Chemical Reactions and Redox Reactions – NDA Chemistry PYQs

Q1. Match List I with List II (reaction types):
A. CaO + H₂O → Ca(OH)₂ — B. CaCO₃ → CaO + CO₂ — C. Pb + CuCl₂ → PbCl₂ + Cu — D. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

1. Single displacement — 2. Double displacement — 3. Combination — 4. Decomposition

Answer: A-3, B-4, C-1, D-2
CaO + H₂O = two substances combining into one = combination (3). CaCO₃ → CaO + CO₂ = one substance breaking into two = decomposition (4). Pb + CuCl₂ = lead displaces copper = single displacement (1). Na₂SO₄ + BaCl₂ = partners exchange = double displacement (2).
[NDA 2025-II]

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Q2. Reaction of quicklime (CaO) with water to produce slaked lime (Ca(OH)₂) is an example of:
(a) Displacement reaction (b) Endothermic reaction (c) Decomposition reaction (d) Exothermic reaction

Answer: (d) Exothermic reaction
CaO + H₂O → Ca(OH)₂ releases a large amount of heat — the mixture becomes very hot and a hissing sound is produced. It is also a combination reaction (two substances combining). Exothermic means heat is released to the surroundings.
[NDA 2021-I]

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Q3. Which one of the following is an endothermic reaction?
(a) CH₄ + 2O₂ → CO₂ + 2H₂O
(b) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
(c) N₂ + 3H₂ → 2NH₃
(d) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

Answer: (d) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Decomposition of lead nitrate requires heat to break the compound apart. It is endothermic — heat is absorbed from surroundings. Options (a), (b), and (c) are all exothermic — combustion of methane, respiration of glucose, and formation of ammonia all release heat.
[NDA 2025-II]

Q4. In the following redox reaction, which compound is acting as a reducing agent?
N₂H₄ + 2H₂O₂ → N₂ + 4H₂O
(a) H₂O₂ (b) N₂H₄ (c) N₂ (d) H₂O

Answer: (b) N₂H₄
N₂H₄ (hydrazine) loses hydrogen — it is oxidised → becomes N₂. A reducing agent gets oxidised. H₂O₂ gains hydrogen — it is reduced → becomes H₂O. H₂O₂ is the oxidising agent. N₂ and H₂O are products — not agents.
[NDA 2025-II]

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Q5. Consider the following redox statements:
I. Fe + 2HCl → FeCl₂ + H₂ → Fe is reductant
II. Zn + CuSO₄ → ZnSO₄ + Cu → Cu is being reduced
III. Br₂ + 2I⁻ → 2Br⁻ + I₂ → Br₂ is reductant
IV. CH₄ + 2O₂ → CO₂ + 2H₂O → C atom in CH₄ is oxidised
Which are correct?
(a) I and II only (b) II, III and IV (c) I, II and IV (d) I, III and IV

Answer: (c) I, II and IV
Statement I ✓ — Fe loses electrons (0 to +2) = reductant.
Statement II ✓ — Cu²⁺ gains electrons → Cu⁰ = Cu is reduced.
Statement III ✗ — Br₂ GAINS electrons (0 to −1) = Br₂ is the oxidant, NOT reductant. I⁻ loses electrons = I⁻ is reductant.
Statement IV ✓ — C in CH₄ = −4, C in CO₂ = +4. Oxidation state increases = oxidised.
[NDA 2025-II]

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Q6. The following equation is an example of a redox reaction in which Cl₂ is the oxidising agent and FeBr₃ is the reducing agent:
2FeBr₃ + 3Cl₂ → 2FeCl₃ + 3Br₂
Which one of the following statements is INCORRECT for this redox reaction?
(a) Oxidising agents are themselves reduced
(b) Reducing agents gain or appear to gain electrons
(c) Reducing agents are themselves oxidised
(d) Oxidising agents oxidise other substances

Answer: (b) Reducing agents gain or appear to gain electrons
This is the false statement. Reducing agents LOSE electrons (they are oxidised). Oxidising agents GAIN electrons (they are reduced). Option (a) is correct — Cl₂ gets reduced. Option (c) is correct — FeBr₃ gets oxidised. Option (d) is correct — Cl₂ causes oxidation.
[NDA 2012-I]

Q7. Which one of the following is NOT an example of a redox reaction?
(a) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
(b) 2NaH → 2Na + H₂
(c) 4Fe + 3O₂ → 2Fe₂O₃
(d) CuSO₄ + Zn → Cu + ZnSO₄

Answer: (a) AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl
In this hydrolysis reaction, no oxidation states change. Al remains +3, Cl remains −1, H remains +1, O remains −2. No electron transfer occurs. In options (b), (c), and (d) — oxidation states change, making them redox reactions.
[NDA 2022-I]

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Q8. The poisonous nature of carbon monoxide (CO) is due to its:
(a) Insolubility in water
(b) Ability to form a complex with haemoglobin
(c) Ability to reduce some metal oxides
(d) Property of having one sigma bond

Answer: (b) Ability to form a complex with haemoglobin
CO binds to haemoglobin approximately 250 times more strongly than oxygen, forming carboxyhaemoglobin. This stable complex prevents oxygen from binding — blocking oxygen transport to body tissues. The result is suffocation at the cellular level.
[NDA 2018-I]

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Q9. Which one of the following compounds is used in black and white photography?
(a) AgF (b) AgBr (c) AgCl (d) Ag₂SO₄

Answer: (b) AgBr
Silver bromide (AgBr) is light-sensitive — it decomposes when light hits it: 2AgBr → 2Ag + Br₂. The silver metal formed creates dark areas. The pattern of light and dark silver atoms corresponds to the image. AgF is too soluble, AgCl is less sensitive, Ag₂SO₄ is not used in photography.
[NDA 2021-II]

Q10. Zinc is used to protect iron from corrosion because zinc is:
(a) More electropositive than iron
(b) Cheaper than iron
(c) A bluish-white metal
(d) A good conductor of heat and electricity

Answer: (a) More electropositive than iron
Zinc is higher in the reactivity series than iron. When zinc and iron are both exposed to moisture and air, zinc corrodes preferentially (sacrificial corrosion) — protecting the iron beneath. The reason is that zinc is more reactive (more electropositive) than iron.
[NDA 2017-II]

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Q11. Statement I: After cutting an apple or banana, the colour of the cut surface becomes brown.
Statement II: Polyphenolic compounds present in fruits get oxidised in air and show colour.
(a) Both true, II correctly explains I
(b) Both true, II does not explain I
(c) I true, II false
(d) I false, II true

Answer: (a) Both true, Statement II correctly explains Statement I
When fruit is cut, the inner surface is exposed to oxygen. Polyphenol oxidase enzyme converts polyphenolic compounds into dark brown quinones. This is an oxidation reaction — it is a chemical change, it is irreversible, and it requires oxygen.
[NDA 2012-I]

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Q12. Consider the thermite reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃
Which one of the following statements is NOT correct?
(a) It is an example of displacement reaction
(b) It is highly endothermic reaction
(c) Fe₂O₃ acts as oxidising agent
(d) This reaction is applicable to joining of railway tracks

Answer: (b) It is highly endothermic reaction
The thermite reaction is highly EXOTHERMIC — it releases enormous heat (temperatures up to 2500°C). This heat is what makes it useful for welding railway tracks. Options (a), (c), and (d) are all correct — it is a displacement reaction, Fe₂O₃ is oxidised, and it is used in thermite welding.
[NDA 2023-II]

Q13. Hydrogenation of vegetable oils using nickel catalyst is an example of:
(a) Substitution reaction (b) Elimination reaction (c) Addition reaction (d) Free-radical polymerisation

Answer: (c) Addition reaction
In hydrogenation, hydrogen (H₂) is added across the double bonds (C=C) of unsaturated vegetable oils. This converts liquid oils into solid fats (vanaspati). Nickel acts as a catalyst to speed up the addition of H₂. No atoms are removed — hydrogen is added across the double bond.
[NDA 2021-I]