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

Gujarat BoardEnglish MediumSTD 11 ScienceChemistry5. Thermodynamics and thermochemistry1 Mark
MCQ
Enthalpy change of the reaction $4H(g) \to 2H_2(g)$ is $-869.6\, kJ$. The bond dissociation energy of $H-H$ bond is .....$kJ$
  • A
    $-434.8$
  • ✓
    $+ 434.8$
  • C
    $-869.6$
  • D
    $+ 217.4$
✓

Answer

Correct option: B.
$+ 434.8$
b
$4 \mathrm{H} \longrightarrow 2 \mathrm{H}_{2}, \quad \Delta \mathrm{H}=-869.6\, \mathrm{kJ}$

$2 \mathrm{H}_{2} \longrightarrow 4 \mathrm{H}, \quad \Delta \mathrm{H}=+869.6\, \mathrm{kJ}$

$2 \times \mathrm{BE}_{\mathrm{H}-\mathrm{H}}=869.6 \,\mathrm{kJ}$

$\mathrm{BE}_{\mathrm{H}-\mathrm{H}}=\frac{869.6}{2}=434.8 \,\mathrm{kJ}$

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