Bond energies

1
Bond energies

• Define the term average bond enthalpy
• Explain, in terms of average and enthalpies, why
  some reactions are exothermic and others are
  endothermic.

2
Average Bond Enthalpy

• The standard molar enthalpy change of bond
  dissociation is the energy change when 1 mole of
  bonds is broken.
• At a gas state at 298 K and a pressure of 100
  kPa.
• Use a data table for values

3

• You have to be able to draw the structural
  chemical formula to see all the bonds and add
  them up properly.
• Bond strength is associated with how much energy
  it takes to break the bond

4

• Breaking bonds requires energy (Endothermic)
• Making bonds releases energy (Exothermic)
• Reactants products this time!

5
?H (5 x OO) ( 8 x C-H) (2 x C-C) (3
x 2 x CO) (4 x 2 x H-O) ?H (5 x 498) (8
x 413) (2 x 347) (3 x 2 x 805) (4 x 2 x
464) ?H -2054 kJ
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7
CH4 Cl2 ? CH3Cl HCl

• This reaction is overall exothermic because the
  final product is lower in energy than the
  reactants.
• Calculate the ?Hrxn using bond enthalpies.
• Look them up on the table.

8
Practice

• H2 Cl2 ? 2HCl
• CH4 2O2 ? CO2 2H2O
• N2 3H2 ? 2NH3

9

• http//www.matter.org.uk/Schools/Content/Reactions
  /BE_enthalpyH2O.html
• Try this site to practice at home
• Read 688
• Practice Pg 690 23,25 and 26
• Pg 691 7