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Question: Answered & Verified by Expert
In the reaction $\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{s})$ at $0^{\circ} \mathrm{C}$ and $1 \mathrm{~atm}$, the internal energy change is $-41 \mathrm{~kJ} / \mathrm{mol}$. What will be the value of molar enthalpy change?
ChemistryThermodynamics (C)TS EAMCETTS EAMCET 2022 (20 Jul Shift 1)
Options:
  • A $-41 \mathrm{~kJ} / \mathrm{mol}$
  • B $41 \mathrm{~kJ} / \mathrm{mol}$
  • C $30 \mathrm{~kJ} / \mathrm{mol}$
  • D $-30 \mathrm{~kJ} / \mathrm{mol}$
Solution:
2967 Upvotes Verified Answer
The correct answer is: $-41 \mathrm{~kJ} / \mathrm{mol}$
$\mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{s})$ at $0^{\circ} \mathrm{C}, 1 \mathrm{~atm}$
This process of phase change from liquid to solid happens at constant temperature and pressure. So the change in internal energy will be the molar enthalpy change.
$$
\Delta \mathrm{H}=\Delta \mathrm{U}=-41 \mathrm{~kJ} / \mathrm{mol}
$$

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