Q1. Choose the correct option from the brackets and justify your answer.
(Oxidation, displacement, electrolysis, reduction, zinc, copper, double displacement, decomposition)
a. To avoid rusting, a coating of zinc metal is applied on iron sheets.
Explanation:
This process is known as galvanisation.
- In galvanisation, iron sheets are covered with a thin zinc layer.
- Zinc is more electropositive than iron, so zinc corrodes first.
- After a long time, the zinc layer slowly wears off.
- Only after this does iron start rusting.
b. The change of ferrous sulphate into ferric sulphate is a oxidation reaction.
Explanation:
Oxidation refers to the loss of electrons.
Ionic form: Fe²⁺ → Fe³⁺
Here, the charge on iron increases from +2 to +3, indicating loss of one electron. Hence, the reaction is oxidation.
c. When electric current is passed through acidulated water, electrolysis of water occurs.
Explanation:
Acidulated water breaks down into hydrogen and oxygen gases when electric energy is supplied. This electrically driven decomposition is called electrolysis.
d. Mixing aqueous ZnSO₄ with aqueous BaCl₂ is an example of a double displacement reaction.
Explanation:
In this reaction, ions exchange places and form a precipitate. Such reactions are called double displacement reactions.
Q2. Answer the following questions
A. What is a reaction called when oxidation and reduction occur together? Explain with an example.
Answer:
A reaction in which oxidation and reduction take place simultaneously is called a redox reaction.
Oxidation → loss of electrons
Reduction → gain of electrons
- Copper oxide loses oxygen, so CuO is reduced.
- Hydrogen gains oxygen, forming water.
- Hence, hydrogen undergoes oxidation.
- Since both oxidation and reduction occur together, it is a redox reaction.
B. How can the rate of decomposition of hydrogen peroxide be increased?
Answer:
Hydrogen peroxide decomposes slowly as shown:
The reaction rate can be increased by adding manganese dioxide (MnO₂) as a catalyst. A catalyst speeds up a reaction without undergoing any permanent chemical change.
C. Explain the terms reactant and product with an example.
Answer:
Reactants are substances that participate in a chemical reaction.
Products are new substances formed as a result of the reaction.
Example:
Here, hydrogen and oxygen are reactants, while water is the product.
D. Explain types of reactions related to oxygen and hydrogen with examples.
Answer:
- Decomposition reaction:
2H₂O₂ → 2H₂O + O₂
Hydrogen peroxide breaks down into water and oxygen.
- Redox reaction:
CuO + H₂ → Cu + H₂O
Oxidation and reduction occur together.
- Electrolysis (decomposition):
2H₂O → 2H₂ + O₂
Electric current decomposes water into gases.
E. Explain the similarity and difference between adding NaOH to water and adding CaO to water.
Answer:
Both reactions produce alkaline solutions, but adding CaO releases heat, making it an exothermic reaction.
Q3. Explain the following terms with examples
A. Endothermic reaction
Answer:
An endothermic reaction absorbs heat from its surroundings.
Example:
Heat absorption causes the container to feel cold.
B. Combination reaction
Answer:
A reaction where two or more reactants combine to form a single product is called a combination reaction.
Example:
Magnesium and oxygen combine to form magnesium oxide.
C. Balanced equation
Answer:
A chemical equation in which the number of atoms of each element is equal on both sides is called a balanced equation.
Example:
Atoms are equal on both sides; hence the equation is balanced.
D. Displacement reaction
Answer:
A reaction in which a more reactive element replaces a less reactive one from its compound is called a displacement reaction.
Example:
Zinc displaces copper because it is more reactive.
Q4. Give scientific reasons
A. Lime water turns milky when CO₂ is passed through it.
Reason:
CO₂ reacts with lime water to form calcium carbonate, which is insoluble and appears milky. This is a confirmatory test for carbon dioxide gas.
B. Powdered Shahabad tile reacts faster with HCl than solid pieces.
Reason:
Smaller particles have a larger surface area, increasing collision frequency. Hence, powdered tile reacts faster than solid pieces.
C. Concentrated sulphuric acid is added slowly to water with stirring.
Reason:
Dilution releases a large amount of heat. Adding acid slowly with constant stirring prevents splashing and accidents.
D. Oil should be stored in airtight containers.
Reason:
Exposure to air causes oxidation, leading to rancidity. Airtight containers prevent contact with oxygen.
Q5. Observe the picture and explain the reaction.
Answer: The picture shows rusting of iron, a form of corrosion.
- Rust is hydrated iron(III) oxide: Fe₂O₃·H₂O
- Rusting is an electrochemical process.
- Iron oxidises at the anode and oxygen reduces at the cathode.
- A reddish-brown layer forms on iron, damaging it over time.
This process is known as corrosion.
Q6. Identify oxidation and reduction in the following reactions.
| Reaction | Oxidised Element | Reduced Element |
|---|---|---|
| Fe + S → FeS | Iron (Fe) | Sulphur (S) |
| 2Ag₂O → 4Ag + O₂ | Oxygen (O) | Silver (Ag) |
| 2Mg + O₂ → 2MgO | Magnesium (Mg) | Oxygen (O) |
| NiO + H₂ → Ni + H₂O | Hydrogen (H) | Nickel oxide (NiO) |
Q7. Balance the equations stepwise
A. H₂S₂O₇ + H₂O → H₂SO₄
Balanced:
B. SO₂ + H₂S → S + H₂O
Balanced:
C. Ag + HCl → AgCl + H₂
Balanced:
D. NaOH + H₂SO₄ → Na₂SO₄ + H₂O
Balanced:
Q8. Identify endothermic and exothermic reactions
a. Burning of wood
Type: Exothermic
b. Photosynthesis
Type: Endothermic
c. Respiration
Type: Exothermic
d. Melting of ice
Type: Endothermic
Q9. Match the reactions with their types
| Reaction | Type |
|---|---|
| BaCl₂ + ZnSO₄ → BaSO₄ + ZnCl₂ | Double displacement |
| 2AgCl → 2Ag + Cl₂ | Decomposition |
| CuSO₄ + Fe → FeSO₄ + Cu | Displacement |
| H₂O + CO₂ → H₂CO₃ | Combination |