In which case is the change in entropy negative?

Solution & Explanation

\textbf{Answer:} (D) $2\mathrm{H}(\mathrm{g})\to \mathrm{H}{}_2(\mathrm{g})$ \textbf{Solution:} Entropy decreases ($\Delta S<0$) when randomness or the number of gaseous particles decreases. In $2\mathrm{H}(\mathrm{g})\to \mathrm{H}{}_2(\mathrm{g})$, two moles of gaseous atoms combine into one mole of gas, so $\Delta n_g=-1$ and $\Delta S<0$. Evaporation, isothermal gas expansion, and sublimation all increase disorder ($\Delta S>0$). Mapped: source numeric key "4" → option D.