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D H-Enthalpy using Bond Energies

D H-Enthalpy using Bond Energies. A theoretical approach to calculating enthalpy values for a chemical reaction can be approximated by using the potential energies associated with specific chemical bonds in a compound.

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D H-Enthalpy using Bond Energies

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  1. DH-Enthalpy using Bond Energies A theoretical approach to calculating enthalpy values for a chemical reaction can be approximated by using the potential energies associated with specific chemical bonds in a compound.

  2. Average bond dissociation energy has been tabulated for many different chemical bonds through experiments. For example, CH4 is comprised of 4 C—H bonds, each with an average energy of 413 kJ. This means that 1652 kJ of energy (413 kJ x 4 bonds) was needed or absorbed by the molecule in order to totally break apart (dissociate). *Larger values indicate stronger bonds in a molecule

  3. From a balanced chemical equation and knowledge of Lewis Diagrams for molecules, we can determine the enthalpy of the reaction using: Heat energy is the sum of all bonds broken and bonds formed in the process or DH = Sbonds broken - Sbonds formed Since energy is needed to break bonds (endothermic – positive value) and energy is released when making bonds (exothermic – negative value)

  4. Example: CH4 + 3Cl2 CHCl3 + 3HCl DH = Sbonds broken – Sbonds formed =[4 C--H + 3 Cl—Cl] – [C—H + 3 C—Cl + 3 H—Cl] = [4(413) + 3(239)] – [413 + 3(339) + 3(427)] = -342 kJ

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