Bond Enthalpies Practice Test

Use average bond enthalpies to estimate $\Delta H$ for the reaction $\mathrm{H_2(g) + Cl_2(g) \rightarrow 2HCl(g)}$. The average bond enthalpies are: $\mathrm{H-H}=436\ \text{kJ/mol}$, $\mathrm{Cl-Cl}=243\ \text{kJ/mol}$, and $\mathrm{H-Cl}=431\ \text{kJ/mol}$. What is the estimated enthalpy change, $\Delta H$, for the reaction?

Use average bond enthalpies (in kJ/mol) to estimate $\Delta H$ for combustion: $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)}$. Bonds broken: 4 C–H (413), 2 O=O (498). Bonds formed: 2 C=O in CO$_2$ (799), 4 O–H (463). What is the estimated enthalpy change?