NEET Previous Year Questions (2014-2025): Redox Reactions

Table of Contents
1. 2025
2. 2024
3. 2023
4. 2022
5. 2020
6. 2019
7. 2018
8. 2016
9. 2014
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2025

Q1: Consider the following compounds:
 
The oxidation states of the underlined elements in them are, respectively:      (NEET 2025)
(a)  +1, -2 and +4 
(b) +4 , -4 and +6 
(c) +1, -1 and +6 
(d) +2, -2 and +6
Ans: (c)
In KO₂ (potassium superoxide):

• Potassium (K) has an oxidation state of +1.
• Oxygen in superoxide has an oxidation state of -½.

In H₂O₂ (hydrogen peroxide):

• Hydrogen (H) has an oxidation state of +1.
• Oxygen in peroxide has an oxidation state of -1.

In H₂SO₄ (sulfuric acid):

• Hydrogen (H) has an oxidation state of +1.
• Oxygen has an oxidation state of -2.
• To balance the molecule:
• 2(+1) + x + 4(-2) = 0
• x = +6

Therefore, sulfur (S) has an oxidation state of +6.
Therefore, the oxidation states of the underlined elements in KO₂, H₂O₂, and H₂SO₄ are respectively +1, -1, and +6.

2024

Q2: The products A and B obtained in the following reactions, respectively, are    (NEET 2024)

(a)
(b)
(c)
(d) 
Ans: (d)
Let's analyze the given reactions and determine the products A and B.
Reaction with PCl₃:
When an alcohol (ROH) reacts with phosphorus trichloride (PCl3), the hydroxyl group (OH) of the alcohol is replaced by a chloro group (Cl), resulting in the formation of an alkyl chloride (RCl). Additionally, the reaction produces phosphorous acid as a byproduct. This can be represented as:
3ROH + PCl₃ → 3RCl + H₃PO₃
Here, A is H₃PO₃ (phosphorous acid).
Reaction with PCl₅:
When an alcohol reacts with phosphorus pentachloride (PCl₅), it also replaces the hydroxyl group with a chloro group. The reaction differs
slightly in the byproducts; phosphorus oxychloride (POCl₃) and hydrogen chloride (HCl) are produced:
ROH + PCl₅ → RCl + HCl + POCl₃
Here, B is POCl3 (phosphorus oxychloride).
Thus, in the reactions provided:
A = H₃PO₃
B = POCl₃
Matching these findings against the provided options, we see that Option D correctly identifies A as H₃PO₃ and B as POCl₃:
Option D: H₃PO₃ and POCl₃
This is the correct answer.

Q3: Which of the following reactions is not a disproportionation reaction?               (NEET 2024)
(a) 2 F₂_(g) + 2 OH⁻_(aq) → 2 F⁻(aq) + OF₂_(g) + H₂O(l)
(b) Cl₂_(g)+ 2 OH⁻_(aq) → ClO⁻_(aq) + Cl⁻_(aq) + H₂O(l)
(c) 2 NO₂_(g) + 2 OH⁻_(aq)→ NO₃⁻_(aq) + NO₂⁻_(aq) + H₂O(l)
(d) 2 H₂O₂_(aq) → 2 H₂O(l) + O₂_(g)
Ans: (a)
A disproportionation reaction is a type of redox reaction where a single substance undergoes both oxidation and reduction, forming two different products.
Let's analyze each option:
(a) 2 F₂_(g) + 2 OH⁻_(aq) → 2 F⁻_(aq) + OF₂_(g) + H₂O(l)

• This reaction involves fluorine (F₂), which is reduced to F⁻ and oxidized to OF₂. Therefore, fluorine undergoes both oxidation and reduction.
• This is a disproportionation reaction.

(b) Cl₂_(g) + 2 OH⁻_(aq) → ClO⁻_(aq) + Cl⁻(aq) + H₂O(l)

• In this reaction, chlorine (Cl₂) is reduced to Cl⁻ and oxidized to ClO⁻.
• This is a disproportionation reaction.

(c) 2 NO₂_(g) + 2 OH⁻_(aq)→ NO₃⁻_(aq) + NO₂⁻_(aq) + H₂O(l)

• In this reaction, NO₂ undergoes both oxidation and reduction to form NO₃⁻ and NO₂⁻.
• This is a disproportionation reaction.

(d) 2 H₂O₂_(aq)→ 2 H₂O(l) + O₂_(g)
In this reaction, hydrogen peroxide (H₂O₂) decomposes into water (H₂O) and oxygen (O₂). Here, hydrogen peroxide undergoes reduction to form H₂O and oxidation to form O₂, but since it does not involve two different products from a single reactant, it is not a disproportionation reaction. It's a simple decomposition reaction.
Conclusion:The reaction in (a) is the disproportionation reaction, so the correct answer is (d) 2 H₂O₂_(aq) → 2 H₂O(l) + O₂_(g).

Q4: Given the following reaction involving manganese (Mn):          (NEET 2024)
3 MnO₄²⁻ + 4 H⁺ → 2 MnO₄⁻ + MnO₂ + 2 H₂O
MnO₄²⁻ + 4 H⁺ → 2 MnO₄⁻ + MnO₂ + 2 H₂O
Which oxidation states of manganese are not observed in the above reaction?
A. +6
B. +2
C. +4
D. +7
E. +3
Mark the most appropriate answer from the options given below:
(a) D and E only
(b) B and D only
(c) A and B only
(d) B and E only
Ans: (d)
3 MnO₄²⁻ + 4 H⁺ → 2 MnO₄⁻ + MnO₂ + 2 H₂O

Step 1: Assign oxidation states in the reactants:
MnO₄²⁻: Manganese in MnO₄²⁻ is in the +6 oxidation state.

• Oxygen is typically -2, and since the overall charge on the ion is -2, the charge on Mn must be +6 to balance the charges.
• H⁺: This is a proton, so its oxidation state is +1.

Step 2: Assign oxidation states in the products:
MnO₄⁻: Manganese in MnO₄⁻ is in the +7 oxidation state.

• Oxygen is -2, and since the overall charge on the ion is -1, the charge on Mn must be +7 to balance the charges.

MnO₂: Manganese in MnO₂ is in the +4 oxidation state.

• Oxygen is -2, and for the compound to be neutral, Mn must have a charge of +4.

• H₂O: Water, which does not affect the oxidation state of manganese.

Step 3: Identify which oxidation states are missing:

• +6 is observed in MnO₄²⁻.
• +7 is observed in MnO₄⁻.
• +4 is observed in MnO₂.
• +2 and +3 are not present in the reaction.

Conclusion:The +2 and +3 oxidation states of manganese are not observed in the given reaction.
The correct answer is (d) B and E only.

2023

Q1: On balancing the given redox reaction,      (NEET 2023)

the coefficients a, b and c are found to be, respectively- 
(a) 1, 3, 8 
(b) 3, 8, 1 
(c) 1, 8, 3
(d) 8, 1, 3 

Ans: (a)
Using Ion electron method:

Balancing Redox Reactions by Oxidation Number Method

2022

Q1: Which of the following reactions is a decomposition redox reaction?     (NEET 2022 Phase 2)
(a) P₄(s) + 3OH^$-$(aq) + 3H₂O(l) $\to$ PH₃(g) + 3H₂PO$2-$(aq)
(b) 2Pb(NO₃)₂(s) $\to$ 2PbO(s) + 4NO₂(g) + O₂(g)
(c) N₂(g) + O₂(g) $\to$ 2NO(g)
(d) Cl₂(g) + 2OH^$-$(aq) $\to$ ClO^$-$(aq) + Cl^$-$(aq) + 4H₂O(l)
Ans: (b)
Decomposition redox reaction leads to breakdown of a compound into two or more compounds at least one of which must be in the elemental state with change in oxidation number.

2020

Q1: What is the change in oxidation number of carbon in the following reaction?
CH₄(g) + 4Cl₂(g) $\to$ CCl₄(l) + 4HCl(g) (NEET 2020)
(a) 0 to + 4
(b) -4 to + 4
(c) 0 to - 4
(d) + 4 to + 4
Ans: (b)

Change in oxidation state of carbon is from - 4 to + 4

2019

Q1: Which of the following reactions are disproportionation reaction?    (NEET 2019)
(a) 2Cu⁺ → Cu²⁺ + Cu
(b)
(c)
(d)
Select the correct option from the following
(a) (a) and (b) only
(b) (a), (b) and (c)
(c) (a), (c) and (d)
(d) (a) and (d) only
Ans: (a)
In a disproportionation reaction, same substance undergoes oxidation (increase in oxidation number) and reduction (decrease in oxidation number forming two different products.

2018

Q1: The correct order of N-compounds in its decreasing order of oxidation states is    (NEET 2018)
(a) HNO₃, NO, N₂, NH₄Cl
(b) HNO₃, NO, NH₄Cl, N₂
(c) HNO₃, NH₄Cl, NO, N₂
(d) NH₄Cl, N2, NO, HNO₃

Ans: (a)

Q2: For the redox reaction
MnO₄^- + C₂O₄^2-+ H⁺ →Mn²⁺ + CO₂ + H₂O
the correct coefficients of the reactants for the balanced equation are        (NEET 2018)
MnO₄-     C₂O₄ ^2-      H⁺
(1) 16          5               2
(2) 2           5              16
(3) 2          16              5
(4) 5          16              2
(a) 1
(b) 2
(c) 3
(d) 4

Ans: (b)
The correct balanced equation is
2MnO₄^- + 5C₂O₄ ^2- + 16H⁺ $\to$ 2Mn²⁺ + 10CO₂ + 8H₂O 

2016

Q1: Hot concentrated sulphate acid is a moderately strong oxidizing agent. Which of the following reactions does not show oxidizing behaviour?
(a) Cu + 2H₂SO₄ $\to$ CuSO₄ + SO₂ + 2H₂O
(b) S + 2H₂SO₄ $\to$ 3SO₂ + 2H₂O
(c) C + 2H₂SO₄ $\to$ CO₂ + 2SO₂ + 2H₂O
(d) CaF₂ + H₂SO₄ $\to$ CaSO₄ + 2HF       (NEET 2016 Phase 2 )
Ans: (d)
CaF₂ + H₂SO₄ $\to$ CaSO₄ + 2HF
Here, the oxidation state of every atom remains the same so, it is not a redox reaction. 

2014

Q1: The pair of compounds that can exist together is :    (NEET 2014)
(a) HgCl₂ ,SnCl₂
(b) FeCl₂ ,KI
(c) FeCl₃,SnCl₂
(d) FeCl₂ ,SnCl₂ 
Ans: (a)
The compounds with lower oxidation number and which cannot reduce by one another can exist together. Thus, FeCl₂ and SnCl₂ can exist together as Fe²⁺ can not be reduced by Sn²⁺.

Q2: In acidic medium H₂O₂ changes Cr₂O₇ to CrO₅ which has two -O-O- bonds. Oxidation state of Cr in CrO₅ is : (2014)
(a) +6
(b) -10
(c) +5
(d) +3
Ans: (a)
CrO₅ has butterfly structure having two peroxo bonds.

Peroxo oxygen has -1 oxidation state.
Let oxidation state of Cr be 'x'
CrO₅ : x + 4(-1) + 1 (-2) = 0 ⇒ x = + 6