Chemical Reactions
Overview: Study of chemical changes, balancing equations, and types of reactions like combination, decomposition, displacement, and redox.
1. Chemical Reactions & Equations
Chemical Reaction: A process in which new substances with new properties are formed.
- Reactants: Substances which take part in a chemical reaction.
- Products: New substances formed after a chemical reaction.
Magnesium + Oxygen → Magnesium Oxide
2Mg + O2 → 2MgO
2Mg + O2 → 2MgO
Characteristics: Evolution of gas, Change in color, Change in state, Change in temperature.
2. Balanced Chemical Equations
Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction. Total mass of reactants = Total mass of products.
To satisfy this law, we balance chemical equations.
Fe + H2O → Fe3O4 + H2 (Unbalanced)
3Fe + 4H2O → Fe3O4 + 4H2 (Balanced)
3. Types of Chemical Reactions
A. Combination Reaction
Two or more reactants combine to form a single product.
CaO (Quick Lime) + H2O → Ca(OH)2 (Slaked Lime) + Heat
This is an exothermic reaction (heat is released).
B. Decomposition Reaction
A single reactant breaks down to form two or more products. Requires energy (Heat, Light, or Electricity).
- Thermal Decomposition: CaCO3 → CaO + CO2 (Heat)
- Electrolytic Decomposition: 2H2O → 2H2 + O2 (Electricity)
- Photolytic Decomposition: 2AgCl → 2Ag + Cl2 (Sunlight) - Used in B&W photography.
C. Displacement Reaction
A more reactive element displaces a less reactive element from its compound solution.
Fe + CuSO4 (Blue) → FeSO4 (Green) + Cu
Iron displaces Copper because Iron is more reactive.
D. Double Displacement Reaction
Exchange of ions between reactants takes place.
Na2SO4 + BaCl2 → BaSO4 (White ppt) + 2NaCl
Precipitation Reaction: Any reaction that produces a precipitate (solid insoluble substance).
4. Oxidation and Reduction
- Oxidation: Gain of Oxygen or Loss of Hydrogen.
- Reduction: Loss of Oxygen or Gain of Hydrogen.
- Redox: Both oxidation and reduction occur simultaneously.
CuO + H2 → Cu + H2O
CuO is reduced to Cu. H2 is oxidized to H2O.
5. Effects of Oxidation in Daily Life
Corrosion
Metals are eaten up by action of air, moisture, or acid. E.g., Rusting of Iron (Reddish-brown coating), Black coating on Silver, Green coating on Copper.
Rancidity
Oxidation of fats and oils when exposed to air, causing bad smell and taste.
Prevention: Adding antioxidants, Packaging in Nitrogen gas like chips packets.
NCERT In-Text & Exercise Questions
Q1. Why should a magnesium ribbon be cleaned before burning in air?
To remove the protective layer of magnesium oxide formed on its surface so that it can burn continually.
Q2. What is a balanced chemical equation? Why should chemical equations be balanced? CBSE 2018
An equation having equal number of atoms of each element on both sides. It is balanced to satisfy the Law of Conservation of Mass (Mass can neither be created nor destroyed).
Q3. Translate: Hydrogen gas combines with nitrogen to form ammonia.
3H2 + N2 → 2NH3
Q4. A solution of substance 'X' is used for whitewashing. Name 'X' and write its formula.
'X' is Calcium oxide (Quick Lime). Formula: CaO. Reaction with water forms Slaked Lime for whitewashing.
Q5. Why does the color of copper sulfate solution change when an iron nail is dipped in it? CBSE 2019
Iron is more reactive than copper. It displaces copper from CuSO4 solution to form Iron Sulfate (FeSO4) which is green in color. The blue color fades.
Q6. Oil and fat containing food items are flushed with Nitrogen. Why? CBSE 2020
To prevent oxidation (rancidity) which causes bad taste and smell. Nitrogen is an inert gas and prevents contact with oxygen.
Balancing & Reasoning
Mastering this chapter requires practice in balancing equations and identifying reaction types.
1. Balancing Chemical Equations
Question: Balance the following equation:
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Solution:
Step 1: Balance Calcium (Ca) and Nitrogen (N).
LHS: 1 Ca, 1 N. RHS: 1 Ca, 2 N.
Multiply HNO3 by 2.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Step 2: Balance Hydrogen (H) and Oxygen (O).
LHS: 2 H + 2 H = 4 H. RHS: 2 H.
Multiply H2O by 2.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
Step 1: Balance Calcium (Ca) and Nitrogen (N).
LHS: 1 Ca, 1 N. RHS: 1 Ca, 2 N.
Multiply HNO3 by 2.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Step 2: Balance Hydrogen (H) and Oxygen (O).
LHS: 2 H + 2 H = 4 H. RHS: 2 H.
Multiply H2O by 2.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
2. Identifying Reaction Types
Question: Identify the type of reaction:
(a) Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(b) 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
(a) Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(b) 2Pb(NO3)2(s) → 2PbO(s) + 4NO2(g) + O2(g)
Solution:
(a) Iron displaces Copper from Copper Sulfate.
Type: Displacement Reaction
(b) A single reactant breaks down into simpler products on heating.
Type: Thermal Decomposition Reaction
(a) Iron displaces Copper from Copper Sulfate.
Type: Displacement Reaction
(b) A single reactant breaks down into simpler products on heating.
Type: Thermal Decomposition Reaction
3. Redox Reactions
Question: Identify the substance oxidized and reduced in:
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2
Solution:
1. Identify loss/gain of Oxygen/Hydrogen.
MnO2 loses Oxygen → Reduced.
HCl loses Hydrogen (to form Cl2) → Oxidized.
Substance Oxidized: HCl
Substance Reduced: MnO2
1. Identify loss/gain of Oxygen/Hydrogen.
MnO2 loses Oxygen → Reduced.
HCl loses Hydrogen (to form Cl2) → Oxidized.
Substance Oxidized: HCl
Substance Reduced: MnO2
4. Practical Based Questions
Question: What happens when dilute Hydrochloric acid is added to Iron filings?
Solution:
Iron reacts with dilute acid to liberate Hydrogen gas.
Equation: Fe + 2HCl → FeCl2 + H2
Result: Hydrogen gas and Iron chloride are produced.
Iron reacts with dilute acid to liberate Hydrogen gas.
Equation: Fe + 2HCl → FeCl2 + H2
Result: Hydrogen gas and Iron chloride are produced.
5. Diagram Based Analysis
[ Test Tube ]
|
|-- [Lead Nitrate]
| (Heating)
|
(Emission of brown fumes)
Question: Identify the brown fumes evolved on heating Lead Nitrate. Write the reaction.
Solution:
The brown fumes are of Nitrogen Dioxide (NO2).
Reaction:
2Pb(NO3)2 (Heat) → 2PbO + 4NO2 (Brown Fumes) + O2
The brown fumes are of Nitrogen Dioxide (NO2).
Reaction:
2Pb(NO3)2 (Heat) → 2PbO + 4NO2 (Brown Fumes) + O2
Key Reactions & Facts
List of important chemical reactions, color changes, and facts.
Important Reactions
Magnesium Burning
2Mg + O2 → 2MgO (White Powder)
Quick Lime + Water
CaO + H2O → Ca(OH)2 + Heat
Respiration
C6H12O6 + 6O2 → 6CO2 +
6H2O
Decomp. of FeSO4
2FeSO4 → Fe2O3 + SO2 +
SO3
Decomp. of Pb(NO3)2
2Pb(NO3)2 → 2PbO + 4NO2 +
O2
Silver Chloride in Sun
2AgCl → 2Ag + Cl2 (Grey)
Iron + Copper Sulfate
Fe + CuSO4 → FeSO4 + Cu
50 Important Facts
1. Magnesium ribbon burns with a dazzling white
flame.
2. Magnesium Oxide is a white powder.
3. A chemical change is a permanent change.
4. Evolution of gas indicates a chemical reaction.
5. Change in color indicates a chemical reaction.
6. Change in state indicates a chemical reaction.
7. Change in temperature indicates a chemical
reaction.
8. Reactants are on the left side of the arrow.
9. Products are on the right side of the arrow.
10. Law of Conservation of Mass governs balancing.
11. Skeleton equation is an unbalanced equation.
12. (s) stands for Solid state.
13. (l) stands for Liquid state.
14. (g) stands for Gaseous state.
15. (aq) stands for Aqueous (solution in water).
16. Calcium Oxide is Quick Lime.
17. Calcium Hydroxide is Slaked Lime.
18. Reaction of CaO with water is highly exothermic.
19. Slaked lime is used for whitewashing.
20. CaCO3 is Limestone/Marble/Chalk.
21. Combination reaction: A + B -> AB.
22. Burning of coal is a combination reaction.
23. Formation of water is a combination reaction.
24. Exothermic reactions release heat.
25. Respiration is an exothermic process.
26. Decomposition: AB -> A + B.
27. Endothermic reactions absorb energy.
28. Ferrous Sulfate crystals are Green (FeSO4.7H2O).
29. Heating FeSO4 gives brown Ferric Oxide.
30. Smell of burning sulfur is SO2 gas.
31. Lead Nitrate is white powder.
32. Brown fumes on heating Lead Nitrate is NO2.
33. Electrolysis of water is decomposition.
34. Volume of Hydrogen is double of Oxygen in
electrolysis.
35. Silver Chloride turns Grey in sunlight.
36. Silver Bromide is used in photography.
37. Displacement: A + BC -> AC + B.
38. Iron nail becomes brownish in CuSO4.
39. Blue CuSO4 turns Green (FeSO4) with Iron.
40. Zinc is more reactive than Copper.
41. Double Displacement exchanges ions.
42. Barium Sulfate (BaSO4) is a white precipitate.
43. Precipitation reactions produce insoluble salts.
44. Oxidation is gain of Oxygen.
45. Reduction is loss of Oxygen.
46. Redox = Reduction + Oxidation.
47. Corrosion is degradation of metals.
48. Rust is hydrated iron oxide.
49. Rancidity spoils taste of fats/oils.
50. Antioxidants prevent rancidity.
📱 Practice MCQs for this topic inside our App
📱 Practice MCQs for this topic inside our App
📱 Practice MCQs for this topic inside our App
📱 Practice MCQs for this topic inside our App