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Chapter 3 - Chemical Reactions and Equations

Chapter 3 : Chemical Reactions and Equations

Textbook Exercise Solutions

Q1) Explain the statement with reason:
1. To consider the chemical equation balanced, what should be the number of atoms of elements on both sides?
Answer :

To consider a chemical equation balanced, the number of atoms of each element in the reactants (left-hand side) must be equal to the number of atoms of those elements in the products (right-hand side).

Reason: According to the Law of Conservation of Mass, matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products, which necessitates having an identical atom count for each element on both sides of the equation.

2. Rusting of iron is an electrochemical reaction. Explain.
Answer :

Rusting does not occur simply by direct chemical combination of iron and oxygen; it proceeds through mini electrochemical cells formed on the uneven surface of iron:

  • Anode region: Metallic iron oxidizes and dissolves, releasing electrons:
    Fe(s) → Fe²⁺(aq) + 2e⁻   (Oxidation)
  • Cathode region: Oxygen dissolved in the moisture droplet accepts these electrons and gets reduced:
    O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)   (Reduction)
  • Precipitation of Rust: Fe²⁺ ions migrate through the water droplet, react with water, and oxidize further by atmospheric oxygen to form hydrated iron(III) oxide (rust):
    2Fe²⁺(aq) + 4H₂O(l) + ½O₂(g) → Fe₂O₃ · xH₂O(s) + 4H⁺(aq)

Since distinct oxidation (anode) and reduction (cathode) zones are formed with an electron flow through iron and ion migration through water, rusting is classified as an electrochemical process.

3. When edible oil is left aside for a long time, it turns rancid. Explain.
Answer :

When edible oil or fat is left exposed to air for a long time, it slowly reacts with oxygen in the air (oxidation). This oxidation gives the oil a foul smell and bad taste. This condition is called rancidity. Rancidity can be prevented by adding antioxidants and keeping oil in airtight containers.

Q2) Write scientific reasons:
1. White silver chloride turns grey in sunlight.
Answer :
  • In the presence of sunlight, silver chloride (AgCl) undergoes a photochemical decomposition reaction.
  • It breaks down into grey silver metal and chlorine gas:
    2AgCl(s) —[Sunlight]➔ 2Ag(s) [grey] + Cl₂(g)↑
  • The chlorine gas escapes into the air, leaving behind grey metallic silver.
2. Lime water turns milky when carbon dioxide gas is passed through it.
Answer :
  • Lime water is a solution of calcium hydroxide [Ca(OH)₂].
  • When carbon dioxide (CO₂) gas is passed through it, insoluble calcium carbonate (CaCO₃) is formed:
    Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s)↓ [white precipitate] + H₂O(l)
  • The insoluble particles of calcium carbonate make the solution look milky (white).
3. Respiration is an exothermic reaction.
Answer :
  • During respiration, glucose (C₆H₁₂O₆) from food reacts with inhaled oxygen inside cells:
    C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + Energy
  • This reaction releases a large amount of energy and heat for our body activities.
  • Since heat and energy are released, cellular respiration is an exothermic reaction.
4. It is recommended to use air-tight containers for storing oil for long time.
Answer :
  • Airtight containers prevent air and moisture from coming in contact with cooking oil.
  • This stops the oxidation (rancidity) of the oil, keeping it fresh and safe to use for a longer time.
Q3) Explain the following terms with example:
1. Endothermic reaction
Answer :

A chemical reaction accompanied by the absorption of heat energy from the surroundings is called an endothermic reaction.

Example: Thermal decomposition of calcium carbonate (limestone):

CaCO₃(s) + Heat → CaO(s) + CO₂(g)↑
2. Combination reaction
Answer :

A reaction in which two or more reactants combine chemically to form a single product is called a combination reaction.

Example: Reaction between ammonia gas and hydrogen chloride gas:

NH₃(g) + HCl(g) → NH₄Cl(s)   (Ammonium chloride)
3. Balanced chemical equation
Answer :

A chemical equation in which the total number of atoms of each constituent element on the reactant side is exactly equal to that on the product side is called a balanced chemical equation.

Example: Balanced combustion of methane:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
4. Displacement reaction
Answer :

A chemical reaction in which a more electropositive (more reactive) element displaces a less reactive element from its aqueous salt solution is called a displacement reaction.

Example: Displacement of copper by iron in copper sulphate solution:

Fe(s) + CuSO₄(aq) [blue] → FeSO₄(aq) [pale green] + Cu(s)↓ [reddish-brown]
Q4) Give one example for each of the following:
1. Addition of oxygen (Oxidation)
Answer :
2Mg(s) + O₂(g) → 2MgO(s)   (Magnesium burns in oxygen)
2. Removal of hydrogen (Oxidation)
Answer :
MgH₂(s) → Mg(s) + H₂(g)↑   (Dehydrogenation)
3. Addition of hydrogen (Reduction)
Answer :
C(s) + 2H₂(g) → CH₄(g)   (Synthesis of methane)
4. Removal of oxygen (Reduction)
Answer :
CuO(s) + H₂(g) → Cu(s) + H₂O(l)   (Reduction of black cupric oxide to copper)
Q5) Balance the following equations step by step:
1. H₂S₂O₇(l) + H₂O(l) → H₂SO₄(l)
Answer :
Step 1: Count atoms of each element on both sides:
Left (Reactants): H = 4, S = 2, O = 8
Right (Products): H = 2, S = 1, O = 4
Step 2: To balance S, H, and O, multiply H₂SO₄ by coefficient 2:
H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)
Check: Left has (H=4, S=2, O=8); Right has (H=4, S=2, O=8). Both sides are equal.
Balanced Equation: H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)
2. SO₂(g) + H₂S(aq) → S(s) + H₂O(l)
Answer :
Step 1: Count atoms on both sides:
Reactants: S = 2 (1 in SO₂ + 1 in H₂S), O = 2, H = 2
Products: S = 1, O = 1, H = 2
Step 2: Balance Oxygen by multiplying H₂O by 2:
SO₂ + H₂S → S + 2H₂O (O = 2 on both sides; H becomes 4 on right)
Step 3: Balance Hydrogen by multiplying H₂S by 2:
SO₂ + 2H₂S → S + 2H₂O (H = 4 on both sides; S on left becomes 1 + 2 = 3)
Step 4: Balance Sulfur by multiplying S on the right by 3:
Balanced Equation: SO₂(g) + 2H₂S(aq) → 3S(s) + 2H₂O(l)
3. Ag(s) + HCl(aq) → AgCl(s) + H₂(g)
Answer :
Step 1: Count atoms: Left: Ag=1, H=1, Cl=1; Right: Ag=1, Cl=1, H=2.
Step 2: Multiply HCl by 2 to balance Hydrogen: Ag + 2HCl → AgCl + H₂
Step 3: Chlorine on right is 1; multiply AgCl by 2: Ag + 2HCl → 2AgCl + H₂
Step 4: Silver on right is 2; multiply Ag on left by 2:
Balanced Equation: 2Ag(s) + 2HCl(aq) → 2AgCl(s) + H₂(g)↑
4. NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l)
Answer :
Step 1: Count atoms: Na=1 on left, Na=2 on right.
Step 2: Multiply NaOH by 2: 2NaOH + H₂SO₄ → Na₂SO₄ + H₂O
Now H on left = 2 + 2 = 4, while H on right = 2.
Step 3: Multiply H₂O on right by 2 to balance H and O:
Balanced Equation: 2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
Q6) Identify the oxidising agent and reducing agent in the following reactions:
1. 2H₂S + SO₂ → 3S + 2H₂O
Answer :
  • Reducing Agent: H₂S (Hydrogen sulfide loses hydrogen / increases oxidation state of S from -2 to 0; hence it is oxidized and acts as the reducing agent).
  • Oxidising Agent: SO₂ (Sulfur dioxide loses oxygen / decreases oxidation state of S from +4 to 0; hence it is reduced and acts as the oxidising agent).
2. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
Answer :
  • Oxidising Agent: MnO₂ (Manganese dioxide loses oxygen, reducing Mn from +4 to +2 in MnCl₂).
  • Reducing Agent: HCl (Hydrochloric acid loses hydrogen and is oxidized to Cl₂ gas).
Q7) What is the difference between physical change and chemical change?
Answer :
Physical Change Chemical Change
Only physical properties (state, size, shape) change; no new substance is formed. Chemical composition changes and one or more completely new substances are formed.
It is usually temporary and easily reversible. It is permanent and generally irreversible.
Chemical bonds are not broken or formed. Existing chemical bonds break and new chemical bonds form.
Example: Melting of ice, dissolution of salt in water. Example: Burning of coal, rusting of iron, digestion of food.
Q8) What is corrosion? How can it be prevented?
Answer :

Corrosion is the slow, continuous, and spontaneous destruction of metals caused by chemical or electrochemical reactions with components of the surrounding environment (atmospheric moisture, oxygen, acids, and gases like CO₂ and H₂S).

Methods of Preventing Corrosion:

  1. Barrier Coatings: Applying paint, oil, grease, or varnish on metal surfaces to block moisture and oxygen contact.
  2. Galvanizing: Coating iron and steel articles with a thin protective layer of molten zinc (Zn). Zinc corrodes preferentially (sacrificial protection).
  3. Tinning: Coating copper and brass utensils with molten tin (Sn) to prevent toxic green verdigris formation.
  4. Anodizing: Electrolytically forming a thick, uniform protective oxide layer on metals like aluminium.
  5. Alloying: Mixing metals with other elements to form corrosion-resistant alloys (e.g. Stainless steel: Iron + Chromium + Nickel).
Q9) Explain the electrochemical mechanism of rusting of iron with the help of a neat labelled diagram.
Answer :

The rusting of iron is an electrochemical redox process occurring within a water droplet on the metal surface:

  1. Anodic Region: Iron atoms in contact with water oxidize to ferrous ions:
    Fe(s) → Fe²⁺(aq) + 2e⁻   (Oxidation)
  2. Cathodic Region: Electrons travel through the bulk metal to the edge of the droplet, where dissolved atmospheric oxygen is reduced in an acidic or neutral environment:
    O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)   (Reduction)
  3. Rust Deposition: Ferrous ions migrate through the droplet, react with water, and oxidize further to form hydrated ferric oxide [Fe₂O₃ · xH₂O] (rust precipitate):
    2Fe²⁺(aq) + 4H₂O(l) + ½O₂(g) → Fe₂O₃ · xH₂O(s)↓ + 4H⁺(aq)
Iron Surface (Fe Metal) Water Droplet with Dissolved O₂ Anode (Oxidation) Fe → Fe²⁺ + 2e⁻ Cathode (Reduction) O₂ + 4H⁺ + 4e⁻ → 2H₂O Electron Flow (e⁻) in Metal ➔ Rust (Fe₂O₃ · xH₂O)
Fig 3.1: Electrochemical Mechanism of Rusting of Iron
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Chapter Overview: Chemical Reactions and Equations

Class 10 Maharashtra State Board Science 1 Chapter 3 covers the core principles of chemical equations, writing and balancing chemical equations by the hit-and-trial method, types of chemical reactions (combination, decomposition, displacement, double displacement, endothermic, and exothermic), rate of reaction factors (concentration, temperature, particle size, catalyst), redox reactions (oxidation and reduction), and chemical decay processes including corrosion and rancidity.

Frequently Asked Questions (FAQs)

Q1. Why must a chemical equation always be balanced?

To adhere to the Law of Conservation of Mass, which states that atoms cannot be created or destroyed during a chemical change; hence atom counts on both sides must be equal.

Q2. What is a catalyst and how does it influence a reaction?

A catalyst is a substance that accelerates the rate of a chemical reaction without undergoing any permanent chemical alteration itself (e.g. MnO₂ in hydrogen peroxide decomposition).

Q3. What is the difference between displacement and double displacement reactions?

In a single displacement reaction, a more reactive element displaces a less reactive element from its compound. In a double displacement reaction, two compounds exchange their ions to form two new compounds, often forming a precipitate.

Q4. What is rancidity and how is it prevented?

Rancidity is the aerial oxidation of fats and oils leading to unpleasant odor and taste. It is prevented by airtight packaging, refrigeration, and adding antioxidants.

Related Study Topics

Chemical Reactions Class 10 Balancing Chemical Equations Redox Reactions Electrochemical Rusting Endothermic vs Exothermic Corrosion Prevention Methods SSC Board Science 1 Solutions
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