A Possible Reaction of Ethane with Chlorine Is Shown Below

Multiple Choice

A possible reaction of ethane with chlorine is shown below. $$\mathrm { CH } _ { 3 } \mathrm { CH } _ { 3 } + \mathrm { Cl } _ { 2 } \longrightarrow \mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } \mathrm { Cl } + \mathrm { HCl }$$ This reaction could conceivably occur by the following chain mechanisms [1], [2], and [3]. Determine ?H for step [2]. [1] $\quad \mathrm { Cl } _ { 2 } \longrightarrow 2 \mathrm { Cl } \cdot$
[2] $\mathrm { Cl } ^ { \cdot } + \mathrm { CH } _ { 3 } \mathrm { CH } _ { 3 } \stackrel { \text { slow } } { \longrightarrow } \mathrm { HCl } + \mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } ^ \cdot$
$[ 3 ] \mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } ^ \cdot + \mathrm { Cl } _ { 2 } \stackrel { \text { fast } } { \longrightarrow } \mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } \mathrm { Cl } + \mathrm { Cl } ^\cdot$ Bond dissociation energies (kcal/mol) : $$\begin{array} { | l l | } \hline \text { Bond A-B } & \Delta \mathrm { H } \\\hline \mathrm { Cl } - \mathrm { Cl } & 58 \\\mathrm { CH } _ { 3 } - \mathrm { H } & 104 \\\mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } - \mathrm { H } & 98 \\\mathrm { CH } _ { 3 } \mathrm { CH } _ { 3 } & 88 \\\mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } - \mathrm { Cl } & 81 \\\mathrm { H } - \mathrm { Cl } & 103 \\\hline\end{array}$$

A) -5 kcal/mol
B) +58 kcal/mol
C) -28 kcal/mol
D) None of the choices are correct.