Question

Which of the following is(are) an endothermic process(es)?
(i) Dilution of sulphuric acid
(ii) Sublimation of dry ice
(iii) Condensation of water vapours
(iv) Evaporation of water
(A) (i) and (iii)
(B) (ii) only
(C) (iii) only
(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.
There are two types of reactions, one is exothermic and the other is endothermic. Let us discuss both one by one.

Complete 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.
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 vapour releases heat and changes its state from vapour to liquid to through condensation. Thus, condensation of water is possible through the evolution of heat energy. Thus, condensation of water 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 dry ice 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 (D) (iii) and (iv).

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.