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8 Chemical Bonding Solutions to Exercises (d) The value of 509.5 kJ is between the average value for a C-C single bond (348 kJ) and a C=C double bond (614 kJ). It is somewhat greater than the average of these two values, indicating that the carbon-carbon bond in benzene is a bit stronger than we might expect. 8.112 (a) 2Br(g) = 2AH Br(g) = 2(111.8) kJ = 223.6 kJ (b) C(g) + 4Cl(g) = C(g) + Cl(g) - = + 4(121.7) kJ - (-139.3) kJ = 1344.5 1344.5kJ = 336.1 kJ - bonds (c) 2OH(g) 2OH(g) D(O - = H(g) + O(g) - H = 2(217.94) kJ + 2(247.5) kJ - (-187.8) kJ - 2(463) kJ = 193 kJ (d) The data are listed below. bond D gas kJ/mol D liquid kJ/mol Br-Br 193 223.6 328 336.1 0-0 146 192.7 Breaking bonds in the liquid requires more energy than breaking bonds in the gas phase. For simple molecules, bond dissociation from the liquid phase can be thought of in two steps: molecule (1) molecule (g) molecule (g) atoms (g) The first step is evaporation or vaporization of the liquid and the second is bond dissociation in the gas phase. Average bond enthalpy in the liquid phase is then the sum of the enthalpy of vaporization for the molecule and the gas phase bond dissociation enthalpies, divided by the number of bonds dissociated. This is greater than the gas phase bond dissociation enthalpy owing to the contribution from the enthalpy of vaporization. 229