# Tang 06 bond energy

Mar. 26, 2013
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### Tang 06 bond energy

• 2. BOND ENERGY All chemical reactions involve the breaking of old bonds followed by the making of new bonds. The heat absorbed or released in a reaction comes from the chemical bonds being broken or made respectively.
• 3. BOND ENERGY Bond breaking is endothermic. C-H + energy  C H Bond formation is exothermic. H H  H-H + energy The greater the bond energy is, the stronger the bond
• 4. BOND ENERGY Table of average bond energies:
• 5. BOND ENERGY The bond energies on the table are averages, because they may differ depending on adjacent bonds. i.e. the C-H bond requires 435kJ to break, but breaking all 4 bonds of CH4 requires 1652kJ of energy, this the average is 413kJ/mol 435kJ/mol H H C H 1652kJ/mol = 413kJ/mol 4 bonds H
• 6. BOND ENERGY Use bond energies to find the ΔH° of a reaction: Ex 1. H2(g) + Br2(g)  2HBr(g) Bond energies 432kJ/mol 193kJ/mol 2 x -363kJ/mol There are 2 Bond formation moles of H-Br is exothermic Add the bond energies together: ΔH° = 432kJ/mol + 193kJ/mol + 2 x (-363kJ/mol) ΔH° = -101kJ/mol .: ΔH° is -101kJ/mol
• 7. BOND ENERGY Use bond energies to find the ΔH° of a reaction: Ex 1. H2(g) + Br2(g)  2HBr(g) 432kJ/mol 193kJ/mol 363kJ/mol Can be solved using this formula as well ΔH° = Σ bond energy of reactants –Σ bond energy of products ΔH° = (432kJ/mol + 193 kJ/mol) – (2 x 363kJ/mol) ΔH° = -101kJ/mol .: ΔH° is -101kJ/mol
• 8. BOND ENERGY For more complex molecules… Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(g) Draw the structures to determine the types and number of bonds 347
• 9. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. 347 + (1 x C-C) + (1 x 347) + 347 + 347 kJ/mol = 4731 kJ/mol
• 10. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. = 4731 kJ/mol – 5782 kJ/mol = -1051 kJ/mol .: ΔH° is -1.05x103kJ The molar enthalpy of combustion of ethanol based on bond energies is -1051 kJ/mol. The accepted value is -1368 kJ/mol (using Hess’ Law). There difference is due to the use of average bond energies.
• 11. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH3OCH3) using average bond energies. CH3OCH3(l) + 3O2(g)  2CO2(g) + 3H2O(g)
• 12. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH3OCH3) using average bond energies.
• 13. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH3OCH3) using average bond energies. .: ΔH° is -1.09x103kJ
• 14. BOND ENERGY Bond dissociation energy (D) is also a measure of bond strength in a chemical bond. It is the change in enthalpy of a homolysis reaction at abolute zero (0 kelvin) where a molecule is broken down into two free radicals. ΔH = __kJ/mol It is not the same as average bond energy.