🔷Chapter 1: Chemical Reactions and Equations
A chemical reaction is a process in which one or more substances (reactants) change into new substances (products) with different properties. Chemical reactions occur in daily life such as rusting of iron, burning of fuels, digestion of food, respiration, and fermentation.
Indicators of a Chemical Reaction
A chemical reaction can be identified by:
- Change in colour
- Change in state
- Evolution of gas
- Change in temperature (heat released or absorbed)
Chemical Equations
Chemical reactions are represented using chemical equations.
- Word Equation
Example:
Magnesium + Oxygen → Magnesium oxide - Chemical Equation
Mg + O₂ → MgO - Balanced Chemical Equation
According to the Law of Conservation of Mass, mass can neither be created nor destroyed. Therefore, the number of atoms of each element must be equal on both sides of the equation.
Balancing of Chemical Equations
- Done using the hit and trial method
- Coefficients are adjusted, formulas are not changed
- Physical states may be shown:
(s), (l), (g), (aq)
🔷 TYPES OF CHEMICAL REACTIONS
1. Combination Reaction
Two or more substances combine to form a single product.
Example:
CaO + H₂O → Ca(OH)₂ + Heat
These are often exothermic reactions (release heat).
2. Decomposition Reaction
A single compound breaks down into simpler substances. Energy is required.
Types:
- Thermal decomposition (Heat)
CaCO₃ → CaO + CO₂ - Photolytic decomposition (Light)
2AgCl → 2Ag + Cl₂ - Electrolytic decomposition (Electricity)
2H₂O → 2H₂ + O₂
These are usually endothermic reactions.
3. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
Example:
Fe + CuSO₄ → FeSO₄ + Cu
4. Double Displacement Reaction
Exchange of ions between compounds.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
If an insoluble solid is formed, it is called a precipitation reaction.
Oxidation and Reduction (Redox Reactions)
- Oxidation: Gain of oxygen / Loss of hydrogen
- Reduction: Loss of oxygen / Gain of hydrogen
Both occur simultaneously in a reaction.
Effects of Oxidation in Everyday Life
1. Corrosion
Gradual destruction of metals by reaction with air, moisture, or chemicals.
Example: Rusting of iron.
2. Rancidity
Oxidation of fats and oils causing bad smell and taste.
Prevented by:
- Airtight containers
- Refrigeration
- Adding antioxidants
- Flushing food packets with nitrogen
🔷 IMPORTANT QUESTIONS WITH ANSWERS
🔹 Very Short Answer (1 Mark)
Q1. Why should magnesium ribbon be cleaned before burning?
Ans: To remove the protective layer of magnesium oxide so that magnesium burns easily.
🔹 Short Answer (2–3 Marks)
Q2. Define a balanced chemical equation.
Ans: A chemical equation in which the number of atoms of each element is equal on both sides of the equation.
Q3. Why is respiration considered an exothermic reaction?
Ans: Because energy is released when glucose reacts with oxygen during respiration.
Q4. What is a precipitation reaction? Give an example.
Ans: A reaction in which an insoluble solid (precipitate) is formed.
Example:
Na₂SO₄ + BaCl₂ → BaSO₄ ↓ + 2NaCl
🔹 Long Answer (5 Marks)
Q5. Explain decomposition reactions with examples of heat, light, and electricity.
Ans:
A decomposition reaction is one in which a single compound breaks down into two or more simpler substances.
- Heat: CaCO₃ → CaO + CO₂
- Light: 2AgCl → 2Ag + Cl₂
- Electricity: 2H₂O → 2H₂ + O₂
Q6. Differentiate between displacement and double displacement reactions.
| Displacement | Double Displacement |
|---|---|
| One element replaces another | Exchange of ions |
| One compound involved | Two compounds involved |
| Example: Fe + CuSO₄ | Example: Na₂SO₄ + BaCl₂ |
Q7. What is corrosion? How can it be prevented?
Ans:
Corrosion is the slow destruction of metals due to chemical reactions with air and moisture.
Prevention methods include painting, oiling, galvanisation, alloying, and coating.
🔹 Case-Based / Competency Questions (Frequently Asked)
Q8. A brown metal turns black on heating in air. Identify the metal and compound formed.
Ans:
Metal: Copper
Compound: Copper(II) oxide (CuO)
🔹 HOTS Question
Q9. Why are chips packets flushed with nitrogen gas?
Ans:
To prevent oxidation of fats and oils and avoid rancidity.
Short Answer Type (1–2 Marks)
1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: It is cleaned to remove the layer of magnesium oxide/carbonate so it can burn properly.
2. Define a precipitation reaction.
Answer: Any reaction that produces an insoluble substance (precipitate) is called a precipitation reaction.
3. Why is respiration considered an exothermic reaction?
Answer: During digestion, food forms glucose. Glucose combines with oxygen in cells to release energy for our body, making it exothermic.
4. Oil and fat-containing food items are flushed with nitrogen. Why?
Answer: Nitrogen is an inert gas that prevents the oxidation of fats and oils, thus preventing rancidity (bad smell/taste).
5. Identify the substance oxidised and reduced in:
Answer: Hydrogen (H2) is oxidised (gains O), and Copper Oxide (CuO) is reduced (loses O).
Medium Answer Type (3 Marks)
6. Distinguish between Displacement and Double Displacement reactions with equations.
Answer: In displacement, a more reactive element displaces a less reactive one:
In double displacement, there is an exchange of ions between reactants:
7. A shiny brown element ‘X’ on heating in air becomes black. Name the element and the compound formed.
Answer: The element ‘X’ is Copper (Cu). The black compound is Copper(II) Oxide (CuO) formed by oxidation.
8. Write balanced chemical equations for the following:
Long Answer Type (5 Marks)
9. Explain Decomposition reactions. Provide one balanced equation for decomposition by:
(i) Heat, (ii) Light, and (iii) Electricity.
Answer: A reaction where a single reactant breaks down into simpler products.
10. Describe an activity to show the reaction between Iron nails and Copper Sulphate solution. What is the color change and why?
Activity: Clean iron nails and dip them in blue copper sulphate solution for 20 minutes.
Observation: The blue colour of the solution fades to pale green, and the iron nail becomes brownish.
Reason: Iron is more reactive than copper and displaces it.