ChemistryCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2024-25: question 22

From the official CBSE sample question paper for 2024-25, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerElectrochemistry

a. Write down the reaction occurring on two inert electrodes when electrolysis of copper chloride is done. What will happen if a concentrated solution of copper sulphate is replaced with copper chloride? (2)
b. Write an expression for the molar conductivity of aluminium sulphate at infinite dilution according to Kohlrausch law. (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

Open one hint at a time and try again before the next.

Hint 1

At the cathode, which is easier to reduce: Cu²⁺ or H₂O?

Hint 2

At the anode, compare what can be oxidised when Cl⁻ is present and when only SO₄²⁻ and water are present.

Hint 3

For (b), count how many cations and anions one formula unit gives.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Electrolysis of aqueous $C u C l_{2}$ with inert (Pt) electrodes:
Cathode (–): $C u^{2 +} (a q) + 2 e^{-} \to C u (s)$ (copper is deposited)
Anode (+): $2 C l^{-} (a q) \to C l_{2} (g) + 2 e^{-}$ (chlorine gas evolves)
1 mark
With concentrated $C u S O_{4}$ instead: copper is still deposited at the cathode, $C u^{2 +} + 2 e^{-} \to C u (s)$ .
There is no $C l^{-}$ , so $C l_{2}$ is no longer formed. In concentrated solution, sulphate ions are oxidised at the anode to peroxodisulphate: $2 S O_{4}^{2 -} (a q) \to S_{2} O_{8}^{2 -} (a q) + 2 e^{-}$ . In dilute solution, water is oxidised to $O_{2}$ instead.
1 mark
(b) By Kohlrausch's law, each formula unit gives 2 $A l^{3 +}$ and 3 $S O_{4}^{2 -}$ ions:
$Λ_{m}^{\circ} [A l_{2} (S O_{4})_{3}] = 2 λ^{\circ} (A l^{3 +}) + 3 λ^{\circ} (S O_{4}^{2 -})$
1 mark

Answer: (a) CuCl₂: Cu at cathode, Cl₂ at anode; conc. CuSO₄: Cu at cathode, SO₄²⁻ → S₂O₈²⁻ at anode (no Cl₂); (b) Λ°m = 2λ°(Al³⁺) + 3λ°(SO₄²⁻)

Where the marks are

Cathode and anode reactions for CuCl₂ (1 mark)
For conc. CuSO₄: Cu at cathode and anode product S₂O₈²⁻, no chlorine (1 mark)
Kohlrausch expression with coefficients 2 and 3 (1 mark)

Revise the chapter

Chapter: Electrochemistry (notes, formulas, where marks are lost and every past question)
Topic: Conductivity, Molar Conductivity and Kohlrausch's Law

Practise this chapterBoard-style questions from Electrochemistry, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2024-25: every question, by section.

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CBSE Class 12 Chemistry sample paper 2024-25: question 22

Hints

Step-by-step solution

Revise the chapter

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