Questions on Oxygen in Reactions — Oxidation and Reduction
Multiple-Choice Questions: Oxygen in Reactions — Oxidation and Reduction
1. What is oxidation in terms of oxygen?
A) Loss of oxygen
B) Gain of oxygen
C) Loss of electrons
D) Gain of hydrogen
E) Formation of water
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2. In a redox reaction, the substance that is oxidized:
A) Gains electrons
B) Loses electrons
C) Gains oxygen
D) Both B and C
E) Both A and C
3. What is reduction in terms of oxygen?
A) Loss of oxygen
B) Gain of electrons
C) Gain of hydrogen
D) All of the above
E) None of the above
4. In the reaction: 2Mg + O₂ → 2MgO, which species is reduced?
A) Mg
B) O₂
C) MgO
D) Both Mg and O₂
E) None
5. In the same reaction (2Mg + O₂ → 2MgO), what happens to magnesium?
A) It is reduced
B) It gains electrons
C) It loses electrons
D) It forms hydrogen gas
E) It undergoes no change
6. Which of the following is an example of an oxidation reaction?
A) Rusting of iron
B) Photosynthesis
C) Fermentation
D) Electrolysis of water
E) Melting of ice
7. What is the oxidation number of oxygen in most of its compounds?
A) 0
B) +2
C) –2
D) +1
E) –1
8. In which compound does oxygen have an oxidation number different from –2?
A) H₂O
B) CO₂
C) Na₂O
D) H₂O₂
E) SO₂
9. The term "redox" is a combination of which two processes?
A) Reduction and oxidation
B) Reactivity and decomposition
C) Respiration and oxidation
D) Radiolysis and hydrolysis
E) Reaction and diffusion
10. In combustion reactions involving hydrocarbons, oxygen is typically:
A) Oxidized
B) Reduced
C) Decomposed
D) Evaporated
E) Neutral
11. Which of the following best defines a reducing agent?
A) A substance that gains oxygen
B) A substance that loses electrons
C) A substance that is oxidized
D) Both B and C
E) Both A and C
12. In the reaction CuO + H₂ → Cu + H₂O, which substance is oxidized?
A) CuO
B) H₂
C) Cu
D) H₂O
E) Both CuO and H₂
13. In the above reaction, what is the role of CuO?
A) Oxidizing agent
B) Reducing agent
C) Catalyst
D) Base
E) Acid
14. What is the oxidation state of oxygen in the peroxide ion (O₂²⁻)?
A) 0
B) –1
C) –2
D) +1
E) +2
15. Which of the following statements about oxidation numbers is correct?
A) The oxidation number of a free element is always +1
B) Oxygen always has an oxidation number of 0
C) Hydrogen always has an oxidation number of –1
D) The oxidation number of oxygen is usually –2
E) The oxidation number of a compound is always negative
16. What is the oxidizing agent in the reaction: 2H₂ + O₂ → 2H₂O?
A) H₂
B) O₂
C) H₂O
D) None
E) Both H₂ and O₂
17. In cellular respiration, oxygen acts as:
A) A reducing agent
B) A product
C) An oxidizing agent
D) An enzyme
E) A neutral molecule
18. Which of the following reactions involves oxidation of an element?
A) 2Na + Cl₂ → 2NaCl
B) H₂ + Cl₂ → 2HCl
C) C + O₂ → CO₂
D) Fe²⁺ → Fe³⁺ + e⁻
E) All of the above
19. Which of these does not involve redox?
A) Precipitation of silver chloride
B) Rusting of iron
C) Combustion of methane
D) Reaction of Zn with HCl
E) Electrolysis of water
20. In redox reactions involving oxygen, which statement is generally true?
A) The element that gains oxygen is reduced
B) The element that gains oxygen is oxidized
C) The element that loses oxygen is oxidized
D) Oxygen always acts as a reducing agent
E) Oxygen loses electrons
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Answers with Explanations
1. B – Oxidation involves gain of oxygen.
2. D – Oxidation = loss of electrons and/or gain of oxygen.
3. D – Reduction = loss of oxygen, gain of electrons, or gain of hydrogen.
4. B – O₂ is reduced as it gains electrons and becomes oxide (O²⁻).
5. C – Magnesium is oxidized as it loses electrons.
6. A – Rusting is an oxidation process involving oxygen gain.
7. C – Oxygen is usually –2 in its compounds.
8. D – In H₂O₂, oxygen is –1, not –2.
9. A – “Redox” = Reduction + Oxidation.
10. B – Oxygen is reduced (gains electrons) in combustion.
11. D – A reducing agent loses electrons and is oxidized.
12. B – H₂ is oxidized to H⁺ in H₂O.
13. A – CuO is the oxidizing agent (it accepts electrons).
14. B – In peroxides, oxygen is –1.
15. D – Oxygen usually has oxidation number –2.
16. B – O₂ is the oxidizing agent (gains electrons).
17. C – In respiration, oxygen accepts electrons, acting as oxidizing agent.
18. E – All involve electron transfer → all are redox.
19. A – Precipitation reactions like Ag⁺ + Cl⁻ → AgCl do not involve oxidation states changing.
20. B – Gaining oxygen = oxidation.
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