Q48 / 150 · MHT CET 2025 (19 Apr Shift 2) · Chemistry

Which of the following reactions exhibits decrease in entropy?

  • A
    $2 \mathrm{H}_2 \mathrm{O}_{2(l)} \longrightarrow 2 \mathrm{H}_2 \mathrm{O}_{(l)}+\mathrm{O}_{2(\mathrm{~g})}$
  • B
    $\mathrm{H}_{2(\mathrm{~g})} \longrightarrow 2 \mathrm{H}_{(\mathrm{g})}$
  • C
    $\mathrm{CaCO}_{3(\mathrm{~s})} \xrightarrow{\Delta} \mathrm{CaO}_{(\mathrm{s})}+\mathrm{CO}_{2(\mathrm{~g})}$
  • Correct answer
    D
    $2 \mathrm{H}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})} \longrightarrow 2 \mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}$

Solution

Decrease in Entropy Analysis

Entropy decreases when the number of gaseous moles reduces or when matter transitions to a more ordered phase.

For reaction D: $2\mathrm{H}_{2(\mathrm{g})} + \mathrm{O}_{2(\mathrm{g})} \longrightarrow 2\mathrm{H}_{2}\mathrm{O}_{(\mathrm{l})}$, reactants total 3 moles of gas, while products yield 2 moles of liquid. The reduction from 3 gaseous moles to 0 coupled with the gas-to-liquid phase transition results in decreased entropy.

Thus, the correct choice is $\boxed{\text{D}}$.

Hence, the correct answer is option: 'D'.

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