher conductivity.

Answer: (c) 1.56 V

Explanation:
1. Ag⁺/Ag is the cathode and Zn²⁺/Zn is the anode.
2. E°cell = 0.80 − (−0.76) = 1.56 V.

Teacher's Note:
1. Cathode minus anode.
2. Do not change E° for the number of electrons.

Answer: (b) 10²⁰

Explanation:
1. log K = nE° / 0.0591 = 2 × 0.591 / 0.0591 = 20.
2. K = 10²⁰.

Teacher's Note:
1. Use n = 2.
2. The numbers were chosen so that log K is exactly 20.

Answer: (c) CuCl₂ can be stored in a silver vessel

Explanation:
1. Cu (0.34 V) reduces Ag⁺ (0.80 V) and Hg²⁺ (0.79 V), so (a) and (d) are wrong.
2. Cu cannot reduce Mg²⁺, so Mg(NO₃)₂ can be stored in copper and (b) is wrong, and Ag cannot reduce Cu²⁺, so CuCl₂ can be stored in a silver vessel.

Teacher's Note:
1. A metal vessel reacts with a solution of ions that have a higher reduction potential.
2. Check each pair separately.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Fused CaH₂ contains Ca²⁺ and H⁻ ions, and H⁻ is oxidised at the anode (2H⁻ → H₂ + 2e⁻).
2. The assertion is true and it follows from the reason, which is also true.

Teacher's Note:
1. Negative ions are attracted to the anode and are oxidised there.
2. Calcium metal forms at the cathode.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Molar conductance of a weak electrolyte increases on dilution.
2. This happens because the degree of ionisation rises, so the reason correctly explains the assertion.

Teacher's Note:
1. Ostwald's dilution law says α increases as concentration decreases.
2. More ions per mole means a higher molar conductance.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. A salt bridge completes the circuit and prevents the liquid junction potential, so the assertion is true.
2. The reason is also true because ions of equal mobility avoid the junction potential, but it is a requirement for the choice of electrolyte rather than the explanation of why a bridge is used.

Teacher's Note:
1. Typical salt bridge electrolytes are KCl, KNO₃ and NH₄NO₃.
2. Their cations and anions have almost equal speeds.

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. Iron is more reactive than copper and reduces Cu²⁺ (Fe + Cu²⁺ → Fe²⁺ + Cu), so the vessel would be corroded and the assertion is true.
2. The reason is false, because it is iron atoms that are oxidised by copper ions.

Teacher's Note:
1. A more reactive metal displaces a less reactive one from its salt solution.
2. Check which species gets oxidised.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. The acidic NH₄Cl paste slowly attacks the zinc container even when the cell is idle.
2. This explains why a dry cell does not last indefinitely.

Teacher's Note:
1. Dry cells have a limited shelf life.
2. The electrolyte is acidic because of NH₄⁺ hydrolysis.

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. A weak electrolyte is only partly ionised at moderate concentration, so it has fewer ions and a lower molar conductance.
2. The assertion is true, but the reason is false because weak electrolytes dissociate to a smaller extent.

Teacher's Note:
1. Strong electrolytes are fully ionised at every concentration.
2. The reason states the opposite of the truth.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Zinc is more easily oxidised than iron (−0.76 V against −0.44 V).
2. It acts as a sacrificial anode and protects the iron even if the coating is scratched, so the reason explains the assertion.

Teacher's Note:
1. A more reactive metal protects the less reactive one by cathodic protection.
2. Tin plating protects only while the coating is intact.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. E = E° + (0.059/2) log([Cu²⁺]/[Zn²⁺]) is the Nernst equation for the Daniell cell.
2. A higher [Cu²⁺] increases the log term and the emf, so the reason explains the assertion.

Teacher's Note:
1. Increasing the concentration of a reactant raises the emf.
2. Cu²⁺ is the reactant at the cathode.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. On a mercury cathode H⁺ has a very high overpotential, and Na⁺ forms a stable sodium amalgam.
2. So Na⁺ is discharged first, which shows that the nature of the electrode affects the order of discharge.

Teacher's Note:
1. Overpotential of hydrogen on mercury is very high.
2. This is the principle of the mercury cathode cell for NaOH.

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. Cu²⁺ is deposited as copper at the cathode, so the blue colour disappears, and the solution becomes sulphuric acid.
2. The reason is false because no Cu(OH)₂ forms.

Teacher's Note:
1. Blue colour is due to hydrated Cu²⁺ ions.
2. When all Cu²⁺ ions are deposited, the solution is colourless.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Ag⁺ needs 1 F per mole and Cu²⁺ needs 2 F per mole, so the assertion is true.
2. The atomic weights are different, but that is not the reason. The difference is in the charges on the ions.

Teacher's Note:
1. Charge needed depends on the number of electrons per ion.
2. Atomic weights decide the mass deposited, not the charge.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. OH⁻ ions accumulate near the cathode (2H₂O + 2e⁻ → H₂ + 2OH⁻), so the pH rises.
2. Electrolysis is non-spontaneous, which is true, but this does not explain the rise in pH.

Teacher's Note:
1. The pH rises because NaOH is formed.
2. A true fact need not be the explanation.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. ΔG = −nFEcell, so a positive Ecell means a negative ΔG.
2. A reaction is spontaneous when ΔG is negative, so the reason explains the assertion.

Teacher's Note:
1. ΔG = −nFE connects the two quantities.
2. Positive emf means feasible reaction.

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. E°cell = 0.34 − (−0.76) = 1.10 V, so the assertion is false.
2. The cell potential at standard conditions is not zero. (It is zero only at equilibrium.)

Teacher's Note:
1. Standard conditions of 1 M do not mean the equilibrium state.
2. A cell has zero emf only at equilibrium.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Conductance per unit volume falls on dilution, but Λm = κ × V and the volume containing one mole increases by a larger factor.
2. So molar conductivity rises, and the reason explains the assertion.

Teacher's Note:
1. Λm = κ × 1000 / M.
2. κ falls on dilution but 1/M rises faster.

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. Impure copper is the anode and dissolves, while pure copper is deposited at the cathode.
2. The assertion is true but the reason is false.

Teacher's Note:
1. Oxidation takes place at the anode, so the anode dissolves.
2. Pure metal is always deposited at the cathode.