Chemical Reactions and Equations (Class 10): Notes और MCQ Practice

आसान भाषा में

जब एक या अधिक substances आपस में react करके बिल्कुल नए properties वाले नए substances बनाते हैं, तो उसे Chemical Reaction कहते हैं। इसमें पुराने chemical bonds टूटते हैं और नए bonds बनते हैं। इसी reaction को symbols और chemical formulas की मदद से short में लिखना Chemical Equation कहलाता है।

Most Important Points & Definitions

1. Law of Conservation of Mass: किसी भी chemical reaction में mass create या destroy नहीं हो सकता। इसलिए Reaction हमेशा Balanced होनी चाहिए (LHS और RHS में atoms की संख्या बराबर हो)।
2. Combination Reaction: दो या अधिक reactants मिलकर केवल एक single product बनाते हैं।

• Example: CaO (Quick lime) + H₂O → Ca(OH)₂ (Slaked lime) + Heat

1. Decomposition Reaction: एक single reactant टूटकर दो या अधिक products देता है (Heat, Light या Electricity से)।

• Example (Thermal): 2FeSO₄ + Heat → Fe₂O₃ + SO₂ + SO₃

1. Displacement Reaction: ज़्यादा reactive element, कम reactive element को उसके solution से displace कर देता है (Fe + CuSO₄ → FeSO₄ + Cu).
2. Double Displacement: Reactants के बीच ions का exchange होता है। इससे अक्सर insoluble precipitate बनता है (Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl).
3. Redox Reactions:

• Oxidation: Oxygen का जुड़ना या Hydrogen का हटना।
• Reduction: Hydrogen का जुड़ना या Oxygen का हटना।

1. Corrosion & Rancidity: Metals का moisture/air से खराब होना Corrosion है (e.g., Rusting of iron)। Fats और oils का oxidize होकर smelly होना Rancidity कहलाता है (इसे रोकने के लिए Nitrogen gas use होती है)।

Memory Tricks

• Trick 1 (OIL RIG): Oxidation Is Loss (of electrons/gain of O), Reduction Is Gain (of electrons/loss of O).
• Trick 2 (CDR): Reactions याद रखने के लिए — Combination (जुड़ना), Decomposition (टूटना), Displacement (जगह छीनना)।

Common Board-Exam Mistakes

1. Formula का Subscript बदलना: Equation balance करते वक्त कभी formula मत बदलो (e.g., H₂O को balance करने के लिए H₂O₂ मत लिखो, हमेशा coefficient आगे लगाओ: 2H₂O)।
2. Oxidising vs Reducing Agent confusion: जिस reactant का Oxidation होता है, वही Reducing Agent होता है; और जिसका Reduction होता है, वही Oxidising Agent होता है। ये दोनों हमेशा Reactants साइड से ही चुने जाते हैं, Products से नहीं!

Chemical Reactions and Equations: Class 10 Study Notes

1. Chapter in Short

This chapter introduces the language of chemistry. Whenever a chemical change occurs, a chemical reaction takes place. In a reaction, old chemical bonds break and new bonds form, producing completely new substances with different properties.

Because matter cannot be created or destroyed (Law of Conservation of Mass), chemical equations must always be balanced so that the number of atoms of each element remains identical before and after the reaction.

The chapter classifies chemical reactions into five main categories:

1. Combination reactions
2. Decomposition reactions
3. Displacement reactions
4. Double displacement reactions
5. Oxidation-Reduction (Redox) reactions

Finally, it explores how oxidation affects our daily lives through corrosion (like rusting of iron) and rancidity (spoilage of oily food).

2. Key Concepts Explained Simply

A. What is a Chemical Reaction?

A chemical reaction is a process in which one or more substances (called reactants) change into new substances with new chemical properties (called products).

Everyday Examples:

• Cooking food.
• Souring of milk when left at room temperature in summer.
• Digestion of food in our stomach.
• Rusting of an iron nail exposed to moist air.

Signs that a Chemical Reaction has occurred:

• Change in state (e.g., gas formed from liquids)
• Change in colour (e.g., blue copper sulphate solution turns green when iron is added)
• Evolution of a gas (e.g., bubbles of hydrogen gas when zinc reacts with acid)
• Change in temperature (e.g., container gets hot when quicklime reacts with water)

B. Writing and Balancing Chemical Equations

• Word Equation: Magnesium + Oxygen → Magnesium oxide
• Skeletal Equation (Unbalanced): Mg + O₂ → MgO
• Balanced Equation: 2Mg + O₂ → 2MgO

Why balance? According to the Law of Conservation of Mass, the total mass of reactants must equal the total mass of products. You balance by adding whole numbers (coefficients) in front of formulas. Never change the small subscript numbers inside a formula!

C. Types of Chemical Reactions

1. Combination Reaction

Two or more reactants combine to form a single product.

• General formula: A + B → AB
• Example (White washing): Quicklime (calcium oxide) reacts vigorously with water to form slaked lime (calcium hydroxide):

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

2. Decomposition Reaction

A single reactant breaks down to give two or more simpler products. This reaction requires energy (heat, light, or electricity).

• General formula: AB → A + B
• Thermal decomposition (by heat):
• Heating green ferrous sulphate crystals:

2FeSO₄(s) + Heat → Fe₂O₃(s) + SO₂(g) + SO₃(g)

• Limestone turning into quicklime:

CaCO₃(s) + Heat → CaO(s) + CO₂(g)

• Heating lead nitrate (produces brown fumes of NO₂):

2Pb(NO₃)₂(s) + Heat → 2PbO(s) + 4NO₂(g) + O₂(g)

• Electrolytic decomposition (by electricity):
• Splitting of water into hydrogen and oxygen:

2H₂O(l) + Electricity → 2H₂(g) + O₂(g)

• Photolytic decomposition (by sunlight):
• White silver chloride turns grey in sunlight (used in black & white photography):

2AgCl(s) + Sunlight → 2Ag(s) + Cl₂(g)

3. Displacement Reaction

A more reactive element kicks out (displaces) a less reactive element from its compound solution.

• General formula: A + BC → AC + B
• Example: An iron nail placed in blue copper sulphate solution:

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

Observation: Blue solution turns pale green, and a reddish-brown coating of copper deposits on the iron nail.

4. Double Displacement Reaction

Two compounds exchange their ions to form two new compounds. Often, an insoluble solid called a precipitate settles down (known as a Precipitation Reaction).

• General formula: AB + CD → AD + CB
• Example: Sodium sulphate reacts with barium chloride:

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Observation: A white precipitate of barium sulphate (BaSO₄) is formed.

5. Oxidation and Reduction (Redox Reactions)

• Oxidation: Gain of oxygen OR loss of hydrogen.
• Reduction: Loss of oxygen OR gain of hydrogen.
• When both happen simultaneously, it is called a Redox Reaction.
• Example:

CuO + H₂ + Heat → Cu + H₂O

• Here, CuO loses oxygen to become Cu (CuO is reduced).
• H₂ gains oxygen to become H₂O (H₂ is oxidised).

D. Energy Changes: Exothermic vs. Endothermic

• Exothermic Reactions: Reactions in which heat is released along with products.
• Examples: Burning of natural gas (CH₄ + 2O₂ → CO₂ + 2H₂O + Heat), Respiration, Decomposition of vegetable matter into compost.
• Endothermic Reactions: Reactions in which energy is absorbed from the surroundings.
• Examples: All thermal decomposition reactions, photosynthesis.

E. Effects of Oxidation in Daily Life

1. Corrosion: The slow eating away of metals by the action of air, moisture, or chemicals on their surface.

• Rusting of iron: Iron develops a reddish-brown flaky coating in moist air.
• Silver: Develops a black coating (silver sulphide, Ag₂S).
• Copper: Develops a green coating (basic copper carbonate).
• Prevention: Painting, greasing, galvanisation (coating with zinc).

1. Rancidity: When fats and oils present in food are oxidised, their smell and taste change, making the food bad.

• Prevention: Adding antioxidants, storing in airtight containers, flushing potato chip bags with inert nitrogen gas (N₂).

3. Important Definitions, Formulas, and Facts

• Reactants: Substances that take part in a chemical reaction (written on the left side).
• Products: New substances formed in a chemical reaction (written on the right side).
• Precipitate: An insoluble solid formed during a chemical reaction in a liquid solution.
• Quicklime: Calcium oxide, formula: CaO.
• Slaked lime: Calcium hydroxide, formula: Ca(OH)₂.
• Limestone / Marble / Chalk: Calcium carbonate, formula: CaCO₃.
• Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. Total mass of reactants = Total mass of products.
• Ratio of gases in electrolysis of water: The volume of hydrogen gas collected is double the volume of oxygen gas (Ratio is 2 : 1) because a water molecule (H₂O) contains 2 atoms of hydrogen for every 1 atom of oxygen.

4. Solved Examples & Board-Style Questions

Question 1: Balancing a Chemical Equation

Problem: Balance the skeletal equation: Fe + H₂O → Fe₃O₄ + H₂

Step-by-step Solution:

1. Count atoms on both sides:

• Left (Reactants): Fe = 1, H = 2, O = 1
• Right (Products): Fe = 3, H = 2, O = 4

1. Balance Oxygen first: Put 4 in front of H₂O:

Fe + 4H₂O → Fe₃O₄ + H₂

(Now O is 4 on both sides, but H becomes 4 × 2 = 8 on the left).

1. Balance Hydrogen: Put 4 in front of H₂ on the right:

Fe + 4H₂O → Fe₃O₄ + 4H₂

1. Balance Iron (Fe): Put 3 in front of Fe on the left:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

1. Final Check: Fe: 3 = 3; H: 8 = 8; O: 4 = 4. The equation is balanced!

Question 2: The White-Wash Phenomenon

Problem: A solution of substance 'X' is used for whitewashing walls.

(a) Name substance 'X' and write its chemical formula.

(b) Write the reaction of substance 'X' with water.

(c) Why do walls develop a shiny finish after 2 to 3 days of whitewashing?

Answer:

(a) Substance 'X' is Calcium oxide (Quicklime). Formula: CaO.

(b) CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

(c) Slaked lime [Ca(OH)₂] applied to the wall reacts slowly with carbon dioxide (CO₂) present in the air to form a thin, hard layer of calcium carbonate (CaCO₃), which gives the walls a shiny white appearance:

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Question 3: Identifying Oxidation and Reduction

Problem: In the reaction: MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

Identify:

(a) The substance oxidised

(b) The substance reduced

(c) The oxidising agent

(d) The reducing agent

Answer:

• Substance oxidised: HCl (loses hydrogen to form Cl₂).
• Substance reduced: MnO₂ (loses oxygen to form MnCl₂).
• Oxidising agent: MnO₂ (it gives oxygen / causes oxidation).
• Reducing agent: HCl (it removes oxygen / causes reduction).

Question 4: Conceptual Reasoning

Problem: Why is respiration considered an exothermic reaction?

Answer:

During digestion, carbohydrates are broken down into glucose. In our body cells, this glucose combines with inhaled oxygen to produce carbon dioxide, water, and releases a large amount of energy that keeps our body working:

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

Because energy is released in this process, respiration is classified as an exothermic reaction.

5. Common Mistakes and Quick Revision Points

Common Mistakes to Avoid:

• Changing subscripts while balancing: Writing H₂ + O₂ → H₂O₂ to balance oxygen is wrong. You must only add coefficients in front: 2H₂ + O₂ → 2H₂O.
• Confusing Quicklime and Slaked lime: Remember, Quicklime is dry (CaO). Add water and it becomes Slaked lime [Ca(OH)₂].
• Confusing Displacement with Double Displacement:
• Displacement involves an element and a compound (A + BC → AC + B).
• Double displacement involves two compounds exchanging ions (AB + CD → AD + CB).
• Muddling up Oxidising Agent and Substance Oxidised: The substance that gets reduced acts as the oxidising agent. The substance that gets oxidised acts as the reducing agent.

Quick Revision Points (Memory Hacks):

• Exothermic = Heat Exits (given out).
• Endothermic = Heat Enters (absorbed).
• OIL RIG (alternative memory rule):
• Oxidation Is Loss of electrons / gain of oxygen.
• Reduction Is Gain of electrons / loss of oxygen.
• Colour Changes to Remember:
• FeSO₄ crystals: Green → Brown-red (Fe₂O₃) upon heating.
• CuSO₄ solution: Blue → Pale green (FeSO₄) when iron nails are added.
• Pb(NO₃)₂: White powder → Yellow residue (PbO) + Brown gas (NO₂) upon heating.