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Thermodynamics — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryThermodynamics1 Mark
MCQ
$\Delta_\text{f}\text{U}^\ominus$ of formation of $\text{CH}_4\ (\text{g})$ at certain temperature is $-393\ \text{kJ}\ \text{mol}^{-1}.$ The value of $\Delta_\text{f}\text{H}^\ominus$ is:
  • A
    Zero
  • ✓
    $<\Delta_\text{f}\text{U}^\ominus$
  • C
    $>\Delta_\text{f}\text{U}^\ominus$
  • D
    Equal to $\Delta_\text{f}\text{U}^\ominus$
✓

Answer

Correct option: B.
$<\Delta_\text{f}\text{U}^\ominus$
$\Delta_\text{f}\text{H}^\ominus=\Delta_\text{f}\text{U}^\ominus+\Delta\text{n}_\text{g}\text{RT}$
For the reaction,
$\text{C}(\text{s})+2\text{H}_2(\text{g})\rightarrow\text{CH}_4$
$\therefore\ \Delta\text{n}_\text{g}=1-2=-1$
$\therefore\ \Delta_\text{f}\text{H}^\ominus= \Delta_\text{f}\text{U}^\ominus-1\text{RT}$
or$\ \Delta_\text{f}\text{U}^\ominus=\Delta_\text{f}\text{H}^\ominus+\text{RT}$
or$\ \Delta_\text{f}\text{H}^\ominus<\Delta_\text{f}\text{U}^\ominus$

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