Use bond-dissociation enthalpies (Table 4-2, p. 136) to...

Use bond-dissociation enthalpies (Table 4-2, p. 136) to calculate values of $\Delta H^{\circ}$ for the following reactions. (a) $\mathrm{CH}_{3}-\mathrm{CH}_{3}+\mathrm{I}_{2} \longrightarrow \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{HI}$ (b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}+\mathrm{HI} \longrightarrow \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{HCl}$ (c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}+\mathrm{HCl} \longrightarrow\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Cl}+\mathrm{H}_{2} \mathrm{O}$ (d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{3}+\mathrm{CH}_{4}$ (e) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{Br}+\mathrm{H}_{2} \mathrm{O}$

Use bond-dissociation enthalpies (Table 4-2, p. 136) to calculate values of $\Delta H^{\circ}$ for the following reactions.
(a) $\mathrm{CH}_{3}-\mathrm{CH}_{3}+\mathrm{I}_{2} \longrightarrow \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{HI}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}+\mathrm{HI} \longrightarrow \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{I}+\mathrm{HCl}$
(c) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}+\mathrm{HCl} \longrightarrow\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{Cl}+\mathrm{H}_{2} \mathrm{O}$
(d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{3}+\mathrm{CH}_{4}$
(e) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{Br}+\mathrm{H}_{2} \mathrm{O}$

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Key Concepts

Bond Dissociation Enthalpy

This concept refers to the energy required to break a specific chemical bond in a molecule in the gas phase, measured under standard conditions. Bond dissociation enthalpies are typically given in kJ/mol and provide an estimate of the strength of a particular bond. They are central to understanding the energy changes involved when bonds are broken during chemical reactions.

Reaction Enthalpy Calculation Using Bond Energies

This concept involves estimating the overall enthalpy change (?H°) of a reaction by summing the energies required to break bonds in the reactants and subtracting the energies released upon forming bonds in the products. Essentially, ?H° is approximated as the total bond dissociation energies of bonds broken minus the bond formation energies of bonds made, following an energy balance approach rooted in Hess's Law.

Hess's Law

Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway taken, as long as the initial and final conditions are the same. This principle allows us to calculate reaction enthalpies by breaking down complex reactions into simpler steps or by using bond energies, reinforcing the idea of energy conservation in chemical processes.

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