Q57 / 200 · NEET 2007 · Chemistry

Given that bond energies of $\mathrm{H}-\mathrm{H}$ and $\mathrm{Cl}-\mathrm{Cl}$ are $430 \mathrm{~kJ} \mathrm{~mol}^{-1}$ and $240 \mathrm{~kJ}$ $\mathrm{mol}^{-1}$ respectively and $\Delta \mathrm{H}_{\mathrm{f}}$ for $\mathrm{HCl}$ is $-\mathrm{kJ}$ $\mathrm{mol}^{-1}$, bond enthalpy of $\mathrm{HCl}$ is:

  • A
    $380 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • Correct answer
    B
    $425 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • C
    $245 \mathrm{~kJ} \mathrm{~mol}^{-1}$
  • D
    $290 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Solution

$\frac{1}{2} \mathrm{H}_2+\frac{1}{2} \mathrm{Cl}_2 \longrightarrow \mathrm{HCl}$ $\Delta \mathrm{H}_{\mathrm{HCl}}=\Sigma$ B.E. of reactant $-\Sigma$ B.E. of products $-90=\frac{1}{2} \times 430+\frac{1}{2} \times 240-$ B.E. of $\mathrm{HCl}$ $\therefore \quad$ B.E. of $\mathrm{HCl}=215+120+90$ $=425 \mathrm{~kJ} \mathrm{~mol}^{-1}$

Hence, the correct answer is option: 'B'.

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