Electrolysis: Quiz

Question 1EasyIn an electrolytic cell, the anode is:

negative, where reduction happenspositive, where oxidation happensnegative, where oxidation happenspositive, where reduction happens

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Answer: positive, where oxidation happens

Oxidation always happens at the anode; in electrolysis it is connected to the positive terminal.

Question 2MediumWhat forms at the cathode when aqueous KNO₃ is electrolysed (inert electrodes)?

potassiumhydrogenoxygennitrogen

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Answer: hydrogen

Potassium is far more reactive than hydrogen, so water is reduced: 2H₂O + 2e⁻ → H₂ + 2OH⁻.

Question 3MediumWhat forms at the anode when aqueous CuSO₄ is electrolysed (inert electrodes)?

coppersulfuroxygenhydrogen

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Answer: oxygen

Sulfate is not discharged; water is oxidized: 2H₂O → O₂ + 4H⁺ + 4e⁻. Copper forms at the cathode.

Question 4MediumWhy is sodium made from molten NaCl and not from salt solution?

molten NaCl is cheaperin solution, water is reduced instead of Na⁺sodium dissolves in molten NaClchlorine does not form in solution

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Answer: in solution, water is reduced instead of Na⁺

Na⁺ is very hard to reduce; in water, hydrogen forms at the cathode instead.

Question 5MediumA current of 1.00 A flows for 965 s. How many moles of electrons pass?

0.0100 mol0.100 mol965 mol1.00 mol

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Answer: 0.0100 mol

Q = 1.00 C/s × 965 s = 965 C; n = 965 C ÷ 96 485 C/mol = 0.0100 mol.

Question 6HardThe same charge deposits 0.0300 mol of Ag. How many moles of Cu would it deposit?

0.0150 mol0.0300 mol0.0600 mol0.0100 mol

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Answer: 0.0150 mol

0.0300 mol Ag needs 0.0300 mol e⁻; Cu²⁺ needs 2 e⁻ per atom, so 0.0300 ÷ 2 = 0.0150 mol Cu.

Question 7EasyIn copper refining, the impure copper is:

the cathodethe anodethe electrolytenot used

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Answer: the anode

The impure anode is oxidized and dissolves (Cu → Cu²⁺ + 2e⁻); pure copper is deposited on the cathode.