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# Tang 06 bond energy

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• 1. 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 . Bond formation is exothermic . C-H + energy  C H 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 CH 4 requires 1652kJ of energy, this the average is 413kJ/mol 435kJ/mol 1652kJ/mol 4 bonds = 413kJ/mol C H H H H
• 6. BOND ENERGY Ex 1. H 2(g) + Br 2(g)  2HBr (g) Use bond energies to find the Δ H° of a reaction: Bond energies 432kJ/mol 193kJ/mol 2 x - 363kJ/mol Add the bond energies together: There are 2 moles of H-Br Bond formation is exothermic Δ H° = 432kJ/mol + 193kJ/mol + 2 x (-363kJ/mol) Δ H° = -101kJ/mol .: Δ H° is -101kJ/mol
• 7. BOND ENERGY Use bond energies to find the Δ H° of a reaction: 432kJ/mol 193kJ/mol 363kJ/mol Δ H° = Σ bond energy of reactants – Σ bond energy of products Can be solved using this formula as well Δ H° = (432kJ/mol + 193 kJ/mol) – (2 x 363kJ/mol) Δ H° = -101kJ/mol .: Δ H° is -101kJ/mol Ex 1. H 2(g) + Br 2(g)  2HBr (g)
• 8. BOND ENERGY For more complex molecules… Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. C 2 H 5 OH (l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g) Draw the structures to determine the types and number of bonds
• 9. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies.
• 10. BOND ENERGY Ex 2. Calculate the enthalpy of combustion for ethanol using average bond energies. The molar enthalpy of combustion of ethanol based on bond energies is -1052 kJ/mol. The accepted value is -1368 kJ/mol. There difference is due to the use of average bond energies. .: Δ H° is -1.40x10 3 kJ
• 11. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies. CH 3 OCH 3(l) + 3O 2(g)  2CO 2(g) + 3H 2 O (g)
• 12. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies.
• 13. BOND ENERGY Ex 3. Calculate the enthalpy of combustion for methoxy methane (CH 3 OCH 3 ) using average bond energies. .: Δ H° is -1.09x10 3 kJ
• 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. Δ H = __kJ/mol It is not the same as average bond energy.