Which of the following are exothermic processes ? i. Evaporation of water. ii. Dilution of an acid (H2SO4). iii. Reaction of water with quick lime. iv. Sublimation of camphor (crystals).
ii and iii
The dilution of sulphuric acid and the slaking of quicklime are exothermic; evaporation and sublimation are endothermic. The answer is ii and iii, option 2.
| Process | Heat | Why |
|---|---|---|
| i. Evaporation of water | Endothermic | Energy must be supplied to break the hydrogen bonds holding molecules in the liquid — which is why sweating and an earthen pot both cool |
| ii. Dilution of sulphuric acid | Exothermic | Hydration of H+ and SO42− ions releases a large amount of energy — the mixture can boil and spit |
| iii. Water with quicklime | Exothermic | CaO + H2O → Ca(OH)2, the slaking of lime, gives out enough heat to make the water hiss and steam |
| iv. Sublimation of camphor | Endothermic | A solid becoming vapour directly requires energy to overcome the forces holding the crystal together |
The general rule. Any change in which particles are pulled further apart — melting, evaporation, sublimation — absorbs energy and is endothermic. Any change in which particles come closer together or in which new bonds form — condensation, freezing, hydration, most combination reactions — releases energy and is exothermic. Options 1, 3 and 4 each pair one process of each kind, so applying the rule twice eliminates all three.
The practical warning. Because dilution of concentrated sulphuric acid is so strongly exothermic, the acid must always be added slowly to water, with stirring, and never water to acid. Adding water to the acid concentrates the heat at the surface, and the water can flash to steam and spray acid out of the vessel.
The equation for slaking :
\(\text{CaO}+\text{H}_{2}\text{O}\rightarrow\text{Ca(OH)}_{2}+\text{heat}\)
Hence, the answer is ii and iii.
Similar Questions
Which of the following pairs will give displacement reactions ?
Standard electrode potential of three metals A, B and C are respectively 0.5 V, −3 V and −1.2 V. Reducing power of these metals would be
Catalyst used in hydrogenation of vegetable oil into vanaspati ghee is
Important Questions from Chemical Reaction
Which of the following statement is/are true?
I. In physical change new substance is formed. substance is formed.
II. In chemical change no new substance is formed.
Which of the following cells involves similar chemical reactions?
The information not obtained from balanced chemical equation is
Oxygen cannot be obtained by which one of the following methods?
Which is the most reactive metal?
