The Foundations of Chemistry

CXH bond energies differ slightly from compound to compound, as in CH 4 , CH 3 Cl,
CH 3 NO 2 , and so on. Nevertheless, they are sufficiently constant to be useful in estimating
thermodynamic data that are not readily available by another approach. Values of
H^0 rxn
estimated in this way are not as reliable as those obtained from
H^0 fvalues for the
substances involved in the reaction.
A special case of Hess’s Law involves the use of bond energies to estimateheats of reac-
tion. Consider the enthalpy diagrams in Figure 15-5. In general terms,
H^0 rxnis related
to the bond energies of the reactants and products in gas phase reactionsby the following
version of Hess’s Law.

H^0 rxnB.E.reactantsB.E.products in gas phase reactions only

The net enthalpy change of a reaction is the amount of energy required to break all
the bonds in reactant molecules minusthe amount of energy required to break all the
bonds in product molecules. Stated in another way, the amount of energy released when
a bond is formed is equal to the amount absorbed when the same bond is broken. The
heat of reaction for a gas phase reaction can be described as the amount of energy released
in forming all the bonds in the products minus the amount of energy released in forming
all the bonds in the reactants (see Figure 15-5). This heat of reaction can be estimated
using the average bond energies in Tables 15-2 and 15-3.
The definition of bond energies is limited to the bond-breaking process only,and does
not include any provision for changes of state. Thus, it is valid only for substances in the
gaseous state. The calculations of this section therefore apply onlywhen all substances in
the reaction are gases. If liquids or solids were involved, then additional information such
as heats of vaporization and fusion would be needed to account for phase changes.

Figure 15-5 A schematic representation of the relationship between bond energies and

Hrxnfor gas phase reactions. (a) For a general reaction (exothermic). (b) For the gas phase
reaction

H 2 (g)Br 2 (g)88n2HBr(g)

As usual for such diagrams, the value shown for each change refers to the number of moles
of substances or bonds indicated in the diagram.

Hrxn  103 kJ

HBrBr 193 kJ

 2 
HHBr 732 kJ


HHH 436 kJ

2HBr

2H  2Br

2H  Br 2

H 2  Br 2

Atoms (g)

Reactants (g)

(a) (b)

Hrxn

Energy

released in

bond

formation

Energy

required to

break bonds

Products (g)

Note that this equation involves bond

energies of reactantsminus bond

energies of products.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 9.10, Bond Energy and 
H^0 rxn.