Methane undergoes several different exothermic reactions...

Methane undergoes several different exothermic reactions with gaseous chlorine. One of these forms chloroform, $\mathrm{CHCl}_{3}(\mathrm{~g})$. $\begin{aligned} \mathrm{CH}_{4}(\mathrm{~g})+3 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CHCl}_{3}(\mathrm{~g})+3 \mathrm{HCl}(\mathrm{g}) \\ \Delta H_{\mathrm{rxn}}^{0} &=-305.2 \mathrm{~kJ} / \mathrm{mol} \mathrm{rxn} \end{aligned}$ Average bond energies per mole of bonds are: $\mathrm{C}-\mathrm{H}=$ $413 \mathrm{~kJ} ; \mathrm{Cl}-\mathrm{Cl}=242 \mathrm{~kJ} ; \mathrm{H}-\mathrm{Cl}=432 \mathrm{~kJ} .$ Use these to calculate the average $\mathrm{C}-\mathrm{Cl}$ bond energy in chloroform. Compare this with the value in Table $15-2$.

Methane undergoes several different exothermic reactions with gaseous chlorine. One of these forms chloroform, $\mathrm{CHCl}_{3}(\mathrm{~g})$.
$\begin{aligned} \mathrm{CH}_{4}(\mathrm{~g})+3 \mathrm{Cl}_{2}(\mathrm{~g}) \longrightarrow \mathrm{CHCl}_{3}(\mathrm{~g})+3 \mathrm{HCl}(\mathrm{g}) \\ \Delta H_{\mathrm{rxn}}^{0} &=-305.2 \mathrm{~kJ} / \mathrm{mol} \mathrm{rxn} \end{aligned}$
Average bond energies per mole of bonds are: $\mathrm{C}-\mathrm{H}=$ $413 \mathrm{~kJ} ; \mathrm{Cl}-\mathrm{Cl}=242 \mathrm{~kJ} ; \mathrm{H}-\mathrm{Cl}=432 \mathrm{~kJ} .$ Use these to
calculate the average $\mathrm{C}-\mathrm{Cl}$ bond energy in chloroform. Compare this with the value in Table $15-2$.

Key Concepts

Substitution Reactions

Substitution reactions involve replacing one atom or group in a molecule with another. In these reactions, certain bonds in the original compound are broken and new bonds are formed, leading to a change in the chemical structure and energy profile of the molecule. This concept is crucial when analyzing the energy changes that occur during the conversion of one molecular species into another.

Bond Formation and Bond Breaking

Chemical reactions involve breaking chemical bonds in the reactants and forming new bonds in the products. Breaking bonds is an endothermic process, meaning it requires energy input, while forming bonds is exothermic, releasing energy. The delicate balance between these processes determines whether a reaction is overall exothermic or endothermic.

Bond Dissociation Energy

Bond dissociation energy is the energy required to break one mole of a specific type of bond in gaseous molecules. These average values provide a useful, though approximate, way to estimate the energy change associated with breaking particular bonds, and they are typically derived from a variety of compounds containing that bond. In thermochemistry, these energies are central to calculating the enthalpy changes of reactions where bonds are broken and formed.

Reaction Enthalpy Calculation Using Bond Energies

In chemical reactions, reaction enthalpy can be estimated by considering the energy needed to break bonds in the reactants and the energy released when new bonds are formed in the products. The overall enthalpy change is calculated by subtracting the total energy released in bond formation from the total energy required to break bonds, a method that provides a useful approximation especially when using average bond energies.

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