CAPF AC 2021 Q12: Science & Technology – Chemistry
Which one of the following is an example of endothermic reaction?
(a) CaO(s) + H2O(l) → Ca(OH)2(aq)
(b) CaCO3(s) → CaO(s) + CO2(g)
(c) C(s) + O2(g) → CO2(g)
(d) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
- (a) CaO(s) + H2O(l) → Ca(OH)2(aq)
- (b) CaCO3(s) → CaO(s) + CO2(g)
- (c) C(s) + O2(g) → CO2(g)
- (d) CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
Answer: (b) CaCO3(s) → CaO(s) + CO2(g)
Explanation
Endothermic reaction
How to think: ask of each reaction, does it need heating or does it give out heat?
Burning always gives out heat.
Breaking one compound into two usually needs heat.
| Type | Meaning | Heat |
|---|---|---|
| Exothermic | Heat is released | Surroundings get hot |
| Endothermic | Heat is absorbed | Must be heated, or surroundings get cold |
The options checked:
| Reaction | What it is | Type |
|---|---|---|
| CaO + H₂O → Ca(OH)₂ | Slaking of lime; the vessel becomes hot | Exothermic |
| CaCO₃ → CaO + CO₂ | Thermal decomposition of limestone; needs strong heating | Endothermic |
| C + O₂ → CO₂ | Burning of coal | Exothermic |
| CH₄ + 2O₂ → CO₂ + 2H₂O | Burning of natural gas | Exothermic |
Answer: (b) CaCO₃(s) → CaO(s) + CO₂(g).
Why each option is right or wrong
- (a) Incorrect. Adding water to quicklime releases a lot of heat; it is exothermic.
- (b) CORRECT. Limestone breaks down only on strong heating, so it absorbs heat.
- (c) Incorrect. Burning of carbon is combustion, which releases heat.
- (d) Incorrect. Burning of methane is combustion, which releases heat.
Key point
Decomposition reactions are endothermic (need heat, light or electricity). Combustion, respiration, slaking of lime and neutralisation are exothermic.
Remember
Decomposition needs heat: endothermic.
Common mistakes
- Thinking slaking of lime absorbs heat because water is added; it releases heat.
- Forgetting that all combustion reactions are exothermic.
- Confusing quicklime (CaO) with limestone (CaCO₃) and slaked lime (Ca(OH)₂).
Exam tip
Educated guess: two options are burning (exothermic), so they go. Between the other two, the one that splits a compound is the decomposition, and decomposition needs heat.
Sources
- NCERT Class X Science — Chemical Reactions and Equations (combination and decomposition reactions)
- NCERT Class XI Chemistry — Thermodynamics (enthalpy change of a reaction)