Q8 / 125 · BITSAT 2009 · Chemistry

At $25^{\circ} \mathrm{C}$ and 1 bar which of the following has a non-zero $\Delta \mathrm{H}_{\mathrm{f}}^{\circ} ?$

  • A
    $\operatorname{Br}_{2}(\ell)$
  • B
    C (graphite)
  • Correct answer
    C
    $\mathrm{O}_{3}(\mathrm{~g})$
  • D
    $\mathrm{I}_{2}(\mathrm{~s})$

Solution

Ozone is allotropic form of oxygen and is of higher energy (by $68 \mathrm{~K}$ Cal mol $^{-1}$ ) than $\mathrm{O}_{2}$. Hence it can not be taken as the reference in standard state.

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

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