QUIZ : CHEMICAL REACTIONS, BALANCING & REDOX

CLASS 10 SCIENCE: CHEMICAL REACTIONS, BALANCING & REDOX

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PRACTICE QUESTIONS (Q1 - Q10)

Q1. When the equation aAl + bHCl → cAlCl₃ + dH₂ is balanced using the smallest whole-number stoichiometric coefficients, what are the values of a, b, c, and d?

(A) a = 1, b = 3, c = 1, d = 3 (B) a = 2, b = 6, c = 2, d = 3 (C) a = 2, b = 3, c = 2, d = 3 (D) a = 1, b = 6, c = 1, d = 3

Detailed Rationale: Correct Answer: (B). Balancing chlorine atoms between HCl and AlCl₃ requires a lowest common multiple of 6 (putting 6 on HCl and 2 on AlCl₃). This yields 2 Aluminum atoms and 6 Hydrogen atoms on the left, requiring 3 molecules of H₂ on the right: 2Al + 6HCl → 2AlCl₃ + 3H₂.

Q2. On strongly heating solid lead(II) nitrate in a dry test tube, a brown gas (X) and a colourless gas (Y) supporting combustion are evolved, leaving behind a yellow residue (Z). Identify X, Y, and Z.

(A) X = NO₂, Y = O₂, Z = PbO (B) X = N₂O, Y = O₂, Z = Pb₃O₄ (C) X = NO, Y = CO₂, Z = PbO₂ (D) X = N₂O₅, Y = O₂, Z = Pb

Detailed Rationale: Correct Answer: (A). Thermal decomposition of lead nitrate follows: 2Pb(NO₃)₂ (s) → 2PbO (s, yellow) + 4NO₂ (g, brown fumes) + O₂ (g). Nitrogen dioxide (NO₂) is reddish-brown, and oxygen supports combustion.

Q3. During the electrolytic decomposition of acidified water, what is the ratio of the volume of gas collected at the cathode to the gas collected at the anode?

(A) 1 : 2 (B) 1 : 1 (C) 2 : 1 (D) 8 : 1

Detailed Rationale: Correct Answer: (C). Water decomposes into Hydrogen and Oxygen according to: 2H₂O (l) → 2H₂ (g, cathode) + O₂ (g, anode). The volume ratio of Hydrogen (cathode) to Oxygen (anode) is strictly 2 : 1.

Q4. In the reaction: MnO₂ (s) + 4HCl (aq) → MnCl₂ (aq) + 2H₂O (l) + Cl₂ (g), which species is oxidized and which substance acts as the oxidizing agent?

(A) Oxidized: MnO₂ ; Oxidizing Agent: HCl (B) Oxidized: HCl ; Oxidizing Agent: MnO₂ (C) Oxidized: HCl ; Oxidizing Agent: Cl₂ (D) Oxidized: MnCl₂ ; Oxidizing Agent: H₂O

Detailed Rationale: Correct Answer: (B). HCl loses hydrogen/electrons to form Cl₂ gas, meaning HCl is oxidized. MnO₂ loses oxygen (oxidation state drops from +4 to +2), undergoing reduction and acting as the oxidizing agent.

Q5. An aqueous solution of lead(II) nitrate is added to an aqueous solution of potassium iodide. What observation and reaction type correctly describe this change?

(A) White precipitate of KNO₃; Combination reaction (B) Yellow precipitate of PbI₂; Double displacement reaction (C) Black precipitate of PbO; Decomposition reaction (D) Reddish precipitate of I₂; Single displacement reaction

Detailed Rationale: Correct Answer: (B). The mutual exchange of ions produces an insoluble bright yellow precipitate of lead iodide: Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ ↓ (yellow) + 2KNO₃ (aq).

Q6. Based on the reactivity series of metals, which of the following mixtures will result in a spontaneous single displacement reaction?

(A) Cu (s) + FeSO₄ (aq) (B) Ag (s) + CuSO₄ (aq) (C) Cu (s) + 2AgNO₃ (aq) (D) Fe (s) + ZnSO₄ (aq)

Detailed Rationale: Correct Answer: (C). Copper is more reactive than silver and can displace silver ions from solution: Cu (s) + 2AgNO₃ (aq) → Cu(NO₃)₂ (aq) + 2Ag (s).

Q7. Which of the following is an endothermic chemical process?

(A) Dilution of concentrated sulphuric acid with water (B) Cellular respiration in living organisms (C) Thermal decomposition of calcium carbonate into quicklime and CO₂ (D) Reaction of quicklime (CaO) with water to form slaked lime

Detailed Rationale: Correct Answer: (C). Decomposing calcium carbonate requires continuous absorption of heat energy: CaCO₃ (s) → CaO (s) + CO₂ (g) (ΔH > 0). The other three processes release heat (exothermic).

Q8. When white silver chloride (AgCl) is exposed to sunlight for a prolonged duration, it turns grey due to:

(A) Formation of silver oxide through atmospheric oxidation (B) Photolytic decomposition yielding elemental silver and chlorine gas (C) Sublimation of chlorine atoms leaving an empty lattice (D) Reaction of silver chloride with atmospheric moisture

Detailed Rationale: Correct Answer: (B). Sunlight provides photon energy to split the salt: 2AgCl (s, white) → 2Ag (s, grey) + Cl₂ (g).

Q9. In the electronic concept of redox reactions, a reducing agent is defined as a substance that:

(A) Accepts electrons and decreases its oxidation number (B) Donates electrons and increases its oxidation number (C) Supplies oxygen without electron movement (D) Gains hydrogen atoms by extracting protons only

Detailed Rationale: Correct Answer: (B). Oxidation is the loss of electrons. A reducing agent donates electrons to another species, causing that species to be reduced while itself becoming oxidized (increasing its oxidation state).

Q10. Food manufacturers flush bags of potato chips with nitrogen gas (N₂) primarily to:

(A) Add essential nutrients and enhance crispiness (B) Prevent rancidity caused by oxidative degradation of unsaturated fats (C) Induce a combination reaction that sterilizes bacteria (D) Accelerate displacement of residual salt crystals

Detailed Rationale: Correct Answer: (B). Nitrogen is an unreactive inert gas that displaces oxygen, preventing the oxidative breakdown (rancidity) of oils and fats that causes foul odor and taste.

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