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Question: Answered & Verified by Expert
At $25^{\circ} \mathrm{C}$ and 1 bar which of the following has a non-zero $\Delta \mathrm{H}_{\mathrm{f}}^{\circ} ?$
ChemistryThermodynamics (C)BITSATBITSAT 2009
Options:
  • A $\operatorname{Br}_{2}(\ell)$
  • B C (graphite)
  • C $\mathrm{O}_{3}(\mathrm{~g})$
  • D $\mathrm{I}_{2}(\mathrm{~s})$
Solution:
2699 Upvotes Verified Answer
The correct answer is: $\mathrm{O}_{3}(\mathrm{~g})$
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.

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