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# Tang 06 bond energy

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### Tang 06 bond energy

1. 1. BOND ENERGY
2. 2. BOND ENERGYAll chemical reactions involve the breaking of oldbonds followed by the making of new bonds.The heat absorbed or released in a reaction comesfrom the chemical bonds being broken or maderespectively.
3. 3. BOND ENERGY Bond breaking is endothermic. C-H + energy  C H Bond formation is exothermic. H H  H-H + energyThe greater the bond energy is, the stronger the bond
4. 4. BOND ENERGYTable of average bond energies:
5. 5. BOND ENERGYThe bond energies on the table are averages, becausethey may differ depending on adjacent bonds.i.e. the C-H bond requires 435kJ to break, butbreaking all 4 bonds of CH4 requires 1652kJ ofenergy, this the average is 413kJ/mol435kJ/mol H H C H 1652kJ/mol = 413kJ/mol 4 bonds H
6. 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. 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/molCan be solved using this formula as wellΔH° = 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. 8. BOND ENERGYFor 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. 9. BOND ENERGYEx 2. Calculate the enthalpy of combustion for ethanolusing average bond energies. 347 + (1 x C-C) + (1 x 347) + 347 + 347 kJ/mol = 4731 kJ/mol
10. 10. BOND ENERGYEx 2. Calculate the enthalpy of combustion for ethanolusing 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. 11. BOND ENERGYEx 3. Calculate the enthalpy of combustion for methoxymethane (CH3OCH3) using average bond energies. CH3OCH3(l) + 3O2(g)  2CO2(g) + 3H2O(g)
12. 12. BOND ENERGYEx 3. Calculate the enthalpy of combustion for methoxymethane (CH3OCH3) using average bond energies.
13. 13. BOND ENERGYEx 3. Calculate the enthalpy of combustion for methoxymethane (CH3OCH3) using average bond energies. .: ΔH° is -1.09x103kJ
14. 14. BOND ENERGYBond dissociation energy (D) is also ameasure of bond strength in a chemical bond.It is the change in enthalpy of a homolysisreaction at abolute zero (0 kelvin) where amolecule is broken down into two free radicals. ΔH = __kJ/mol It is not the same as average bond energy.

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Aug. 5, 2015
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