NEET UG 2008 — Chemistry Physical Chemistry

Key Idea: $\Delta H_{\text {reaction }}=\Sigma$ΔHreaction ​=Σ Bond energy of reactant $-\Sigma$−Σ Bond energy of product $$\begin{array}{ll} \text {Here, } & \Delta \mathrm{H}_{\mathrm{H}-\mathrm{H}}=434 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \Delta \mathrm{H}_{\mathrm{Cl}-\mathrm{Cl}}=242 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ & \Delta \mathrm{H}_{\mathrm{H}-\mathrm{Cl}}=431 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \because & \frac{1}{2} \mathrm{H}_2+\frac{1}{2} \mathrm{Cl}_2 \longrightarrow \mathrm{HCl} \end{array}$$Here, ∵​ΔHH−H​=434 kJ mol−1ΔHCl−Cl​=242 kJ mol−1ΔHH−Cl​=431 kJ mol−121​H2​+21​Cl2​⟶HCl​ $$\begin{aligned} \Delta H_{\text {reaction }} & =\frac{1}{2} \Delta H_{\mathrm{H}-\mathrm{H}}+\frac{1}{2} \Delta \mathrm{H}_{\mathrm{Cl}-\mathrm{Cl}}-\Delta \mathrm{H}_{\mathrm{H}-\mathrm{Cl}} \\ & =\frac{1}{2} \times 434+\frac{1}{2} \times 242-431 \\ & =217+121-431 \\ & =-93 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned}$$ΔHreaction ​​=21​ΔHH−H​+21​ΔHCl−Cl​−ΔHH−Cl​=21​×434+21​×242−431=217+121−431=−93 kJ mol−1​