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Unformatted text preview: Bond H (kJ/mole) Bond Length C=C 614 1.37 C C 839 1.20 C-N 305 1.43 C=N 615 1.38 Bond H (kJ/mole) Bond Length C N 891 1.16 We can use these average bond enthalpy changes to calculate the approximate enthalpy change for reactions. For example, to calculate the change in enthalpy for the following reaction: H 2(g) + F 2(g) 2 HF (g) we identify and count all the bonds that are broken (shown in red) and formed (shown in blue). H H + F F 2 H F Substituting the average bond enthalpies: D H-H = 432 kJ/mole , D F-F = 154 kJ/mole , D H-F = 565 kJ/mole in the expression H = ( H of bonds broken) - ( H of bonds formed) we obtain H = [(1 mole) (432 kJ/mole) + (1 mole) (154 kJ/mole)] - [(2 moles) (565 kJ/mole) ] = -544 kJ...
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