JEE Main · 2021 · 17 Mar 2021 · Shift 2 · Chemical Thermodynamics

Entropy of the system falls whenever its particles lose positional or translational freedom. Water freezing to ice, at $0^{\circ }\mathrm{C}$ or at $-10^{\circ }\mathrm{C}$, replaces a mobile liquid by a fixed crystal lattice, so the entropy of the system falls in both cases; the temperature changes the magnitude, not the sign. In ${\mathrm{N}}_2(\mathrm{g})+3{\mathrm{H}}_2(\mathrm{g})\to 2\mathrm{N}{\mathrm{H}}_3(\mathrm{g})$ four moles of gas become two, so the gaseous disorder falls. Adsorption of carbon monoxide on lead removes molecules from the gas phase and ties them to a surface, again a loss of freedom. Dissolving sodium chloride disperses the ions of the lattice through the solvent and raises the entropy. Entropy decreases in the freezing of water at both temperatures, in the formation of ammonia from its elements and in the adsorption of carbon monoxide.