Page 150 - Physical chemistry understanding our chemical world

ENTHALPY 117

Worked Example 3.13 Use the bond enthalpies in Table 3.3 to calculate the enthalpy
of burning methane (Equation (3.28)). Assume all processes occur in the gas phase.

Strategy. We start by writing a list of the bonds that break and form.

Broken: 4 × C–H and 2 × O=O
Formed: 2 × C=Oand 4 × O–H

so

O
O
O
O
H r O =−[2 × H BE(C=O) + 4 × H BE(O–H) ] − [4 × H BE(C–H) + 2 × H BE(O=O) ]
Inserting values of H O from Table 3.3 into Equation (3.32):
BE
Reminder:all H BE
O
H O =−[(2 × 803) + (4 × 464)]
r values are positive
− [(4 × 414) + (2 × 498)]kJ mol −1 because they relate to
dissociation of bonds.
H O =−(3462 − 2652) kJ mol −1
r
so
H r O =−810 kJ mol −1

which is similar to the value in Worked Example 3.11, but less exothermic.

Aside

O
Calculations with bond enthalpies H BE tend to be relatively inaccurate because each
energy is an average. As a simple example, consider the sequential dissociation of
ammonia.

ž
ž
(1) NH 3 dissociates to form NH and H ,and re- The symbol ‘ž’means
2
−1
quires an energy of 449 kJ mol . a radical species, i.e.
with unpaired elec-
ž
ž
ž
(2) NH dissociates to form NH and H ,and re- tron(s).
2
−1
quires an energy of 384 kJ mol .
−1
ž
ž
ž
(3) NH dissociates to form N and H , and requires an energy of 339 kJ mol .
O
The variations in H BE are clearly huge, so we usually work with an average bond
O
enthalpy, which is sometimes written as BE or H . The average bond enthalpy for
BE
−1
the three processes above is 390.9 kJ mol .