Daily Olympiad: Chemistry - Redox Reactions [20260512] – 6 Practice Problems & Printable PDF

1. In the reaction 2Fe³⁺ + Sn²⁺ → 2Fe²⁺ + Sn⁴⁺, what is the equivalent weight of Fe³⁺ if the molecular weight is 55.85 g/mol?

A. 27.925 g/equivalent B. 18.62 g/equivalent C. 11.17 g/equivalent D. 55.85 g/equivalent

Solution

Correct: D

The oxidation state of Fe decreases from +3 to +2, gaining 1 electron per Fe ion. The equivalent weight is molecular weight divided by n-factor (1). Thus, 55.85/1 = 55.85 g/equivalent. Common mistake: Confusing n-factor with change in oxidation state for multiple atoms.

2. Which of the following reactions will not occur under standard conditions? Given E° values: Fe³⁺/Fe²⁺ = +0.77 V, I₂/I⁻ = +0.54 V.

A. Fe³⁺ + I⁻ → Fe²⁺ + I₂ B. Fe³⁺ + Br⁻ → Fe²⁺ + Br₂ C. Fe³⁺ + Cl⁻ → Fe²⁺ + HClO3 D. Fe³⁺ + S²⁻ → Fe²⁺ + S

Solution

Correct: B

A spontaneous reaction requires E°cell > 0. For Fe³⁺ + I⁻: E°cell = 0.77 - 0.54 = +0.23 V (spontaneous). For Br⁻/Br₂: E°cell = 0.77 - 1.07 = -0.30 V (non-spontaneous). Option C is invalid because Cl⁻ reduction to Cl₂ has E°=1.36 V, making E°cell negative.

3. What is the oxidation state of Cr in [Cr(NH3)4Cl2]⁺?

A. +3 B. +4 C. +6 D. +2

Solution

Correct: A

Let Cr oxidation state be x. NH3 is neutral, Cl⁻ is -1 each. Total charge: x + 4(0) + 2(-1) = +1 → x = +3. Common error: Forgetting that ammonia is a neutral ligand.

4. Balancing the redox reaction MnO4⁻ + H2O2 → Mn²⁺ + O2 (in acidic medium), what are the stoichiometric coefficients of H2O2 and MnO4⁻?

A. 5 and 2 B. 2 and 5 C. 3 and 2 D. 1 and 2

Solution

Correct: A

Oxidation (H₂O₂ → O2: O oxidation state increases from -1 to 0. Mn is reduced from +7 to +2. Balanced equation: 2MnO4⁻ + 5H2O2 + 6H+ → 2Mn²⁺ + 5O2 + 8H2O. Ratio: MnO4⁻:5, H2O2:2.

5. In the disproportionation of H2O2, which statement is incorrect?

A. Oxidation state of O changes from -1 to -2 and 0 B. Reaction is endothermic C. Used in rocket propellants D. Acts as both oxidizing and reducing agent

Solution

Correct: B

The disproportionation of H2O2 is exothermic and acts as both oxidizing and reducing agent. Decomposition into H2O and O2 (O changes -1 → -2 and 0). All correct statements are in options A, C, D except option B.

6. What is the change in pH when 1 mole of Fe³⁺ is reduced to Fe²⁺ in 1 L solution if 0.5 moles are reduced?

A. Remains same B. Increases by 0.3 C. Decreases by 0.3 D. Increases by 0.9

Solution

Correct: A

Fe³⁺ + e⁻ → Fe²⁺ (no H+ or OH- involved). Since H+ concentration isn’t affected directly, pH remains unchanged. Common confusion: Assuming redox affects water autoionization unless specified.