Correct Answer - Option 1 : (i) and (ii)
Concept:
Exothermic reaction:
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An exothermic reaction is a chemical reaction that delivers energy through light or heat.
- It is the opposite of an endothermic reaction.
- Expressed in a chemical equation: reactants → products + energy.
- Exothermic Reaction means "Exo" meaning releases and "thermic" means heat.
- In an exothermic reaction, by definition, the enthalpy change has a negative value ΔH < 0 since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction).
- For example, when hydrogen burns-
2H2(g) + O2(g) → 2H2O(g), ΔH = −483.6 kJ/mol of O2
- In an exothermic reaction, by definition, the enthalpy change has a negative value ΔH < 0 since a larger value (the energy released in the reaction) is subtracted from a smaller value (the energy used for the reaction).
Explanation:
- Reaction of water with quick lime is an exothermic process. When quick lime or CaO is reacted with water, Ca(OH)2 is formed.
- The process releases heat and is exothermic.
- When we add acid to water, heat is released. That is why acid in small amounts is poured into water and not vice versa.
- When water is added to acid, drastic amounts of energy are released which may even cause explosion.
- Evaporation of water is the conversion of liquid to vapour. Conversion of liquid to vapour requires heat energy absorption and not release.
- Hence, evaporation is not exothermic.
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Sublimation of camphor is the turning of camphor directly into vapour from the solid phase which also requires heat absorption. Hence, the sublimation of camphor is not exothermic.
- Hence, the exothermic reaction is the reaction of water with quick lime and sublimation of camphor (crystals).
Thus, the correct option is (i) and (ii).
- An endothermic reaction is a reaction in which heat is absorbed.
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Sublimation and evaporation are endothermic reactions.
