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Unformatted text preview: Bond ΔH (kJ/mole) Bond Length C=C 614 1.37Å C≡C 839 1.20Å C-N 305 1.43Å C=N 615 1.38Å C≡N 891 1.16Å 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 ΔH = Σ (ΔH of bonds broken) - Σ (ΔH of bonds formed) we obtain ΔH = [ (1 mole) (432 kJ/mole) + (1 mole) (154 kJ/mole) ]- [ (2 moles) (565 kJ/mole) ] = -544 kJ...
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- Winter '08
- Enthalpy, Self number, Standard enthalpy change of formation, 1 mole, 2 moles, 4 kJ