Q36 / 150 · MHT CET 2025 (19 Apr Shift 2) · Chemistry

Which of the following statements is correct about $\mathrm{O}_2$ and $\mathrm{O}_3$ molecule?

  • A
    $\mathrm{O}_2$ and $\mathrm{O}_3$ are paramagnetic
  • Correct answer
    B
    The enthalpy change during the formation of $\mathrm{O}_3$ from $\mathrm{O}_2$ is positive
  • C
    The entropy change in the formation of $\mathrm{O}_3$ from $\mathrm{O}_2$ is positive
  • D
    $\mathrm{O}_3$ is more stable than $\mathrm{O}_2$

Solution

A: $\mathrm{O}_2$ is paramagnetic due to two unpaired $\pi^*$ electrons, but $\mathrm{O}_3$ is diamagnetic with all electrons paired. This statement is incorrect.

B: The reaction $\frac{3}{2}\mathrm{O}_2(g) \rightarrow \mathrm{O}_3(g)$ is endothermic ($\Delta H > 0$) since ozone is less stable than dioxygen. This statement is correct.

C: The entropy change for $\frac{3}{2}\mathrm{O}_2(g) \rightarrow \mathrm{O}_3(g)$ is negative ($\Delta S < 0$) due to decreased gas moles. This statement is incorrect.

D: $\mathrm{O}_2$ is more stable than $\mathrm{O}_3$, which readily decomposes to oxygen. This statement is incorrect.

Only statement B is correct.

The final answer is $\boxed{\text{B}}$

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

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