Questions on Assigning Oxidation Numbers and Changes in Reactions
Multiple-Choice Questions – Oxidation Numbers and Their Changes
1. What is the oxidation number of oxygen in most compounds?
A) +2
B) –1
C) 0
D) –2
E) +1
2. What is the oxidation number of hydrogen in H₂O?
A) 0
B) +1
C) –1
D) –2
E) +2
3. What is the oxidation number of a free element like O₂ or Fe?
A) 0
B) +2
C) –2
D) +1
E) –1
4. Which element is oxidized in the reaction: Zn + Cu²⁺ → Zn²⁺ + Cu?
A) Zn
B) Cu²⁺
C) Cu
D) H
E) Both Zn and Cu
5. What is the oxidation number of sulfur in H₂SO₄?
A) +2
B) +4
C) +6
D) –2
E) 0
6. In which compound does nitrogen have the highest oxidation number?
A) NH₃
B) NO₂
C) N₂O
D) HNO₃
E) N₂
7. What is the oxidation number of chlorine in Cl₂?
A) –1
B) +1
C) 0
D) –2
E) +2
8. In the reaction: 2H₂ + O₂ → 2H₂O, which element is reduced?
A) H
B) O
C) H₂O
D) H₂
E) None
9. What is the oxidation number of carbon in CO₂?
A) 0
B) +2
C) –4
D) +4
E) –2
10. Which of the following species has an oxidation number of –1 for chlorine?
A) ClO₃⁻
B) Cl⁻
C) Cl₂
D) HClO
E) ClO₂
11. Oxidation involves:
A) Gaining electrons
B) Losing electrons
C) No change in electrons
D) Gaining protons
E) Formation of neutral atoms
12. What is the sum of oxidation numbers in a neutral compound?
A) –1
B) 0
C) +1
D) Depends on the compound
E) –2
13. In H₂O₂, what is the oxidation number of each oxygen atom?
A) –2
B) –1
C) 0
D) +1
E) +2
14. In which reaction is iron reduced?
A) Fe → Fe²⁺
B) Fe²⁺ → Fe³⁺
C) Fe³⁺ → Fe²⁺
D) Fe → Fe³⁺
E) Fe²⁺ → Fe
15. What is the oxidation number of phosphorus in PO₄³⁻?
A) +5
B) +3
C) –3
D) +2
E) –1
16. In the reaction: H₂ + Cl₂ → 2HCl, which element is oxidized?
A) H
B) Cl
C) HCl
D) Both H and Cl
E) None
17. What is the oxidation number of Mn in KMnO₄?
A) +2
B) +4
C) +5
D) +7
E) +3
18. In Na₂O, what is the oxidation number of sodium?
A) –2
B) 0
C) +2
D) –1
E) +1
19. In a redox reaction, the substance that is oxidized will:
A) Gain electrons
B) Lose protons
C) Lose electrons
D) Gain neutrons
E) Undergo reduction
20. In the reaction: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻, which is the oxidizing agent?
A) Fe²⁺
B) Fe³⁺
C) Cl⁻
D) Cl₂
E) Fe
Answers and Explanations
1. D – Oxygen is usually –2 in compounds (except peroxides or OF₂).
2. B – Hydrogen is +1 when bonded to nonmetals like oxygen.
3. A – Free elements (e.g., O₂, Fe) have oxidation number 0.
4. A – Zn goes from 0 to +2 → oxidized.
5. C – In H₂SO₄, sulfur is +6.
6. D – In HNO₃, nitrogen is +5, its highest common state.
7. C – Cl₂ is elemental → oxidation number 0.
8. B – O₂ goes from 0 to –2 → reduced.
9. D – O is –2, so carbon must be +4 to balance CO₂.
10. B – Cl⁻ is –1, as in ionic salts.
11. B – Oxidation = loss of electrons.
12. B – The sum of oxidation numbers in a neutral molecule is 0.
13. B – In H₂O₂ (peroxide), O is –1.
14. C – Fe³⁺ → Fe²⁺ is a gain of electrons → reduction.
15. A – O is –2, total –8, charge is –3 → P must be +5.
16. A – H goes from 0 to +1 → oxidized.
17. D – In KMnO₄, Mn is +7.
18. E – Na is always +1 in compounds.
19. C – Oxidized species lose electrons.
20. D – Cl₂ is reduced, so it acts as the oxidizing agent.
- Questions on Oxygen in Reactions — Oxidation and Reduction
- Questions on Redox Reactions and Ionic Compounds
- Questions on Redox Reactions and Molecular Compounds
- Chemistry Questions with Answers Key.
Practical Classroom Applications
Share Social !
