E X E R C I S E S CHAPTER 1 Chemical Reactions and Equations SCIENCE NCERT TEXTBOOK (CBSE) CLASS 10:

E X E R C I S E S

CHAPTER 1 Chemical Reactions and Equations

1. Which of the statements about the reaction below are incorrect?

Reaction:

2PbO(s) + C(s) → 2Pb(s) + CO₂(g)

(a) Lead is getting reduced. ✅ Correct. Lead oxide (PbO) is reduced to lead (Pb).

(b) Carbon dioxide is getting oxidised. ❌ Incorrect. Carbon dioxide is a product of the reaction, not a reactant. It does not get oxidized.

(c) Carbon is getting oxidised. ✅ Correct. Carbon (C) is oxidized to carbon dioxide (CO₂).

(d) Lead oxide is getting reduced. ✅ Correct. Lead oxide (PbO) is reduced to lead (Pb).

Answer: (i) (a) and (b) – Incorrect statements are (a) and (b).

2. Reaction:

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

This is an example of a displacement reaction. Aluminum displaces iron from iron(III) oxide, resulting in the formation of aluminum oxide and iron.

Answer: (d) Displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings?

(a) Hydrogen gas and iron chloride are produced. ✅ Correct. When hydrochloric acid reacts with iron, it produces hydrogen gas and iron chloride.

(b) Chlorine gas and iron hydroxide are produced. ❌ Incorrect.

(c) No reaction takes place. ❌ Incorrect.

(d) Iron salt and water are produced. ❌ Incorrect.

Answer: (a) Hydrogen gas and iron chloride are produced.

4. What is a balanced chemical equation? Why should chemical equations be balanced?

A balanced chemical equation has equal numbers of atoms of each element on both sides of the equation. This is because matter cannot be created or destroyed during a chemical reaction, following the law of conservation of mass.

5. Translate the following statements into chemical equations and then balance them:

(a) Hydrogen gas combines with nitrogen to form ammonia.
$\text{N}_2 (g) + 3\text{H}_2 (g) \rightarrow 2\text{NH}_3 (g)$

(b) Hydrogen sulphide gas burns in air to give water and sulfur dioxide.
$2\text{H}_2\text{S} (g) + 3\text{O}_2 (g) \rightarrow 2\text{H}_2\text{O} (l) + 2\text{SO}_2 (g)$

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
$\text{BaCl}_2 (aq) + \text{Al}_2(\text{SO}_4)_3 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{AlCl}_3 (aq)$

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
$2\text{K} (s) + 2\text{H}_2\text{O} (l) \rightarrow 2\text{KOH} (aq) + \text{H}_2 (g)$

6. Balance the following chemical equations:

(a)
$\text{HNO}_3 (aq) + \text{Ca(OH)}_2 (aq) \rightarrow \text{Ca(NO}_3)_2 (aq) + \text{H}_2\text{O} (l)$
Balanced:
$2\text{HNO}_3 (aq) + \text{Ca(OH)}_2 (aq) \rightarrow \text{Ca(NO}_3)_2 (aq) + 2\text{H}_2\text{O} (l)$

(b)
$\text{NaOH} (aq) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{Na}_2\text{SO}_4 (aq) + \text{H}_2\text{O} (l)$
Balanced:
$2\text{NaOH} (aq) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{Na}_2\text{SO}_4 (aq) + 2\text{H}_2\text{O} (l)$

(c)
$\text{NaCl} (aq) + \text{AgNO}_3 (aq) \rightarrow \text{AgCl} (s) + \text{NaNO}_3 (aq)$
Balanced:
$\text{NaCl} (aq) + \text{AgNO}_3 (aq) \rightarrow \text{AgCl} (s) + \text{NaNO}_3 (aq)$
(No change needed; equation is already balanced.)

(d)
$\text{BaCl}_2 (aq) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{HCl} (aq)$
Balanced:
$\text{BaCl}_2 (aq) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{HCl} (aq)$

7. Write the balanced chemical equations for the following reactions:

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
$\text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l)$

(b) Zinc + Silver nitrate → Zinc nitrate + Silver
$\text{Zn} (s) + 2\text{AgNO}_3 (aq) \rightarrow \text{Zn(NO}_3)_2 (aq) + 2\text{Ag} (s)$

(c) Aluminium + Copper chloride → Aluminium chloride + Copper
$2\text{Al} (s) + 3\text{CuCl}_2 (aq) \rightarrow 2\text{AlCl}_3 (aq) + 3\text{Cu} (s)$

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
${BaCl}_{2} (a q) + K_{2} {SO}_{4} (a q) \to {BaSO}_{4} (s) + 2 KCl (a q)$

8. Write the balanced chemical equation for the following and identify the type of reaction in each case:

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
Type: Double displacement reaction
$2 KBr (a q) + {BaI}_{2} (a q) \to 2 KI (a q) + {BaBr}_{2} (s)$

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Type: Decomposition reaction
$\text{ZnCO}_3 (s) \rightarrow \text{ZnO} (s) + \text{CO}_2 (g)$

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Type: Combination reaction
$\text{H}_2 (g) + \text{Cl}_2 (g) \rightarrow 2\text{HCl} (g)$

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Type: Displacement reaction
$\text{Mg} (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + \text{H}_2 (g)$

9. Exothermic and Endothermic Reactions:

Exothermic reactions release energy (usually in the form of heat or light). Example: Combustion of fuels (e.g., burning of methane, CH₄).

Endothermic reactions absorb energy (usually heat) from the surroundings. Example: Photosynthesis in plants.

10. Why is respiration considered an exothermic reaction? Explain.

Respiration is considered exothermic because it releases energy in the form of heat. During cellular respiration, glucose (C₆H₁₂O₆) is broken down with oxygen (O₂) to produce carbon dioxide (CO₂), water (H₂O), and energy:

$\text{C}_6\text{H}_{12}\text{O}_6 (aq) + 6\text{O}_2 (g) \rightarrow 6\text{CO}_2 (g) + 6\text{H}_2\text{O} (l) + \text{energy}$

11. Why are decomposition reactions called the opposite of combination reactions?

In a combination reaction, two or more substances combine to form a single product. In a decomposition reaction, a single compound breaks down into two or more products. Example:

Combination: $\text{A} + \text{B} \rightarrow \text{AB}$

Decomposition: $\text{AB} \rightarrow \text{A} + \text{B}$

12. Decomposition reactions where energy is supplied:

Heat:
${CaCO}_{3} (s) \overset{heat}{\to} CaO (s) + {CO}_{2} (g)$

Light:
$2\text{AgCl} (s) \xrightarrow{\text{light}} 2\text{Ag} (s) + \text{Cl}_2 (g)$

Electricity:
$2\text{H}_2\text{O} (l) \xrightarrow{\text{electricity}} 2\text{H}_2 (g) + \text{O}_2 (g)$

13. Displacement vs. Double Displacement Reactions:

Displacement: One element replaces another in a compound. Example:
$\text{Zn} (s) + 2\text{HCl} (aq) \rightarrow \text{ZnCl}_2 (aq) + \text{H}_2 (g)$

Double Displacement: Two compounds exchange ions to form two new compounds. Example:
$\text{BaCl}_2 (aq) + \text{Na}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{NaCl} (aq)$

14. Refining of silver (displacement by copper):

Reaction: $\text{2AgNO}_3 (aq) + \text{Cu} (s) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2\text{Ag} (s)$

15. Precipitation reaction:

A precipitation reaction occurs when two soluble substances react to form an insoluble solid (precipitate). Example:
$\text{BaCl}_2 (aq) + \text{Na}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{NaCl} (aq)$

16. Oxidation and Reduction:

Oxidation: Loss of electrons or gain of oxygen. Example:
$\text{Fe} (s) \rightarrow \text{Fe}^{2+} (aq) + 2\text{e}^-$

Reduction: Gain of electrons or loss of oxygen. Example:
$\text{Cu}^{2+} (aq) + 2\text{e}^- \rightarrow \text{Cu} (s)$

17. Shiny brown coloured element ‘X’ becomes black on heating:

The element is copper (Cu), and the black compound formed is copper(II) oxide (CuO).

18. Why do we apply paint on iron articles?

Paint is applied to iron articles to protect them from corrosion (rusting) by providing a protective layer that prevents moisture and oxygen from reaching the iron surface.

19. Why are oil and fat containing food items flushed with nitrogen?

Nitrogen is used to flush out oxygen, which helps prevent rancidity by reducing the oxidation of fats and oils.

20. Corrosion and Rancidity:

Corrosion: The deterioration of metals due to chemical reactions, often with oxygen or moisture. Example: Rusting of iron.

Rancidity: The process by which fats and oils spoil due to oxidation, leading to unpleasant tastes and smells. Example: Spoiling of cooking oil.

E X E R C I S E S CHAPTER 1 Chemical Reactions and Equations SCIENCE NCERT TEXTBOOK (CBSE) CLASS 10:

1. Which of the statements about the reaction below are incorrect?

Reaction:

2. Reaction:

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe This is an example of a displacement reaction. Aluminum displaces iron from iron(III) oxide, resulting in the formation of aluminum oxide and iron. Answer: (d) Displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron fillings?

4. What is a balanced chemical equation? Why should chemical equations be balanced?

A balanced chemical equation has equal numbers of atoms of each element on both sides of the equation. This is because matter cannot be created or destroyed during a chemical reaction, following the law of conservation of mass.

5. Translate the following statements into chemical equations and then balance them:

6. Balance the following chemical equations:

7. Write the balanced chemical equations for the following reactions:

8. Write the balanced chemical equation for the following and identify the type of reaction in each case:

9. Exothermic and Endothermic Reactions:

Exothermic reactions release energy (usually in the form of heat or light). Example: Combustion of fuels (e.g., burning of methane, CH₄). Endothermic reactions absorb energy (usually heat) from the surroundings. Example: Photosynthesis in plants.

10. Why is respiration considered an exothermic reaction? Explain.

11. Why are decomposition reactions called the opposite of combination reactions?

12. Decomposition reactions where energy is supplied:

Heat: CaCO3(s)→heatCaO(s)+CO2(g) Light: 2AgCl(s)→light2Ag(s)+Cl2(g)2\text{AgCl} (s) \xrightarrow{\text{light}} 2\text{Ag} (s) + \text{Cl}_2 (g) Electricity: 2H2O(l)→electricity2H2(g)+O2(g)2\text{H}_2\text{O} (l) \xrightarrow{\text{electricity}} 2\text{H}_2 (g) + \text{O}_2 (g)

13. Displacement vs. Double Displacement Reactions:

14. Refining of silver (displacement by copper):

Reaction: 2AgNO3(aq)+Cu(s)→Cu(NO3)2(aq)+2Ag(s)\text{2AgNO}_3 (aq) + \text{Cu} (s) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2\text{Ag} (s)

15. Precipitation reaction:

A precipitation reaction occurs when two soluble substances react to form an insoluble solid (precipitate). Example: BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2NaCl(aq)\text{BaCl}_2 (aq) + \text{Na}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{NaCl} (aq)

16. Oxidation and Reduction:

Oxidation: Loss of electrons or gain of oxygen. Example: Fe(s)→Fe2+(aq)+2e−\text{Fe} (s) \rightarrow \text{Fe}^{2+} (aq) + 2\text{e}^- Reduction: Gain of electrons or loss of oxygen. Example: Cu2+(aq)+2e−→Cu(s)\text{Cu}^{2+} (aq) + 2\text{e}^- \rightarrow \text{Cu} (s)

17. Shiny brown coloured element ‘X’ becomes black on heating:

The element is copper (Cu), and the black compound formed is copper(II) oxide (CuO).

18. Why do we apply paint on iron articles?

Paint is applied to iron articles to protect them from corrosion (rusting) by providing a protective layer that prevents moisture and oxygen from reaching the iron surface.

19. Why are oil and fat containing food items flushed with nitrogen?

Nitrogen is used to flush out oxygen, which helps prevent rancidity by reducing the oxidation of fats and oils.

20. Corrosion and Rancidity:

Corrosion: The deterioration of metals due to chemical reactions, often with oxygen or moisture. Example: Rusting of iron. Rancidity: The process by which fats and oils spoil due to oxidation, leading to unpleasant tastes and smells. Example: Spoiling of cooking oil.

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Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

This is an example of a displacement reaction. Aluminum displaces iron from iron(III) oxide, resulting in the formation of aluminum oxide and iron.

Answer: (d) Displacement reaction.

Exothermic reactions release energy (usually in the form of heat or light). Example: Combustion of fuels (e.g., burning of methane, CH₄).

Endothermic reactions absorb energy (usually heat) from the surroundings. Example: Photosynthesis in plants.

Heat:
${CaCO}_{3} (s) \overset{heat}{\to} CaO (s) + {CO}_{2} (g)$

Light:
$2\text{AgCl} (s) \xrightarrow{\text{light}} 2\text{Ag} (s) + \text{Cl}_2 (g)$

Electricity:
$2\text{H}_2\text{O} (l) \xrightarrow{\text{electricity}} 2\text{H}_2 (g) + \text{O}_2 (g)$

Reaction: $\text{2AgNO}_3 (aq) + \text{Cu} (s) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2\text{Ag} (s)$

A precipitation reaction occurs when two soluble substances react to form an insoluble solid (precipitate). Example:
$\text{BaCl}_2 (aq) + \text{Na}_2\text{SO}_4 (aq) \rightarrow \text{BaSO}_4 (s) + 2\text{NaCl} (aq)$

Oxidation: Loss of electrons or gain of oxygen. Example:
$\text{Fe} (s) \rightarrow \text{Fe}^{2+} (aq) + 2\text{e}^-$

Reduction: Gain of electrons or loss of oxygen. Example:
$\text{Cu}^{2+} (aq) + 2\text{e}^- \rightarrow \text{Cu} (s)$

Corrosion: The deterioration of metals due to chemical reactions, often with oxygen or moisture. Example: Rusting of iron.

Rancidity: The process by which fats and oils spoil due to oxidation, leading to unpleasant tastes and smells. Example: Spoiling of cooking oil.