NEET Previous Year Questions (2014-2024): Redox Reactions | Chemistry Class 11 PDF Download

2024

Q2: The products A and B obtained in the following reactions, respectively, are

(a)
(b)
(c)
(d)      (NEET 2024)
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.

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