Solution : Chapter 1: Chemical Reactions and Equations

Chemical Reactions and Changes - Questions and Answers

Page No-6 Questions

1. Why should a magnesium ribbon be cleaned before burning in air?

A magnesium ribbon should be cleaned before burning to remove the layer of magnesium oxide that forms on its surface due to its reaction with oxygen in the air. This oxide layer prevents the magnesium from burning properly, so it must be removed using sandpaper for an efficient reaction.

2. Write the balanced equation for the following chemical reactions:

(i) Hydrogen + Chlorine → Hydrogen chloride
H₂ + Cl₂ → 2HCl
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na + 2H₂O → 2NaOH + H₂

3. Write a balanced chemical equation with state symbols for the following reactions:

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
BaSO₄ precipitates as an insoluble solid (s).
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
This represents a neutralization reaction forming salt and water.

Page No-10 Questions

1. A solution of a substance ‘X’ is used for whitewashing.

(i) The substance ‘X’ is calcium oxide (quick lime), and its chemical formula is CaO.
(ii) When calcium oxide reacts with water, it forms calcium hydroxide (slaked lime) and releases heat.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.

In Activity 1.7 (Electrolysis of Water), water splits into hydrogen (H₂) and oxygen (O₂) gases.
The balanced equation is:
2H₂O(l) → Electricity 2H₂(g) + O₂(g)
Hydrogen gas (H₂) is collected in double the amount compared to oxygen because for every 2 molecules of water, 2 molecules of hydrogen and only 1 molecule of oxygen are produced.
Gas collected in double quantity: Hydrogen (H₂).

Notes on Displacement Reaction

Activity 1.9: Reaction Between Iron and Copper Sulphate Solution

Observation: The iron nail turns brownish in color. The blue color of copper sulphate solution fades.

Reason: Iron (Fe) is more reactive than copper (Cu). It displaces copper from copper sulphate (CuSO₄) solution. The iron nail gets coated with copper, giving it a brownish appearance. The solution changes from blue to light green due to the formation of iron sulphate (FeSO₄).

Chemical Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

This type of reaction is called a displacement reaction, where a more reactive metal replaces a less reactive metal in a compound.

Other Examples of Displacement Reactions:

• Zinc displacing Copper: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Lead displacing Copper: Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)

Conclusion:

More reactive metals displace less reactive metals from their compounds. Displacement reactions are common in metal reactivity and are used in various industrial processes.

Page No-13 Questions

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

The blue colour of copper sulphate (CuSO₄) solution fades because iron (Fe) displaces copper (Cu) from the solution. This happens because iron is more reactive than copper.
The reaction is:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron sulphate (FeSO₄) is formed, which is pale green, and copper metal is deposited on the nail, giving it a brownish coating.

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Reaction between silver nitrate and sodium chloride:
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
Silver chloride (AgCl) forms a white precipitate, and sodium nitrate (NaNO₃) remains in solution.

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na(s) + O₂(g) → 2Na₂O(s)
Sodium (Na) is oxidised because it gains oxygen to form sodium oxide (Na₂O). Oxygen (O₂) is reduced because it gains electrons from sodium.
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Copper(II) oxide (CuO) is reduced to copper (Cu) because it loses oxygen. Hydrogen (H₂) is oxidised to water (H₂O) because it gains oxygen.

Exercise Questions

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

2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized.
(c) Carbon is getting oxidized.
(d) Lead oxide is getting reduced.
✅ Correct Answer: (i) (a) and (b)
Explanation: Lead oxide (PbO) loses oxygen and forms lead (Pb), so it is reduced (Statement a is correct). Carbon (C) gains oxygen and forms CO₂, so carbon is oxidized (Statement c is correct). Carbon dioxide is not oxidized; it is the product (Statement b is incorrect). Lead oxide is getting reduced (Statement d is correct).

2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

This reaction is an example of:
(a) Combination reaction
(b) Double displacement reaction
(c) Decomposition reaction
(d) Displacement reaction
✅ Correct Answer: (d) Displacement reaction
Explanation: In this reaction, Aluminium (Al) displaces Iron (Fe) from Fe₂O₃ and forms Aluminium Oxide (Al₂O₃). Displacement reaction occurs when a more reactive element replaces a less reactive one. Since Aluminium is more reactive than Iron, it displaces Iron from Iron Oxide.

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

(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
✅ Correct Answer: (a) Hydrogen gas and iron chloride are produced.
Explanation: Fe + 2HCl → FeCl₂ + H₂
Iron reacts with HCl, forming Iron(II) chloride (FeCl₂) and releasing hydrogen gas (H₂).

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

✅ Answer: A balanced chemical equation has equal numbers of atoms of each element on both sides of the equation.
Explanation: Balancing ensures that the law of conservation of mass is followed. Mass cannot be created or destroyed in a reaction, so the total number of atoms before and after the reaction must be the same.
Example:
Unbalanced: H₂ + O₂ → H₂O
Balanced: 2H₂ + O₂ → 2H₂O

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

(a) Hydrogen gas combines with nitrogen to form ammonia.
✅ N₂ + 3H₂ → 2NH₃
(b) Hydrogen sulfide gas burns in air to give water and sulfur dioxide.
✅ 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) Barium chloride reacts with aluminum sulfate to give aluminum chloride and a precipitate of barium sulfate.
✅ 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
✅ 2K + 2H₂O → 2KOH + H₂

6. Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
✅ 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
✅ 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
✅ Already balanced.
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
✅ BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

9. What does one mean by exothermic and endothermic reactions?

✅ Answer:
Exothermic Reaction: A reaction that releases heat (e.g., combustion, respiration).
Endothermic Reaction: A reaction that absorbs heat (e.g., photosynthesis, melting ice).
Example:
Exothermic: C + O₂ → CO₂ + Heat
Endothermic: CaCO₃ → CaO + CO₂ (Heat is absorbed)

10. Why is respiration considered an exothermic reaction?

✅ Answer:
In respiration, glucose reacts with oxygen to produce carbon dioxide, water, and energy:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Since energy is released, respiration is exothermic.

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

✅ Answer:
Combination Reaction: Two or more substances combine to form a single product.
Decomposition Reaction: A single compound breaks down into two or more simpler substances.
Example:
Combination: 2H₂ + O₂ → 2H₂O
Decomposition: 2H₂O → 2H₂ + O₂

13. Difference between displacement and double displacement reactions.

✅ Answer:
Displacement Reaction: One element replaces another in a compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Double Displacement Reaction: Two compounds exchange ions to form new compounds.
Example: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

15. What is a precipitation reaction?

✅ Answer:
A reaction where an insoluble solid (precipitate) forms in a solution.
Example: BaCl₂ + Na₂SO₄ → BaSO₄ (precipitate) + 2NaCl

16. Explain oxidation and reduction.

✅ Answer:
Oxidation: Gain of oxygen/loss of hydrogen.
Reduction: Loss of oxygen/gain of hydrogen.
Example:
Oxidation: Cu → CuO (Copper gains oxygen)
Reduction: CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen)

18. Why do we apply paint on iron articles?

✅ Answer:
Paint prevents rusting by blocking oxygen and moisture from reaching the metal.

19. Why are food items flushed with nitrogen?

✅ Answer:
Nitrogen prevents oxidation and spoilage of fats and oils, keeping food fresh.

20. Define corrosion and rancidity.

✅ Answer:
Corrosion: Gradual destruction of metals (e.g., rusting).
Rancidity: Spoilage of fats due to oxidation (e.g., stale chips).