Chemical Changes & Reactions - Questions & Answers

1. PREVIOUS QUESTIONS

Short Answer Questions

1. Give an example - of an endothermic reaction.
Answer: N₂ + O₂ ⇌ 2NO - Δ

Long Answer Questions

1. Explain : 'Silver nitrate solution is kept in coloured reagent bottles in the laboratory.'
Answer: Silver nitrate undergoes photochemical decomposition in the presence of light to form silver, nitrogen dioxide, and oxygen (2AgNO₃ → 2Ag + 2NO₂ + O₂). Coloured bottles cut off light and prevent this decomposition.

2. Reactions can be classified as : Direct combination, decomposition, simple displacement, double decomposition. State which of the following types, takes place in the reactions given below :
i] Cl₂ + 2KI → 2KCl + I₂
ii] 2Mg + O₂ → 2MgO
iii] SO₂ + 2H₂O + Cl₂ → 2HCl + H₂SO₄
iv] AgNO₃ + HCl → AgCl + HNO₃
v] 4HNO₃ → 4NO₂ + 2H₂O + O₂
Answer:
i] Simple displacement reaction
ii] Direct combination reaction
iii] Oxidation-reduction reaction (Not strictly one of the four types)
iv] Double decomposition reaction
v] Decomposition reaction

2. ADDITIONAL QUESTIONS

Short Answer Questions

1. Give an example - of a photochemical decomposition reaction.
Answer: 2AgCl (in presence of sunlight) → 2Ag + Cl₂

2. Name - a metallic oxide which on thermal decomposition is reduced to a metal.
Answer: Mercury [II] oxide (HgO)

Long Answer Questions

1. Explain the term - chemical reaction with special reference to reactants and products.
Answer: A chemical reaction is a chemical change in which matter changes into a new substance or substances. The substances taking part in the reaction are called reactants, and the new substances formed as a result of the reaction are called products.

2. Give a suitable example - with equation to show the representation of a chemical reaction.
Answer: Ca(OH)₂ + 2HNO₃[dil.] → Ca(NO₃)₂ + 2H₂O. Here, calcium hydroxide and nitric acid are the reactants, and calcium nitrate and water are the products.

3. Give two examples - where a chemical reaction is accompanied by a change in colour of the reactants & products on completion of the reaction.
Answer:
1. Copper [II] nitrate (Blue) on heating gives Copper [II] oxide (Black): 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂
2. Lead [II] nitrate (White) on heating gives Litharge (Yellow): 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂

4. Give balanced equations - for reactions involving evolution of a gas on addition of dilute acid to - (a) sodium sulphite (b) calcium carbonate.
Answer:
(a) Na₂SO₃ + H₂SO₄[dil.] → Na₂SO₄ + H₂O + SO₂(g)
(b) CaCO₃ + 2HCl[dil.] → CaCl₂ + H₂O + CO₂(g)

5. Give a balanced equation - for conversion of: (a) an ammonium salt to a basic gas (b) a soluble lead salt to an insoluble lead salt - formed as a white precipitate.
Answer:
(a) 2NH₄Cl + Ca(OH)₂ → CaCl₂ + 2H₂O + 2NH₃(g)
(b) Pb(NO₃)₂ + 2NaCl → 2NaNO₃ + PbCl₂ (white ppt.)

6. Chemical reactions - may proceed with evolution or absorption of heat. Give an example of each.
Answer:
Evolution of heat (Exothermic): N₂ + 3H₂ ⇌ 2NH₃ + Δ
Absorption of heat (Endothermic): N₂ + O₂ ⇌ 2NO - Δ

7. Define - the following types of chemical changes or reactions with a suitable example each. (a) Direct combination reaction or synthesis (b) Decomposition reaction (c) Displacement reaction or substitution reaction (d) Double decomposition reaction
Answer:
(a) Direct combination reaction: A chemical reaction in which two or more elements or compounds react to form exactly one new compound. Example: 2H₂ + O₂ → 2H₂O
(b) Decomposition reaction: A chemical reaction in which a chemical compound decomposes into two or more simpler substances. Example: CaCO₃ → CaO + CO₂
(c) Displacement reaction: A chemical reaction in which an element or radical has replaced another element in a compound. Example: Mg + CuSO₄ → MgSO₄ + Cu
(d) Double decomposition reaction: A chemical reaction in which two reactants are both decomposed to form new substances by exchanging their radicals. Example: AgNO₃ + NaCl → AgCl + NaNO₃

8. Give balanced equations - for the following: (a) An acid - from sulphur dioxide gas (b) An alkali - from a basic oxide - sodium oxide (c) A salt - from a trivalent metal & a coloured gas.
Answer:
(a) SO₂ + H₂O → H₂SO₃
(b) Na₂O + H₂O → 2NaOH
(c) 2Fe + 3Cl₂ → 2FeCl₃

9. Convert - (a) nitrogen to ammonia (b) hydrogen to hydrogen chloride - by a direct combination reaction.
Answer:
(a) N₂ + 3H₂ ⇌ 2NH₃
(b) H₂ + Cl₂ → 2HCl

10. Give balanced equations - for thermal decomposition of - (a) lead carbonate (b) lead nitrate (c) ammonium dichromate (d) mercury [II] oxide (e) calcium hydroxide
Answer:
(a) PbCO₃ → PbO + CO₂
(b) 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
(c) (NH₄)₂Cr₂O₇ → Cr₂O₃ + 4H₂O + N₂
(d) 2HgO → 2Hg + O₂
(e) Ca(OH)₂ → CaO + H₂O

11. Define - a thermal dissociation reaction with a suitable example.
Answer: It is a reversible decomposition reaction in which a substance dissociates into two or more simpler substances on the application of heat. Example: NH₄Cl ⇌ NH₃ + HCl

12. Define - a displacement reaction with examples. State why Zn displaces H₂ from dil. H₂SO₄ but Cu does not.
Answer: A displacement reaction takes place when an element (or radical) has replaced another element in a compound. Example: Zn + CuSO₄ → ZnSO₄ + Cu. Zinc displaces hydrogen from dilute sulphuric acid because zinc is an active metal placed above hydrogen in the electrochemical series (more electropositive). Copper cannot displace hydrogen because it is placed below hydrogen in the series (less electropositive).

13. Explain the term - double decomposition - precipitation reaction. Give a balanced equation for the preparation of two different insoluble lead salts from their salt solutions by - double decomposition - precipitation.
Answer: It is a reaction between two compounds in the aqueous solution state that gives two new compounds, one of which is insoluble and called a precipitate.
Equations:
1. Pb(NO₃)₂ + 2NaCl → 2NaNO₃ + PbCl₂ (white ppt.)
2. Pb(NO₃)₂ + Na₂SO₄ → 2NaNO₃ + PbSO₄ (white ppt.)

14. With the help of balanced equations - show how precipitation reactions are used for identifying the positive radicals in three different salts, each having a different cation [positive ion].
Answer:
1. Fe²⁺: FeSO₄ + 2NaOH → Na₂SO₄ + Fe(OH)₂ (Dirty green ppt.)
2. Fe³⁺: FeCl₃ + 3NaOH → 3NaCl + Fe(OH)₃ (Reddish brown ppt.)
3. Cu²⁺: CuSO₄ + 2NaOH → Na₂SO₄ + Cu(OH)₂ (Pale blue ppt.)

15. Define the term - double decomposition - neutralization reaction with a suitable representation. Convert - (a) an insoluble base (b) a soluble base to their respective soluble salts by neutralization reaction.
Answer: A neutralization reaction is a double decomposition reaction between a base and an acid to give two new compounds (salt and water) by the interchange of radicals. Representation: Base + Acid → Salt + Water.
(a) Insoluble base: CuO + H₂SO₄ → CuSO₄ + H₂O
(b) Soluble base: NaOH + HCl → NaCl + H₂O

16. Explain the term - energy changes in a chemical change. Give an example with a balanced equation, for each of the following reactions: (a) exothermic (b) endothermic (c) photochemical (d) electrochemical reaction.
Answer: Energy changes in a chemical change refer to the difference between the chemical energy of the reactants and the products, resulting in the evolution or absorption of energy in the form of heat, light, or electricity.
(a) Exothermic: N₂ + 3H₂ → 2NH₃ + Δ
(b) Endothermic: N₂ + O₂ → 2NO - Δ
(c) Photochemical: H₂ + Cl₂ → 2HCl (in the presence of sunlight)
(d) Electrochemical: 2H₂O → 2H₂ + O₂ (acidified water, electric current)

Multiple Choice Questions [MCQs] - Select the correct answer

1. Choose - the reaction which is not a chemical change under prevalent conditions.
(a) Fe + CuSO₄ → (b) FeCO₃ -Δ→ (c) Fe -Δ→ (d) Fe + S -Δ→
Answer: (c) Fe -Δ→

2. Choose - the reaction which is not a precipitation reaction under prevalent conditions.
(a) Pb(NO₃)₂ + NaOH → (b) Pb(NO₃)₂ + Na₂SO₄ (c) Pb(NO₃)₂ -Δ→ (d) Pb(NO₃)₂ + NaCl
Answer: (c) Pb(NO₃)₂ -Δ→

3. Choose - the conversion reaction which is exothermic & takes place by supply of catalytic energy.
(a) Hydrogen to HCl (b) Ammonia to nitric oxide (c) Nitrogen to nitric oxide (d) Oxygen to ozone
Answer: (b) Ammonia to nitric oxide

High Order Thinking Questions

1. Give a balanced equation for - a direct combination reaction involving:
(a) Two elements - one of which is a neutral gas and the other a yellow non-metal
(b) Two elements - one of which is a neutral gas and the other a monovalent metal
(c) Two compounds - resulting in formation of a weak acid
Answer:
(a) S + O₂ → SO₂
(b) 4Na + O₂ → 2Na₂O
(c) SO₂ + H₂O → H₂SO₃ (or CO₂ + H₂O → H₂CO₃)

2. Match the conversion in Column A with correct type of - chemical change or reaction.
Column 'A'
(a) A trivalent metal - to its metal nitride
(b) Hydrous CuSO₄ - to anhydrous CuSO₄
(c) Iron to iron chloride - using a dilute acid
(d) Sodium hydroxide to sodium chloride - using a dilute acid
(e) Iron [III] chloride to iron [III] hydroxide
Column 'B'
i] Displacement
ii] Direct combination
iii] Precipitation
iv] Decomposition
v] Neutralization
Answer:
(a) - ii] Direct combination
(b) - iv] Decomposition
(c) - i] Displacement
(d) - v] Neutralization
(e) - iii] Precipitation

3. TEST - HIGH ORDER & CRITICAL THINKING QUESTIONS
UNIT TEST PAPER - 2

Q.1 Complete the statements by filling in the blank with the correct word/s:

1. Direct combination reaction of sulphur dioxide with water gives H₂SO₃ [H₂SO₄/H₂SO₃/H₂S₂O₇].
2. Formation of hydrogen chloride from hydrogen and chlorine is an example of photochemical reaction [photochemical reaction/electrochemical reaction].
3. The reaction of hydrogen burning in oxygen to give a neutral liquid is an example of exothermic [exothermic/endothermic] reaction.
4. The neutral gas evolved when lead nitrate undergoes thermal decomposition is oxygen [nitrogen dioxide/oxygen/nitrogen].
5. The reddish brown precipitate obtained during a double decomposition - precipitation reaction between an iron salt and an alkali is iron [III] hydroxide [iron [II] hydroxide/iron [III] hydroxide].

Q.2 Select the correct answer from A, B, C, D and E for each statement given below:
A: Ammonia B: Hydrogen chloride C: Hydrogen D: Nitrogen dioxide E: Nitric oxide
1. An active metal reacts with dilute sulphuric acid.
2. A metallic nitrate undergoes thermal decomposition giving a coloured gas.
3. Two gases one of them neutral, combines by absorption of light energy.
4. An ammonium salt reacts with an alkali.
5. An exothermic reaction takes place between ammonia and a neutral gas.
Answer:
1. C: Hydrogen
2. D: Nitrogen dioxide
3. B: Hydrogen chloride
4. A: Ammonia
5. E: Nitric oxide

Q.3 Give a balanced equation for each of the following types of reactions :

1. A direct combination reaction between phosphorus and a neutral gas.
Answer: 4P + 5O₂ → 2P₂O₅

2. A soluble salt of lead formed from an insoluble base by double decomposition - neutralisation.
Answer: PbO + 2HNO₃ → Pb(NO₃)₂ + H₂O

3. A thermal decomposition reaction of a salt - which results in the formation of nitrogen gas.
Answer: (NH₄)₂Cr₂O₇ → Cr₂O₃ + 4H₂O + N₂

4. A synthesis reaction between a metal & a non-metal resulting in formation of an insoluble salt of iron.
Answer: Fe + S → FeS

5. A decomposition reaction of a salt which leaves behind a silvery metal.
Answer: 2AgNO₃ → 2Ag + 2NO₂ + O₂

Q.4 Differentiate between the following :

1. Synthesis reaction & a substitution reaction.
Answer: A synthesis reaction involves two or more elements or compounds combining to form a single new compound (A + B → AB). A substitution (displacement) reaction involves an element or radical replacing another element in a compound (A + BC → AC + B).

2. Electrolytic decomposition & photochemical decomposition.
Answer: Electrolytic decomposition is brought about by the passage of an electric current through a substance (e.g., breaking down acidified water into hydrogen and oxygen). Photochemical decomposition occurs in the presence of light energy (e.g., breaking down silver chloride into silver and chlorine in sunlight).

3. Thermal decomposition & thermal dissociation.
Answer: Thermal decomposition is an irreversible reaction in which a compound breaks down into simpler substances upon heating. Thermal dissociation is a reversible decomposition reaction where a substance dissociates into simpler substances upon heating but recombines when cooled.

4. Decomposition reaction & a double decomposition reaction.
Answer: In a decomposition reaction, a single chemical compound breaks down into two or more simpler substances. In a double decomposition reaction, two reactant compounds mutually exchange their radicals to form two new chemical compounds.

5. Neutralization reaction & a precipitation reaction.
Answer: A neutralization reaction is a specific double decomposition reaction between an acid and a base that produces only salt and water. A precipitation reaction is a double decomposition reaction between two aqueous solutions that yields at least one insoluble product called a precipitate.

Q.5 Match the chemical reactions in List I with the appropriate answer in List II.
List I:
1. AB -heat→ A + B
2. AB ⇌ A + B
3. X⁺Y^- + A⁺B^- → X⁺B^- + A⁺Y^-
4. X + YZ → XZ + Y
5. X + Y -heat→ XY - Δ

List II:
A: Double decomposition
B: Thermal dissociation
C: Endothermic reaction
D: Displacement reaction
E: Decomposition reaction
Answer:
1 - E: Decomposition reaction
2 - B: Thermal dissociation
3 - A: Double decomposition
4 - D: Displacement reaction
5 - C: Endothermic reaction

Q.6 Name the solid residual product formed in each reaction and state its colour during - thermal decomposition of the following substances.

1. Copper nitrate.
Answer: Copper [II] oxide; Colour: Black

2. Ammonium dichromate.
Answer: Chromic oxide; Colour: Green

3. Zinc carbonate.
Answer: Zinc oxide; Colour: Yellow when hot, white when cold

4. Lead nitrate.
Answer: Lead [II] oxide (Litharge); Colour: Yellow

5. Calcium hydroxide.
Answer: Calcium oxide; Colour: White