Organic Chemistry (8th Edition)

Published by Pearson
ISBN 10: 0321768418
ISBN 13: 978-0-32176-841-4

Chapter 4 - The Study of Chemical Reactions - Study Problems - Page 171: 4-40 a

Answer

+42kJ/mol Or +10kcal/mol

Work Step by Step

In the given reaction, bond breaking and bond formation takes place. Bond breaking is endothermic (positive) whereas bond formation is exothermic. In order to find $\Delta$ $H^{\circ}$ of the reaction, we add the bond dissociation energies, keeping in mind that where the bond formation takes place, the bond dissociation energy is negative. Therefore, In kJ/mol Bonds broken= $CH3CH2-H=+410$ $I-I= +151$ Bonds formed= $CH3CH2-I=-222$ $H-I=-297$ $\Delta$$H^{\circ}$= $(+410+151) + (-222) + (-297)$ $\Delta$$H^{\circ}$= $+561+(-519)$ $\Delta$$H^{\circ}$= $+561 -519$ $+42 kJ/mol$ In kcal/mol Bonds broken= $CH3CH2-H= -98$ $I-I= -36$ Bonds formed= $CH3CH2-I=-53$ $H-I=-71$ $\Delta$$H^{\circ}$=$ (+98+36 )+ (-53)+(-71)$ $\Delta$$H^{\circ}$ =$134 + ( -124)$ $\Delta$$H^{\circ}$ = $134 - 124$ $\Delta$$H^{\circ}$= $+10 kcal/mol$
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