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hemistry ecure https/ sessionmastenngchemistry.com/myd/mastering#/assignmert m/s 92701 Secure I https//istry.com Chapter 12 assessmentProblem 12.48 asteringchemistry.com/myct/itemView?assignmentProblemID-8623 1 4&offse; next Time Remaining: 00:44:06 Hide Part A The following mechanism for the monochlorination of methane occurs 2 A possible alternative mechanism would involve the following propagation steps: CHs--H CH--CI +.H H-CI + -CI .CI .H + Cl-CI -_ 12 How do you know that the reaction does not take place by the alternative mechanism? 12 O The first propagation step of the alternative mechanism is very endothermic. The alternative mechanism is very endothermic. O The alternative mechanism is very exothermic. 12 The first propagation step of the alternative mechanism is very exothermic 12. Submit My Answers Give Un 12Explanation / Answer
The first propagation step of the alternate mechanism is very endothermic
Explanation:
Alternate mechanism
CH4 + *Cl----> CH3Cl + H*
H* + Cl2 ----->HCl + Cl*
The bond dissociation energies are:
C-H 413 kJ/mol
Cl-Cl 239 kJ/mol
C-Cl 339 kJ/mol
H-Cl 428 kJ/mol
To determine if the reaction is endothermic or exothermic, calculate H of each step.
H for first step = H° of bonds broken - H° of bonds formed
In first step one C-H bond broken and then one C-Cl bond formed
H = (H° of bonds broken) – (H° of bonds formed)
H = (1mol x 413 kJ/mol) – (1mol x 239 kJ/mol) = 174 kJ
Since H for the first step is positive, the reaction is endothermic. Energetically this reaction is unfavorable
In second step one Cl-Cl bond broken and then one H-Cl bond formed
H for second step = H° of bonds broken - H° of bonds formed
H for second step = 239 kJ/mol - 428 kJ/mol = - 189 kJ
Since H is negative, the reaction is exothermic.
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