Oxidation Numbers and Redox Reactions: Quiz

Oxidation Numbers and Redox Reactions: Quiz

7 questions

1. Question 1EasyWhat is the oxidation number of N in HNO₃?

   A+3B+5C−3D+6
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   Answer: +5

   (+1) + x + 3(−2) = 0, so x = +5.

2. Question 2EasyIn Zn + Cu²⁺ → Zn²⁺ + Cu, which species is the oxidizing agent?

   AZnBCu²⁺CZn²⁺DCu
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   Answer: Cu²⁺

   Cu²⁺ gains electrons (+2 → 0), so it is reduced and acts as the oxidizing agent. Zn is the reducing agent.

3. Question 3MediumWhat is the oxidation number of Cr in Cr₂O₇²⁻?

   A+7B+6C+12D+3
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   Answer: +6

   2x + 7(−2) = −2, so 2x = +12 and x = +6. The answer +12 is the total for both Cr atoms.

4. Question 4MediumWhich reaction is NOT a redox reaction?

   A2Na + Cl₂ → 2NaClBZn + 2HCl → ZnCl₂ + H₂CNaOH + HCl → NaCl + H₂OD2H₂O₂ → 2H₂O + O₂
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   Answer: NaOH + HCl → NaCl + H₂O

   In neutralization no oxidation number changes (Na +1, O −2, H +1, Cl −1 throughout). The others all transfer electrons.

5. Question 5HardWhich is the correctly balanced equation?

   AFe³⁺ + Cu → Fe²⁺ + Cu²⁺B2Fe³⁺ + Cu → 2Fe²⁺ + Cu²⁺CFe³⁺ + 2Cu → Fe²⁺ + 2Cu²⁺D2Fe³⁺ + 2Cu → 2Fe²⁺ + Cu²⁺
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   Answer: 2Fe³⁺ + Cu → 2Fe²⁺ + Cu²⁺

   Cu loses 2 electrons and each Fe³⁺ gains 1, so 2 Fe³⁺ are needed. Charge: +6 on the left, +4 + 2 = +6 on the right.

6. Question 6MediumIn MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺, how many electrons are transferred?

   A1B5C7D8
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   Answer: 5

   Mn goes from +7 to +2 (gains 5 electrons); each of the 5 Fe²⁺ loses 1 electron.

7. Question 7EasyWhat is the oxidation number of O in H₂O₂?

   A−2B−1C0D+1
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   Answer: −1

   H₂O₂ is a peroxide: 2(+1) + 2x = 0, so x = −1.