9 Bond Energies by cuiliqing

VIEWS: 16 PAGES: 8

									Bond Energies
   Thermochemistry
Bond Energy
    is the amount of energy that would be
    required to break the bond.
   Similarly, it is also the amount of energy that
    is released when the bond is formed
   Every chem rxn involves breaking and
    forming bonds
   Bond energies are considered to be
    “average” energies because the actual
    amount varies depending on the bonding
    environment.
   Bond energies can be a useful way of
    predicting the enthalpy of a reaction.

   Consider the reaction:
                  H2 + F2      2HF
       H-H and F-F bonds must be broken (energy
        must be added) and 2 H-F bonds will form
        (energy will be released).
ALL
POSITIVE!!!
• Bond breakage requires energy (sign
positive)
• So from the reactant total subtract the
product total
• For the reaction exo is still (-) and endo (+)
Steps:
1.   Draw Lewis diagrams to determine the
     number and type of bonds in the molecules.
2.   Look up the bond energies for each bond.
3.   Add the reactants (don’t forget to multiple by
     molar coefficients).
4.   Add the products (don’t forget to multiple by
     molar coefficients).
5.   Subtract products from reactants.
Example:
   Use bond energies to estimate the enthalpy
    change for the combustion of methane.

         CH4 + 2O2  CO2 + 2H2O
Try it:
   Page 690 # 23  26
   Worksheet

								
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