CBSE Class 10 Science Advanced Chemistry Chemical Reactions and Equations MCQs Set 02

Answer: (c) Pb

Explanation:
1. Sodium and potassium nitrates give the nitrite and oxygen on heating, without \( \ce{NO2} \).
2. Lead nitrate gives \( \ce{PbO} \), \( \ce{NO2} \) and \( \ce{O2} \): \( \ce{2Pb(NO3)2 -> 2PbO + 4NO2 + O2} \).
3. The answer is Pb.

Teacher's Note:
1. Nitrates of the least reactive metals give the metal oxide, NO2 and O2.
2. Brown fumes tell us NO2 has formed.

Answer: (b) \( \ce{Pb(s) + CuCl2(aq) -> PbCl2(aq) + Cu(s)} \)

Explanation:
1. (A) is a combination reaction, (C) is a redox reaction and (D) is combustion (respiration).
2. In (B) lead, a more reactive metal, takes the place of copper in its salt.
3. So (B) is the displacement reaction.

Teacher's Note:
1. Displacement: an element and a compound give a new element and a new compound.
2. Lead is above copper in the reactivity series.

Answer: (b) \( \ce{N2(g) + O2(g) -> 2NO(g)} \)

Explanation:
1. (A), (C) and (D) are burning (combustion) reactions, which release heat.
2. Nitrogen and oxygen combine only at very high temperature, such as in lightning, and they absorb heat.
3. The answer is (B).

Teacher's Note:
1. All combustion reactions are exothermic.
2. The book gives N2 + O2 as 2NO - 43200 cal, where the minus sign shows heat absorbed.

Answer: (c) ↓

Explanation:
1. A precipitate settles down, so it is shown by a downward arrow (↓) written after its formula.
2. An upward arrow (↑) marks a gas, so the answer is ↓.

Teacher's Note:
1. Gas goes up, precipitate goes down.
2. You may also write (s) or ppt.

Answer: (d) (b) & (d)

Explanation:
1. An insoluble solid forms, so it is a precipitation reaction (b).
2. The ions are exchanged, so it is a double displacement reaction (d).
3. The answer is (b) and (d).

Teacher's Note:
1. Neither a single displacement nor a combination fits, as there are two products.
2. Always check whether a solid is formed.

Answer: (c) neutralization and double displacement

Explanation:
1. A strong acid and a strong base give salt and water: neutralisation.
2. The ions are exchanged too: double displacement.
3. The answer is (C).

Teacher's Note:
1. When both (A) and (B) are correct, look for an option that combines them.
2. Neutralisation is a special case of double displacement.

Answer: (c) Both (A) and (B)

Explanation:
1. Electrolysis of water splits it into hydrogen and oxygen: \( \ce{2H2O -> 2H2 + O2} \).
2. One compound gives two simpler substances, a decomposition, and electrical energy is absorbed, so it is endothermic.
3. The answer is both (A) and (B).

Teacher's Note:
1. Decomposition reactions generally need energy, so most of them are endothermic.
2. Combination is the reverse process.

Answer: (a) exothermic

Explanation:
1. Hydrogen burns in air with a pop: \( \ce{2H2 + O2 -> 2H2O} \).
2. The reaction releases heat energy rapidly, so the sound is due to an exothermic reaction.

Teacher's Note:
1. Burning always releases heat, so it is exothermic.
2. This pop test is used to identify hydrogen gas.

Answer: (c) PbS - Reductant; H₂O₂ - Oxidant

Explanation:
1. PbS gains oxygen to form PbSO4, so PbS is oxidised and acts as the reductant (reducing agent).
2. \( \ce{H2O2} \) gives oxygen and becomes water, so it is reduced and acts as the oxidant.
3. The answer is (C).

Teacher's Note:
1. The oxidant is the substance that gets reduced.
2. Product substances such as PbSO4 and H2O are never the agents.

Answer: (d) a-iv, b-i, c-ii, d-iii

Explanation:
1. a: magnesium gains oxygen, so oxidation (iv).
2. b: slaking of lime gives heat, so exothermic (i).
3. c: AgCl breaks into silver and chlorine, so dissociation (ii).
4. d: CuO loses oxygen, so reduction (iii).
5. The answer is a-iv, b-i, c-ii, d-iii.

Teacher's Note:
1. Start with the pair you are most sure of and remove the options that do not fit.
2. CuO is reduced even though hydrogen is oxidised in the same reaction.

Answer: (b) (i)-q, (ii)-p, (iii)-t, (iv)-s

Explanation:
1. (i) one compound giving elements is decomposition (q).
2. (ii) a metal and a non-metal joining is combination (p).
3. (iii) a hydrocarbon burning in oxygen is combustion (t).
4. (iv) exchange of ions giving a precipitate is double displacement (s).
5. The answer is (B).

Teacher's Note:
1. A hydrocarbon plus oxygen always means combustion.
2. Option (A) wrongly calls the combustion a displacement.

Answer: (b) \( \ce{CuCO3} \), \( \ce{Cu(OH)2} \)

Explanation:
1. Copper reacts slowly with oxygen, carbon dioxide and moisture of air.
2. The green coating is basic copper carbonate, \( \ce{CuCO3.Cu(OH)2} \).
3. The answer is (B).

Teacher's Note:
1. The Statue of Liberty and old copper vessels have the same green layer.
2. CuO is black, not green.

Answer: (c) Physical: b, c, d; Chemical: a, e

Explanation:
1. a: sodium chloride is split into its elements, which is a chemical change.
2. b, c, d: these are only separations of a mixture, so physical.
3. e: zinc displaces copper, so chemical.
4. Physical = b, c, d and chemical = a, e, option (C).

Teacher's Note:
1. Ask whether any new substance is formed; if not, the process is physical.
2. Option (D) lists c both as physical and chemical, so it is absurd.

Answer: (c) White precipitate

Explanation:
1. \( \ce{Pb(NO3)2(aq) + 2HCl(aq) -> PbCl2(s) + 2HNO3(aq)} \).
2. Lead chloride is a white solid.
3. The answer is white precipitate.

Teacher's Note:
1. Lead iodide, formed with KI, is the yellow precipitate. Do not mix the two.
2. Lead sulphide is black.

Answer: (d) loss of water of crystallization

Explanation:
1. Green ferrous sulphate crystals are hydrated, \( \ce{FeSO4.7H2O} \).
2. On heating they first lose their water of crystallisation and become a white powder.
3. The answer is loss of water of crystallization.

Teacher's Note:
1. The first change is dehydration; the decomposition comes on stronger heating.
2. Crystals are not coloured by a dye.

Answer: (c) smell of burning sulphur

Explanation:
1. On strong heating \( \ce{2FeSO4 -> Fe2O3 + SO2 + SO3} \).
2. Sulphur dioxide has the pungent smell of burning sulphur.
3. A solid residue (red \( \ce{Fe2O3} \)) is left, so (D) is wrong. The answer is (C).

Teacher's Note:
1. Rotten egg smell would be H2S, which is not formed here.
2. Heat the crystals gently at first, because they spit.

Answer: (b) (ii) and (iv)

Explanation:
1. Ferrous sulphate crystals are green, not blue, so (iii) is wrong.
2. On heating they end up as reddish brown \( \ce{Fe2O3} \) (ii) and give off \( \ce{SO2} \), with the smell of burning sulphur (iv).
3. The answer is (ii) and (iv).

Teacher's Note:
1. Blue crystals mean copper sulphate, not ferrous sulphate.
2. Match each observation with the product responsible.

Answer: (c) 3

Explanation:
1. A decomposition reaction needs a single compound breaking into simpler substances.
2. Method 1 only boils the water, method 2 gives a displacement reaction and method 4 mixes two salts.
3. Method 3 heats pure iron sulphate crystals alone, so the correct method is 3.

Teacher's Note:
1. A decomposition has one reactant only.
2. Heating in a dry test tube is the standard way to decompose a salt.

Answer: (d) barium chloride and sodium sulphate

Explanation:
1. The pair must be soluble salts that give an insoluble product on exchange of ions.
2. \( \ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)} \) gives white \( \ce{BaSO4} \).
3. Barium sulphate is insoluble, so it cannot be used as a solution (A and C).
4. The answer is (D).

Teacher's Note:
1. Both reacting salts must be in solution.
2. BaCl2 and NaCl give no new product.

Answer: (c) (i), (ii), (iii)

Explanation:
1. The reaction of barium chloride and sodium sulphate gives a white precipitate of barium sulphate.
2. So (i), (ii) and (iii) are right and (iv), which says yellow, is wrong.
3. The answer is (C).

Teacher's Note:
1. Exclude the statement that is wrong, and check which options leave it out.
2. A yellow precipitate is formed with lead nitrate and potassium iodide.