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Oxidation/reduction concepts and oxidation numbers: NEET MCQs with answers

58 questions on this topic of Redox Reactions and Electrochemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following nitrogen oxides contains nitrogen in the same oxidation state as in nitrous acid (HNO₂)?

  1. N₂O
  2. NO
  3. N₂O₅
  4. N₂O₃
Show answer and explanation

Answer: (D) N₂O₃

In HNO₂, the oxidation number of nitrogen is +1 + x + 2(-2) = 0 ⇒ x = +3. In dinitrogen trioxide (N₂O₃), 2x + 3(-2) = 0 ⇒ x = +3.

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2. Given the following two statements: Statement I: Fluorine exhibits both positive and negative oxidation states in its compounds. Statement II: Fluorine is the most electronegative element and lacks vacant d-orbitals in its valence shell. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (C) Statement I is incorrect but Statement II is correct

Fluorine is the most electronegative element and does not possess d-orbitals in its valence shell; therefore, it only shows a -1 oxidation state in its compounds (and 0 in F₂), never positive oxidation states. Thus, Statement I is incorrect, and Statement II is correct.

hard

3. Calculate the oxidation state of iodine in periodic acid (H₅IO₆):

  1. +5
  2. +1
  3. +3
  4. +7
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Answer: (D) +7

In paraperiodic acid (H₅IO₆), hydrogen has an oxidation state of +1 and oxygen has -2. Setting up the equation: 5(+1) + x + 6(-2) = 0, which gives +5 + x - 12 = 0, so x = +7.

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4. In the molecule carbonyl fluoride (COF₂), the oxidation states of carbon, oxygen, and fluorine are respectively:

  1. +4, +2, -3
  2. +2, -2, 0
  3. +4, -2, -1
  4. +2, -1, -1
Show answer and explanation

Answer: (C) +4, -2, -1

Fluorine is the most electronegative element, with an oxidation state of -1. Oxygen has an oxidation state of -2. Setting up the equation: x + (-2) + 2(-1) = 0 gives x = +4 for carbon.

easy

5. Which of the following processes does not represent a redox reaction?

  1. Zn + CuSO₄ → ZnSO₄ + Cu
  2. 2KClO₃ → 2KCl + 3O₂
  3. BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
  4. 2H₂ + O₂ → 2H₂O
Show answer and explanation

Answer: (C) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

The reaction between BaCl₂ and H₂SO₄ is a double displacement (precipitation) reaction. None of the participating atoms undergo a change in their oxidation numbers: Ba(+2), Cl(-1), H(+1), S(+6), and O(-2) remain unchanged throughout the reaction.

hard

6. Given below are two statements: Statement I: In perchloric acid (HClO₄), chlorine exhibits its maximum possible oxidation state of +7, hence it can act only as an oxidizing agent. Statement II: In hydrogen peroxide (H₂O₂), oxygen has an oxidation state of -1, so it can act neither as an oxidizing agent nor as a reducing agent. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
Show answer and explanation

Answer: (C) Statement I is true but Statement II is false

Statement I is true because chlorine in HClO₄ has its maximum oxidation number (+7) and can only decrease its oxidation state (gain electrons), acting solely as an oxidant. Statement II is false because the -1 oxidation state of oxygen in H₂O₂ is intermediate between 0 (in O₂) and -2 (in H₂O), allowing H₂O₂ to act as both an oxidizing and a reducing agent.

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7. Given below are two statements: Statement I: Disproportionation reactions are a special type of redox reaction in which an element in one oxidation state is simultaneously oxidized and reduced. Statement II: Hypochlorous acid (HClO) cannot undergo a disproportionation reaction because chlorine is in its lowest oxidation state. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
Show answer and explanation

Answer: (C) Statement I is true but Statement II is false

Statement I is the correct definition of disproportionation. Statement II is false because in HClO, chlorine has an oxidation state of +1, which is intermediate between its minimum (-1) and maximum (+7) states; hence, it readily disproportionates into Cl⁻ and ClO₃⁻.

easy

8. What is the oxidation number of carbon in glucose (C₆H₁₂O₆)?

  1. +2
  2. -1
  3. 0
  4. +1
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Answer: (C) 0

In glucose (C₆H₁₂O₆), applying standard oxidation state rules gives: 6(x) + 12(+1) + 6(-2) = 0. This simplifies to 6x + 12 - 12 = 0, which yields x = 0. Hence, the average oxidation number of carbon in glucose is 0.

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9. Consider the following statements regarding oxidation numbers: Statement I: The oxidation number of an element in its free or uncombined state is always zero. Statement II: Hydrogen always exhibits an oxidation state of +1 in all of its binary compounds. Choose the correct option:

  1. Statement I is correct, but Statement II is incorrect
  2. Both Statement I and Statement II are correct
  3. Both Statement I and Statement II are incorrect
  4. Statement I is incorrect, but Statement II is correct
Show answer and explanation

Answer: (A) Statement I is correct, but Statement II is incorrect

Statement I is correct by definition of oxidation numbers for elemental forms (e.g., H₂, O₂, P₄). Statement II is incorrect because in binary metallic hydrides such as NaH, CaH₂, and LiH, hydrogen exhibits an oxidation state of -1.

easy

10. What is the oxidation number of chromium in chromium peroxide, CrO₅?

  1. +10
  2. +3
  3. +4
  4. +6
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Answer: (D) +6

CrO₅ has a butterfly structure containing two peroxo groups (-O-O-) and one oxo group (=O). The four peroxo oxygen atoms each have an oxidation state of -1, while the oxo oxygen has -1 × 2 = -2. Therefore, x + 4(-1) + 1(-2) = 0, giving x = +6.

easy

11. What is the oxidation number of phosphorus in hypophosphorous acid (H₃PO₂)?

  1. +5
  2. +2
  3. +3
  4. +1
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Answer: (D) +1

In H₃PO₂, hydrogen is bonded to oxygen and phosphorus with an oxidation state of +1 each, and oxygen is -2. Thus, 3(+1) + x + 2(-2) = 0, which gives x = +1. Structurally, phosphorus is bonded to one -OH, two -H atoms, and one =O atom.

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12. The oxidation number of iron in brown ring complex [Fe(H₂O)₅(NO)]SO₄ is:

  1. 0
  2. +2
  3. +3
  4. +1
Show answer and explanation

Answer: (D) +1

In the brown ring complex, nitric oxide coordinates as the nitrosonium cation (NO⁺). Let the oxidation state of Fe be x: x + 5(0) + 1(+1) - 2 = 0, which yields x = +1.

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