Chemical Reactions and Equations — Previous Year Questions (Class 10 Science)
This chapter covers balancing chemical equations, types of reactions (decomposition, combination, combustion, displacement, double displacement), and redox
TL;DR: This chapter covers balancing chemical equations, types of reactions (decomposition, combination, combustion, displacement, double displacement), and…
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This chapter covers balancing chemical equations, types of reactions (decomposition, combination, combustion, displacement, double displacement), and redox
Chemical Reactions and Equations — Previous Year Questions with Solutions
Q (2023, 2 marks): Balance the equation: Fe + O2 → Fe2O3
Answer: Fe + O2 → Fe2O3
Let coefficients be a, b, c:
aFe + bO2 → cFe2O3
Balance Fe: a = 2c
Balance O: 2b = 3c
Let c = 2: a = 4, b = 3
4Fe + 3O2 → 2Fe2O3
Final Answer: 4Fe + 3O2 → 2Fe2O3
Q (2022, 3 marks): Classify the reaction: CaCO3 → CaO + CO2. Is this a decomposition or redox reaction? Justify.
Answer: Reaction: CaCO3 → CaO + CO2
This is a Decomposition Reaction because one compound breaks down into two simpler products.
Is it redox? Check oxidation states:
In CaCO3: Ca is +2, C is +4, O is -2
In CaO: Ca is +2, O is -2
In CO2: C is +4, O is -2
Oxidation states do NOT change, so it is NOT a redox reaction.
It is a combination of decomposition (physically breaking into two compounds) but NOT a redox process.
Final Answer: Decomposition reaction; NOT redox (no change in oxidation states)
Q (2021, 2 marks): Complete and balance: Zn + HCl → ?
Answer: Zn + HCl → ZnCl2 + H2
Balancing:
Zn + 2HCl → ZnCl2 + H2
This is a Displacement Reaction (single displacement, more reactive Zn displaces H from HCl).
Final Answer: Zn + 2HCl → ZnCl2 + H2
Q (2023, 3 marks): What is the difference between a thermal decomposition and a photochemical decomposition? Give examples.
Answer: Thermal Decomposition: A compound breaks down due to heat.
Example: CaCO3 --heat--> CaO + CO2 (at 825°C)
Photochemical Decomposition: A compound breaks down due to light (photons).
Example: 2AgCl --light--> 2Ag + Cl2 (photography)
Another example: 2H2O --light--> 2H2 + O2 (photolysis, requires photons)
Key difference: Thermal uses heat energy; photochemical uses light energy (photons).
Final Answer: Thermal decomposition uses heat (e.g., CaCO3 → CaO + CO2). Photochemical uses light (e.g., 2AgCl → 2Ag + Cl2). Both are decomposition but differ in the energy source.
Q (2022, 3 marks): Identify the type of reaction and balance: Cu + HNO3 (dilute) → Cu(NO3)2 + NO + H2O
Answer: Cu + HNO3 (dilute) → Cu(NO3)2 + NO + H2O
This is a Redox Reaction (displacement + redox).
Oxidation states:
Cu: 0 → +2 (oxidation, loses 2e-)
N in HNO3: +5 → +2 in NO (reduction, gains 3e-)
Balancing by ion-electron method:
Oxidation: Cu → Cu^2+ + 2e- (×3)
Reduction: NO3^- + 4H+ + 3e- → NO + 2H2O (×2)
3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O
Final Answer: 3Cu + 8HNO3 → 3Cu(NO3)2 + 2NO + 4H2O
Q (2021, 2 marks): What are oxidation and reduction? Define with reference to electron transfer.
Answer: Oxidation: Loss of electrons or increase in oxidation state. The substance being oxidized is called the reducing agent (it reduces the other).
Reduction: Gain of electrons or decrease in oxidation state. The substance being reduced is called the oxidizing agent (it oxidizes the other).
Example: In 2Na + Cl2 → 2NaCl:
Na: 0 → +1 (loses 1e-, oxidized, reducing agent)
Cl: 0 → -1 (gains 1e-, reduced, oxidizing agent)
Final Answer: Oxidation = loss of electrons / increase in oxidation state. Reduction = gain of electrons / decrease in oxidation state. Always occur together (redox).
Frequently Asked Questions
How do you balance a chemical equation systematically?
Start with the most complex compound. Balance metals, then non-metals, then hydrogen, then oxygen last. Check each element count on both sides. Coefficients must be the smallest whole numbers.
What is the difference between a combination and a decomposition reaction?
Combination: two or more substances combine to form one product (e.g., N2 + O2 → 2NO). Decomposition: one compound breaks into two or more simpler products (e.g., CaCO3 → CaO + CO2).
More Class 10 Science PYQs
- Light (Reflection and Refraction)
- Electricity
- Acids, Bases and Salts
- Metals and Non-metals
- Life Processes
- Heredity and Evolution
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