Chemical Reactions and Equations
1. Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different physical and chemical properties.
Characteristics/Indicators (Signs of a reaction):
- Change in state (solid → liquid/gas)
- Change in colour
- Evolution of gas (bubbles/odor)
- Change in temperature (heat released/absorbed)
- Formation of precipitate (insoluble solid)
- Evolution of light
Example: Burning of magnesium ribbon → bright light + white ash (magnesium oxide)
2. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using formulas of reactants and products.
Path: Word equation → Skeleton equation → Balanced equation
Example:
Word: Magnesium + Oxygen → Magnesium oxide
Skeleton: Mg + O₂ → MgO
Balanced: 2Mg + O₂ → 2MgO
Word: Magnesium + Oxygen → Magnesium oxide
Skeleton: Mg + O₂ → MgO
Balanced: 2Mg + O₂ → 2MgO
Physical States & Symbols:
(s)= solid,(l)= liquid,(g)= gas,(aq)= aqueous solution↓= precipitate,↑= gas evolved,Δ= heat
3. Balanced Chemical Equation
Why balance? According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. The number of atoms of each element must be the same on both sides (LHS = RHS).
Steps to Balance (Hit & Trial Method):
- Write correct formulas for reactants and products.
- Write the skeleton equation.
- Count atoms of each element on both sides.
- Balance the most complex molecule first, then single elements, and usually H & O last.
- Verify the balance with small whole number coefficients.
Example:
Unbalanced: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Unbalanced: Fe + H₂O → Fe₃O₄ + H₂
Balanced: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
4. Types of Chemical Reactions
| Type | Definition | General Form | Example Equation | Characteristics |
|---|---|---|---|---|
| Combination | Substances combine to form a single product | A + B → AB | CaO + H₂O → Ca(OH)₂ | Exothermic (usually) |
| Decomposition | Compound breaks down into simpler substances | AB → A + B | 2H₂O → 2H₂ + O₂ | Endothermic (needs energy) |
| Displacement | More reactive element displaces less reactive one | A + BC → AC + B | Fe + CuSO₄ → FeSO₄ + Cu | Based on Reactivity Series |
| Double Displacement | Exchange of ions between two compounds | AB + CD → AD + CB | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl | Often forms a precipitate |
| Redox | Simultaneous Oxidation and Reduction | — | CuO + H₂ → Cu + H₂O | Oxidation = Gain of O / Loss of H |
5. Effects of Oxidation in Everyday Life
Corrosion
Slow eating away of metals due to oxygen, moisture, etc. Example: Rusting of iron.
Prevention: Painting, galvanisation (Zn coating), oiling/greasing.
Rancidity
Oxidation of fats & oils in food leading to bad smell/taste.
Prevention: Adding antioxidants, nitrogen flushing in chip packets, airtight containers.
Quick Exam Tips:
- Always include physical states
(s, l, g, aq). - Respiration is exothermic; Photosynthesis is endothermic.
- Memorize key reactions like
2Mg + O₂ → 2MgO.
MCQs: Chemical Reactions and Equations
1. Which of the following is not a physical change?
2. The reaction 2H₂ + O₂ → 2H₂O is an example of:
3. Which type of reaction usually requires heat, light or electricity?
4. In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, the type of reaction is:
5. The reaction Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl is:
6. The addition of oxygen to a substance is called:
7. Which of the following is an example of a redox reaction?
8. The brown gas evolved when lead nitrate is heated is:
9. Rusting of iron is an example of:
10. Which of these methods helps prevent rancidity in food items?
11. The balanced equation for the reaction of iron with steam is:
12. In which reaction is a precipitate formed?
13. The reaction 2AgCl → 2Ag + Cl₂ (in sunlight) is an example of:
14. Which substance acts as an oxidising agent in the reaction: CuO + H₂ → Cu + H₂O?
15. The chemical name of the white precipitate formed when BaCl₂ is added to Na₂SO₄ is:
16. Which of the following is an exothermic reaction?
17. The process responsible for the grey coating on silver articles is:
18. Which law is satisfied when a chemical equation is balanced?
19. The reaction 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (on heating) is:
20. In chips packets, nitrogen gas is flushed to:
- A) Melting of ice
- B) Evaporation of water
- C) Burning of magnesium ribbon
- D) Dissolving sugar in water
Answer: C
2. The reaction 2H₂ + O₂ → 2H₂O is an example of:
- A) Decomposition reaction
- B) Displacement reaction
- C) Combination reaction
- D) Double displacement reaction
Answer: C
3. Which type of reaction usually requires heat, light or electricity?
- A) Combination reaction
- B) Decomposition reaction
- C) Displacement reaction
- D) Redox reaction
Answer: B
4. In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, the type of reaction is:
- A) Combination
- B) Decomposition
- C) Displacement
- D) Double displacement
Answer: C
5. The reaction Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl is:
- A) Combination reaction
- B) Decomposition reaction
- C) Displacement reaction
- D) Double displacement reaction
Answer: D
6. The addition of oxygen to a substance is called:
- A) Reduction
- B) Oxidation
- C) Redox
- D) Precipitation
Answer: B
7. Which of the following is an example of a redox reaction?
- A) CaCO₃ → CaO + CO₂
- B) CuO + H₂ → Cu + H₂O
- C) NaCl + AgNO₃ → AgCl + NaNO₃
- D) 2H₂O → 2H₂ + O₂
Answer: B
8. The brown gas evolved when lead nitrate is heated is:
- A) SO₂
- B) NO₂
- C) CO₂
- D) O₂
Answer: B
9. Rusting of iron is an example of:
- A) Combination reaction
- B) Slow oxidation
- C) Fast decomposition
- D) Displacement reaction
Answer: B
10. Which of these methods helps prevent rancidity in food items?
- A) Adding antioxidants
- B) Heating strongly
- C) Exposing to sunlight
- D) Keeping in open air
Answer: A
11. The balanced equation for the reaction of iron with steam is:
- A) Fe + H₂O → Fe₃O₄ + H₂
- B) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
- C) 2Fe + 3H₂O → Fe₂O₃ + 3H₂
- D) Fe + 2H₂O → Fe(OH)₂ + H₂
Answer: B
12. In which reaction is a precipitate formed?
- A) Zn + H₂SO₄ → ZnSO₄ + H₂
- B) Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃
- C) CaO + H₂O → Ca(OH)₂
- D) 2Mg + O₂ → 2MgO
Answer: B
13. The reaction 2AgCl → 2Ag + Cl₂ (in sunlight) is an example of:
- A) Thermal decomposition
- B) Electrolytic decomposition
- C) Photolytic decomposition
- D) Displacement reaction
Answer: C
14. Which substance acts as an oxidising agent in the reaction: CuO + H₂ → Cu + H₂O?
- A) H₂
- B) Cu
- C) CuO
- D) H₂O
Answer: C
15. The chemical name of the white precipitate formed when BaCl₂ is added to Na₂SO₄ is:
- A) Barium chloride
- B) Sodium sulphate
- C) Barium sulphate
- D) Sodium chloride
Answer: C
16. Which of the following is an exothermic reaction?
- A) Photosynthesis
- B) Decomposition of CaCO₃
- C) Respiration
- D) Electrolysis of water
Answer: C
17. The process responsible for the grey coating on silver articles is:
- A) Corrosion
- B) Rancidity
- C) Reduction
- D) Precipitation
Answer: A
18. Which law is satisfied when a chemical equation is balanced?
- A) Law of definite proportions
- B) Law of conservation of mass
- C) Law of multiple proportions
- D) Law of combining volumes
Answer: B
19. The reaction 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ (on heating) is:
- A) Combination reaction
- B) Displacement reaction
- C) Thermal decomposition
- D) Double displacement
Answer: C
20. In chips packets, nitrogen gas is flushed to:
- A) Increase taste
- B) Prevent rancidity
- C) Make it heavier
- D) Change colour
Answer: B
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