Ch_322a_10.05

Ch_322a_10.05 - Energetics of the Chlorination of Methane...

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Energetics of the Chlorination of Methane The overall heat of reaction (enthalpy change) for a chemical reaction influences whether a reaction is thermodynamically spontaneous ( ! G is negative), and how fast it proceeds (value of ! G ). The standard enthalpy change ( ! H o ) may be calculated from the bonding changes, values of DH o .
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Standard Enthalpy Changes for the Chlorination of Methane initiation Cl-Cl 2 Cl . (DH o = -58) ! H o = +58 kcal/mol Cl . + CH 4 CH 3 . H-Cl + + Cl 2 CH 3 CH 3 -Cl + Cl . propagation (DH o = -104) (DH o = -103) (DH o = -58) (DH o = -83.5) ! H o = +1 kcal/mol ! H o = -25.5 kcal/mol termination Cl . Cl . + Cl 2 CH 3 CH 3 + CH 3 CH 3 CH 3 + Cl . CH 3 Cl (DH o = -58) (DH o = -88) (DH o = -83.5) ! H o = -58 kcal/mol ! H o = -88 kcal/mol ! H o = -83.5 kcal/mol
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Evaluation of the Energetics The energetics of the chlorination of methane are determined by the two propagating steps that occur thousands of times relative to the initiation and termination reactions. The standard enthalpy change for the overall reaction can be calculated by adding the standard enthalpy changes for the two
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Ch_322a_10.05 - Energetics of the Chlorination of Methane...

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