Question 1

In the reaction 2KMnO₄ + 3H₂O₂ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 5O₂ + 8H₂O, what is the oxidation state change of manganese?

Accepted Answer

Mn in KMnO₄ has oxidation state +7, and in MnSO₄ it has +2, a reduction of 5 electrons per Mn atom.

Question 2

Which species acts as the reducing agent in the reaction: Cl₂ + 2KBr → 2KCl + Br₂?

Accepted Answer

The reducing agent is oxidized; Br⁻ in KBr is oxidized from −1 to 0 in Br₂, so KBr is the reducing agent.

Question 3

In the redox reaction 3Cu + 8HNO₃(dilute) → 3Cu(NO₃)₂ + 2NO + 4H₂O, how many electrons are transferred per mole of Cu oxidized?

Accepted Answer

Cu is oxidized from 0 to +2, losing 2 electrons per Cu atom; total 6 electrons transferred among 3 Cu atoms means 2 electrons per Cu.

Question 4

What is the oxidation state of sulfur in H₂S₂O₃?

Accepted Answer

In H₂S₂O₃, using 2(+1) + 2(x) + 3(−2) = 0, we get 2x = 4, so average oxidation state is +2, but sulfur exists in mixed oxidation states with one at +4 and one at +2, averaging +3.

Question 5

In the reaction Fe²⁺ + MnO₄⁻ + H⁺ → Fe³⁺ + Mn²⁺ + H₂O, what is the ratio of Fe²⁺ to MnO₄⁻ that must be used for complete redox balance?

Accepted Answer

Mn in MnO₄⁻ gains 5 electrons (+7 to +2), and each Fe²⁺ loses 1 electron; therefore 5 Fe²⁺ are needed per MnO₄⁻.

Question 6

In the redox reaction: 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O, what is the oxidation state change of chlorine in HCl that gets oxidized?

Accepted Answer

Some Cl⁻ from HCl is oxidized to Cl₂ (0 oxidation state), changing from -1 to 0.

Question 7

In the reaction: H₂SO₄ + C → CO₂ + SO₂ + H₂O, identify the oxidation state of sulfur in H₂SO₄ and SO₂ respectively.

Accepted Answer

In H₂SO₄, sulfur is +6; in SO₂, sulfur is +4—wait, rechecking: SO₂ has S at +4, not +2; correct answer is +6 to +4, but option shows +6 and +2 as reduction; reviewing: SO₂ is +4, making A incorrect; actual answer should be B (+6 to +4).

Question 8

Which species acts as a reducing agent in the reaction: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O?

Accepted Answer

Cl⁻ is oxidized from -1 to 0 in Cl₂, so it donates electrons and acts as the reducing agent.

Question 9

In the half-reaction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, how many electrons are transferred and what is reduced?

Accepted Answer

Mn oxidation state changes from +7 (in MnO₄⁻) to +2 (in Mn²⁺), gaining 5 electrons.

Question 10

Calculate the oxidation state of nitrogen in NH₄⁺ and NO₃⁻.

Accepted Answer

In NH₄⁺, N is -3 (H is +1, total charge +1); in NO₃⁻, N is +5 (O is -2 each, total charge -1).

Question 11

In a redox reaction, if an element loses electrons, it is said to be:

Accepted Answer

Loss of electrons is oxidation, and the species donating electrons is a reducing agent.

Question 12

In the reaction: Fe²⁺ + MnO₄⁻ + H⁺ → Fe³⁺ + Mn²⁺ + H₂O (unbalanced), identify the oxidizing agent.

Accepted Answer

MnO₄⁻ is reduced from +7 to +2, so it gains electrons and acts as the oxidizing agent.

Question 13

What is the number of electrons transferred when 1 mole of K₂Cr₂O₇ reacts as an oxidizing agent in acidic medium, converting all chromium to Cr³⁺?

Accepted Answer

In K₂Cr₂O₇, Cr is +6; conversion to Cr³⁺ involves 3 electrons per Cr atom; 2 Cr atoms = 6 electrons total transferred.

Question 14

In the redox reaction: 2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 5Cl₂ + 8H₂O, what is the oxidation state change of chlorine in HCl that gets oxidized?

Accepted Answer

In HCl, chlorine has oxidation state -1; the chlorine that undergoes oxidation changes to 0 in Cl₂ gas produced.

Question 15

When potassium dichromate (K₂Cr₂O₇) oxidizes ferrous ion (Fe²⁺) to ferric ion (Fe³⁺) in acidic medium, one Cr₂O₇²⁻ ion oxidizes how many Fe²⁺ ions?

Accepted Answer

Cr₂O₇²⁻ goes from +6 to +3 (6 electrons gained); each Fe²⁺ loses 1 electron, so 6 Fe²⁺ ions are oxidized per Cr₂O₇²⁻.

Question 16

In the reaction: 3Cu + 8HNO₃(dilute) → 3Cu(NO₃)₂ + 2NO↑ + 4H₂O, which species acts as both oxidizing and reducing agent?

Accepted Answer

HNO₃ acts as an oxidizing agent (N: +5 → +2 in NO) and some HNO₃ provides NO₃⁻ ions that do not change oxidation state, while the nitrogen undergoes reduction.

CDS Redox Reactions

Redox Reactions — Practice Questions