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

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
For the reaction

$\mathrm{A}(l) \rightarrow 2 \mathrm{B}(\mathrm{g})$

$\Delta \mathrm{U}=2.1\; \mathrm{kcal}, \Delta \mathrm{S}=20\; \mathrm{cal} \mathrm{K}^{-1}$ at $300\; \mathrm{K}$

Hence $\Delta \mathrm{G}$ in $\mathrm{kcal}$ is

  • A
    $-1.3$
  • ✓
    $-2.7$
  • C
    $-3.7$
  • D
    $-4.2$
✓

Answer

Correct option: B.
$-2.7$
b
$\mathrm{A}(\ell) \longrightarrow 2 \mathrm{B}(\mathrm{g})$

$\Delta \mathrm{U}=2.1 \mathrm{Kcal}, \Delta \mathrm{S}=20 \mathrm{cal} \mathrm{K}^{-1}$ at $300 \mathrm{K}$

$\Delta \mathrm{H}=\Delta \mathrm{U}+\Delta \mathrm{n}_{\mathrm{g}} \mathrm{RT}$

$\Delta \mathrm{G}=\Delta \mathrm{H}-\mathrm{T} \Delta \mathrm{S}$

$\Delta \mathrm{G}=\Delta \mathrm{U}+\Delta \mathrm{n}_{\mathrm{g}} \mathrm{RT}-\mathrm{T} \Delta \mathrm{S}$

$=2.1+\frac{2 \times 2 \times 300}{1000}-\frac{300 \times 20}{1000}$

$\left(\mathrm{R}=2 \mathrm{cal} \mathrm{K}^{-1} \mathrm{mol}^{-1}\right)$

$=2.1+1.2-6=-2.70 \mathrm{Kcal} / \mathrm{mol}$

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