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5. Thermodynamics and thermochemistry — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
The molar heat of formation of $NH_4NO_3(s)$ is $-367.54\, kJ$ and those of $N_2O(g)$ and $H_2O(l)$ are $+ 81.46 \,kJ$ and $-285.78\, kJ$ respectively at $25\,^oC$ and $1.0$ atmospheric pressure. Calculate $\Delta U$ at $25\,^oC$ for the reaction .....$kJ$

$NH_4NO_3(s) \rightarrow N_2O(g) + 2H_2O(l)$

  • ✓
    $-122.56$
  • B
    $-125.04$
  • C
    $-140.34$
  • D
    $-112.04$
✓

Answer

Correct option: A.
$-122.56$
a
$\Delta \mathrm{H}=\left(\Delta \mathrm{H}_{f}\right)_{\mathrm{N}_2O}+2\left(\Delta \mathrm{H}_{f}\right)_{\mathrm{H}_2O}-\left(\Delta \mathrm{H}_{f}\right)_{\mathrm{NH}_4 \mathrm{NO}_3}$

$=-122.56\, \mathrm{kJ}$

$\because \Delta \mathrm{H}=\Delta \mathrm{U}+\Delta \mathrm{Hg} \mathrm{RT}$

$-122.56=\Delta \mathrm{U}+1 \times 8.314 \times 10^{-3} \times 298$

$\Delta \mathrm{U}=-125.04\, \mathrm{kJ}$

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