The Foundations of Chemistry

EXAMPLE 15-11 Bond Energies

Use the bond energies listed in Table 15-2 to estimate the heat of reaction at 298 K for the

following reaction.

N 2 (g)3H 2 (g)88n2NH 3 (g)

Plan

Each NH 3 molecule contains three NXH bonds, so two moles of NH 3 contain six moles of

NXH bonds. Three moles of H 2 contain a total of three moles of HXH bonds, and one mole

of N 2 contains one mole of NmN bonds. Using the bond energy form of Hess’s Law,

Solution

H^0 rxn[
HNmN 3 
HHXH][6
HNXH]

 945 3(436)6(391)93 kJ/mol rxn

You should now work Exercise 46.

EXAMPLE 15-12 Bond Energies

Use the bond energies listed in Table 15-2 to estimate the heat of reaction at 298 K for the

following reaction.

Plan

Two moles of CXC bonds and seven moles of CXH bonds are the same before and after reac-

tion, so we do not need to include them in the bond energy calculation. The only reactant

bonds that are broken are one mole of CXH bonds and one mole of ClXCl bonds. On the

product side, the only new bonds formed are one mole of CXCl bonds and one mole of HXCl

bonds. We need to take into account only the bonds that are different on the two sides of the

equation. As before, we add and subtract the appropriate bond energies, using values from

Table 15-2.

Solution

H^0 rxn[
HCXH
HClXCl][
HCXCl
HHXCl]

[413242][339432]116 kJ/mol rxn

You should now work Exercises 48 and 50.

CHANGES IN INTERNAL ENERGY, E

The internal energy, E,of a specific amount of a substance represents all the energy

contained within the substance. It includes such forms as kinetic energies of the mole-

cules; energies of attraction and repulsion among subatomic particles, atoms, ions, or

molecules; and other forms of energy. The internal energy of a collection of molecules is

15-10

HHC

H

H

CCl

C 3 H 8 (g) Cl 2 (g) C 3 H 7 Cl(g) HCl(g)

Cl

H

H

C

H

H

H C

H

H

C

H

H

C

H

H





ClHCl



We would get the same value for

H^0 rxnif we used the full bond energy
form of Hess’s Law and assumed that
allbonds in reactants were broken and
then allbonds in products were
formed. In such a calculation the bond
energies for the unchanged bonds
would cancel. Why? Try it!

612 CHAPTER 15: Chemical Thermodynamics

N

H

HH

For each term in the sum, the units
are



m

m

ol

ol

bo

rx

n

n

ds



mol

k

b

J

onds



Internal energy is a state function, so it
is represented by a capital letter.