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).

Question

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Correct — A, ii and iii. An exothermic process gives out heat to its surroundings, so its enthalpy change is negative; an endothermic process takes heat in. Sort the four accordingly. Statement (ii), diluting concentrated sulphuric acid, releases a great deal of heat as the acid molecules become hydrated — enough to make the mixture spit and, if done carelessly, to boil the water at the surface. That is the whole reason for the laboratory rule that acid is poured into water in a thin stream with constant stirring, never water into acid: a large volume of water, with its high specific heat of about 4.18 kilojoules per kilogram per kelvin, can soak up the heat, whereas a splash of water landing on concentrated acid cannot. Statement (iii) is the slaking of quicklime, CaO + H₂O → Ca(OH)₂, the standard school example of a combination reaction that is strongly exothermic; the mixture heats up sharply and can steam. Both release heat, so both are exothermic. Statements (i) and (iv) are not reactions at all but phase changes in the same direction — liquid to vapour for water, solid straight to vapour for camphor — and both require latent heat to be supplied to break the molecules free of one another. Evaporation is why sweat cools the skin and why water stays cool in an earthen pot; sublimation is why a camphor tablet left in the open shrinks away without ever melting into a puddle. Both absorb heat and are therefore endothermic. So the exothermic pair is ii and iii, which is option (a).

Why the other options are wrong — (b) Correctly identifies the slaking of quicklime but pairs it with the sublimation of camphor. Sublimation converts a solid directly to vapour and must absorb the enthalpy of sublimation to do it, so it cools its surroundings rather than warming them — the opposite of what this option claims.  (c) Gets the dilution of sulphuric acid right and then adds the evaporation of water. Evaporation is the classic endothermic phase change; it is the reason a wet cloth on the forehead cools a patient and the reason desert coolers work. Heat flows into the water, not out of it.  (d) The exact inverse of the answer: it selects the only two processes that are unambiguously endothermic and omits both of the ones that release heat. It is what a candidate marks after registering that evaporation and sublimation are the two 'physical' items and assuming the question wanted the pair that behaves alike.

Concept: Every physical or chemical change involves an exchange of heat with its surroundings, and the sign of that exchange is what 'exothermic' and 'endothermic' describe. Exothermic means heat flows out of the system — combustion, respiration, neutralisation, the slaking of lime, and the hydration that occurs when concentrated acids are diluted. Endothermic means heat flows in — photosynthesis, the thermal decomposition of calcium carbonate in a lime kiln, and all phase changes that move matter to a less condensed state. That last group is the reliable rule. Melting, vaporisation and sublimation each pull molecules further apart against the forces holding them together, so each requires latent heat to be supplied; freezing, condensation and deposition run the other way and give the same heat back. Water's latent heat of vaporisation is about 2,260 kilojoules per kilogram, roughly seven times its latent heat of fusion of about 334 kilojoules per kilogram, which is why evaporation is such an effective coolant. Dissolution is the one category that goes either way: acids and alkalis usually dissolve exothermically, while ammonium nitrate and potassium nitrate dissolve endothermically and chill the solution.

State PCS 2024 Q50 — Science & Technology PYQ with Answer

State PCS 2024 Q50 — Science & Technology PYQ with Answer