Question

The amount of energy required to break a bond is same as the amount of energy released when the same bond is formed. In gaseous state, the energy required for homolytic cleavage of a bond is called Bond Dissociation Energy (BDE) or Bond Strength. BDE is affected by $s$-character of the bond and the stability of the radicals formed. Shorter bonds are typically stronger bonds. BDEs for some bonds are given below :
$$
\begin{array}{ll}
\mathrm{H}_{3} \mathrm{C}-\mathrm{H}(\mathrm{~g}) & \longrightarrow \mathrm{H}_{3} \mathrm{C}^{\bullet}(\mathrm{g})+\mathrm{H}^{\bullet}(\mathrm{g}) \Delta \mathrm{H}^{\circ}=105 \mathrm{kcalmol}^{-1} \
\mathrm{Cl}-\mathrm{Cl}(\mathrm{~g}) & \longrightarrow \mathrm{Cl}^{\circ}(\mathrm{g}) \
\mathrm{H}_{3} \mathrm{C}-\mathrm{Cl}(\mathrm{~g}) & \longrightarrow \mathrm{Cl}^{\circ}(\mathrm{g}) \Delta \mathrm{H}^{\circ}=58 \mathrm{k}^{\circ}(\mathrm{g})+\mathrm{Cl}^{\circ}(\mathrm{g}) \Delta \mathrm{H}^{\circ}=85 \mathrm{kcal}^{-1} \mathrm{~mol}^{-1} \
\mathrm{H}-\mathrm{Cl}(\mathrm{~g}) & \longrightarrow \mathrm{H}^{\bullet}(\mathrm{g})+\mathrm{Cl}^{\circ}(\mathrm{g}) \Delta \mathrm{H}^{\circ}=103 \mathrm{kcalmol}^{-1}
\end{array}
$$

For the following reaction
$$
\mathrm{CH}_{4}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g}) \xrightarrow{\text { light }} \mathrm{CH}_{3} \mathrm{Cl}(\mathrm{~g})+\mathrm{HCl}(\mathrm{~g})
$$
the correct statement is

Accepted Answer

(D) The reaction is exothermic with $\Delta \mathrm{H}^{\circ}=-25 \mathrm{kcal} \mathrm{mol}^{-1}$.

Answer

(A) Initiation step is exothermic with $\Delta \mathrm{H}^{\circ}=-58 \mathrm{kcal} \mathrm{mol}^{-1}$

Answer

(B) Propagation step involving ${ }^{\cdot} \mathrm{CH}_{3}$ formation is exothermic with
$$
\Delta \mathrm{H}^{\circ}=-2 \mathrm{kcal} \mathrm{~mol}^{-1} .
$$

Answer

(C) Propagation step involving $\mathrm{CH}_{3} \mathrm{Cl}$ formation is endothermic with
$$
\Delta \mathrm{H}^{\circ}=+27 \mathrm{kcal} \mathrm{~mol}^{-1} .
$$

JEE Advanced · 2021 · 03 Oct 2021 · Paper 2 · Chemical Thermodynamics

Solution

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