Methane undergoes several different exothermic reactions...

Methane undergoes several different exothermic reactions with gaseous chlorine. One of these forms chloroform, $\mathrm{CHCl}_{3}(\mathrm{~g})$. 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, $\mathrm{CHCl}_{3}(\mathrm{~g})$.
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

Comparison with Tabulated Standard Values

Chemical reference tables provide standard bond energy values based on a range of measurements and are used for quick estimation and validation of calculated values. Comparing calculated average bond energies with these tabulated values helps in assessing the accuracy of the computations and in understanding any discrepancies due to experimental factors or approximations inherent in the averaging process.

Average Bond Energy Calculation

Average bond energy is calculated by taking the overall energy change from a reaction and dividing it by the number of similar bonds formed or broken. This method provides an estimate of the energy associated with a specific bond, even though individual bond energies can vary somewhat depending on the surrounding molecular environment. It is a useful tool for comparing bonds in different molecules or under different conditions.

Bond Dissociation Energy

Bond dissociation energy is the amount of energy required to break a specific chemical bond in a gaseous molecule under standard conditions. It is usually expressed in kilojoules per mole (kJ/mol) and provides a measure of bond strength. Understanding these energies is crucial for analyzing reaction enthalpies, reaction mechanisms, and the stability of chemical species.

Energy Balance in Chemical Reactions

In chemical reactions, particularly those involving multiple bond breaking and formation, the overall enthalpy change of the reaction can be estimated by summing the energies required to break the bonds in the reactants and subtracting the energies released from forming the bonds in the products. This energy balance approach is fundamental in thermochemistry and allows one to determine whether a reaction is exothermic or endothermic.

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