Question

Which of the following are exothermic processes?
(i) Reaction of water with quicklime.
(ii) Dilution of an acid.
(iii) Evaporation of water.
(iv) Sublimation of camphor (crystals).
A. (i) and (ii)
B. (ii) and (iii)
C. (i) and (iv)
D. (iii) and (iv)

Answer 1

Hint: Determine in which reactions, heat energy is supplied. Also determine the reaction in which heat energy is needed. Two types of Reactions are there: Exothermic and Endothermic reactions. Let us discuss it one by one.

Complete step by step answer:
An exothermic reaction is one in which heat energy is liberated (given out) during the reaction. In the thermochemical equation for the exothermic reaction, the heat energy is on the product's side. The enthalpy of reactants is higher than the enthalpy of products. The enthalpy change of an exothermic reaction is negative.
An endothermic reaction is one in which heat energy is needed (absorbed) during the reaction. In the thermochemical equation for the endothermic reaction, the heat energy is on the reactants side. The enthalpy of reactants is lower than the enthalpy of products. The enthalpy change of an endothermic reaction is positive.
Quick lime is calcium oxide. It reacts with water to form calcium hydroxide.
The reaction of water with quicklime releases a large amount of energy. Hence, this is an exothermic reaction.
\[{\text{CaO + }}{{\text{H}}_2}{\text{O }} \to {\text{ Ca}}{\left( {{\text{OH}}} \right)_2}{\text{ + heat}}\]
When concentrated sulphuric acid is diluted with water to obtain dilute sulphuric acid, a huge amount of energy is released. Thus, this reaction is an exothermic process.
Water absorbs heat and changes its state from liquid to through evaporation. Thus, evaporation of water is possible through absorption of heat energy. Thus, evaporation is an endothermic process.
You will need heat energy for sublimation. Sublimation is the process in which a substance changes its physical state from solid to gas, without entering liquid state. Thus, sublimation of camphor requires input of heat energy. Hence, it is an endothermic process.
Hence, (i) and (ii) represents exothermic processes whereas (iii) and (iv) represents endothermic processes.

Hence, the correct option is the option (A) (i) and (ii).

Note: For a reaction at equilibrium, if the forward reaction is exothermic, then reverse reaction is endothermic. Also, if the forward reaction is endothermic, then the reverse reaction is exothermic.