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Which of the following are exothermic processes? (i) Reaction of water with quick lime (ii) Dilution of an acid (iii) Evaporation of water (iv) Sublimation of camphor (crystals)

Which of the following are exothermic processes? (i) Reaction of water with quick lime (ii) Dilution of an acid (iii) Evaporation of water (iv) Sublimation of camphor (crystals) Correct Answer (i) and (ii)

Concept:

Exothermic reaction:

  • 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.
  • 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).

Additional Information

  • An endothermic reaction is a reaction in which heat is absorbed.
  • Sublimation and evaporation are endothermic reactions. 

Related Questions

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