Bond Energies and the Enthalpy of Reactions

Bond Energies and the Enthalpy of Reactions - exothermic (...

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Bond Energies and the Enthalpy of Reactions If we know which bonds are broken and which bonds are made during a chemical reaction, we can estimate the enthalpy change of the reaction ( H rxn ) even if we don't know the enthalpies of formation ( H f °)for the reactants and products: H = Σ (bond energies of broken bonds) - Σ (bond energies of formed bonds) Example: The reaction between 1 mol of chlorine and 1 mol methane Bonds broken : 1 mol of Cl-Cl bonds, 1 mol of C-H bonds Bonds formed : 1 mol of H-Cl bonds, 1 mol of C-Cl bonds H = [ D (Cl-Cl) + D (C-H)] - [ D (H-Cl)+ D (C-Cl)] [242 kJ + 413 kJ] - [431 kJ + 328 kJ] -104 kJ Thus, the reaction is
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Unformatted text preview: exothermic ( because the bonds in the products are stronger than the bonds in the reactants ) Example: The combustion of 1 mol of ethane bonds broken : 6 moles C-H bonds, 1 mol C-C bonds, 7 / 2 moles of O=O bonds bonds formed : 4 moles C=O bonds, 6 moles O-H bonds H = [(6*413) + (348) + ( 7 / 2 *495)] - [(4*799) + (6*463)] = 4558 - 5974 = -1416 kJ (the reaction is exothermic) Bond strength and bond length Bond Bond Energy (kJ/mol) Bond Length () C-C 348 1.54 C=C 614 1.34 C= C 839 1.20 As the number of bonds between two atoms increases, the bond grows shorter and stronger...
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Bond Energies and the Enthalpy of Reactions - exothermic (...

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