Topic 5.4: Bond enthalpies

5.4.1 Define the term average bond enthalpy

The standard molar enthalpy change of bond dissociation is the energy change when 1 mole of bonds is broken. At its standard state at 297 degree kelvins and 1 atmospheric pressure.

This is in the data booklet.

5.4.2 Explain, in terms of average bond enthalpies, why some reactions are exothermic and others are endothermic

In an exothermic reaction, the amount of energy required to break the bonds of the reactants is less than the amount of energy released when the bonds form in the products.

This suggests that the bonds in the reactant are weaker than those in the product (and that the product is therefore more stable).

In an endothermic reaction the reverse is true. The reactants have stronger bonds than the products.