Question

Which of the following are exothermic processes?

(1) Dilution of an acid.

(2) Evaporation of water.

(3) Reaction of water with Quicklime.

(4) Sublimation of camphor (crystals).

• A. (1) and (2)
• B. (1) and (3)
• C. (1) and (4)
• D. (3) and (4)

Answer 1

The correct option is B

(1) and (3)

The correct option is B.

Explanation the correct option:

Exothermic reactions:

• The reactions in which the heat energy is given out during the reaction are called exothermic reactions.
• The enthalpy change of an exothermic reaction is negative.

Dilution of an acid:

The dilution of an acid with water is an exothermic reaction as it evolves heat energy.

Example:

The dilution of sulphuric acid with water is an exothermic process.

${\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{H}}_3\mathrm{O}++{{\mathrm{HSO}}_4}^{-}+\mathrm{Heat}$

Reaction of water with quicklime:

• Calcium oxide (quick lime) reacts with water to form calcium hydroxide.
• This reaction releases an enormous amount of energy. Hence, it is an exothermic reaction.

$\mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{Ca}{\left(\mathrm{OH}\right)}_2+\mathrm{Heat}$

Explanation the incorrect option:

Evaporation of water:

• Evaporation is an endothermic process as water must absorb heat from the surroundings to increase its kinetic energy.
• Ex: Sweat which cools the human body as it evaporates from the skin.

Sublimation of camphor crystals:

• The process of changing a solid to gas without becoming liquid is called sublimation.
• The sublimation process takes place by absorbing the heat. Hence, the sublimation of camphor is an endothermic reaction.

Final answer: Dilution of acid and reaction of water with quicklime are exothermic processes.