1. 5.4 Bond enthalpies 5.4.1 Define the term average bond enthalpy 5.4.2 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 O=O) + ( 8 x C-H) + (2 x C-C)] – [(3 x 2 x C=O) + (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

7. CH 4 + Cl 2  CH 3 Cl + HCl  This reaction is overall exothermic because the final product is lower in energy than the reactants.  Calculate the ∆H rxn using bond enthalpies.  Look them up on the table.

8. Practice: 1. H 2 + Cl 2  2HCl 2. CH 4 + 2O 2  CO 2 + 2H 2 O 3. N 2 + 3H 2  2NH 3

9.  http://www.matter.org.uk/Schools/Conten t/Reactions/BE_enthalpyH2O.html http://www.matter.org.uk/Schools/Conten t/Reactions/BE_enthalpyH2O.html  Try this site to practice at home