CAPF AC 2025 Q100: Science & Technology – Chemistry
Which one of the following is not an exothermic reaction?
- (a) Dissolution of calcium oxide in water
- (b) Decomposition of vegetable matter into compost
- (c) Decomposition of lead nitrate
- (d) Dilution of sodium hydroxide with water
Answer: (c) Decomposition of lead nitrate
Explanation
The reaction that is not exothermic
How to think: an exothermic reaction gives out heat.
Only the decomposition of lead nitrate needs heat to go; it is endothermic.
| Reaction | Heat change |
|---|---|
| Dissolving calcium oxide in water (slaking of lime) | Exothermic, the water heats up |
| Decomposition of vegetable matter into compost | Exothermic, composting heap becomes warm |
| Decomposition of lead nitrate | Endothermic, heat is absorbed; gives lead oxide, nitrogen dioxide and oxygen |
| Diluting sodium hydroxide with water | Exothermic, the solution warms up |
Answer: (c) Decomposition of lead nitrate.
Why each option is right or wrong
- (a) Incorrect. Slaking of lime gives out a large amount of heat.
- (b) Incorrect. Composting releases heat as microbes break down the matter.
- (c) CORRECT. The thermal decomposition of lead nitrate absorbs heat, so it is not exothermic.
- (d) Incorrect. Diluting sodium hydroxide releases heat.
Key point
Exothermic: slaking of lime, respiration, decomposition of vegetable matter, neutralisation, dilution of NaOH. Endothermic: thermal decomposition (lead nitrate, calcium carbonate), photosynthesis.
Remember
Thermal decomposition of lead nitrate absorbs heat: endothermic.
Common mistakes
- Thinking every decomposition is exothermic because composting is.
- Missing the word not in the question.
- Forgetting that lead nitrate decomposes only on heating.
Exam tip
Educated guess: lead nitrate has to be heated in the laboratory to decompose. A reaction that needs a burner is not giving out heat.
Sources
- NCERT Class X Science, Chapter 1 (Chemical Reactions and Equations: exothermic reactions and decomposition of lead nitrate)
- NCERT Class XI Chemistry, Part I, Chapter 6 (Thermodynamics)