Physical Chemistry Third Edition

2.8 Calculation of Energy Changes of Chemical Reactions 97

Thermometer

Bucket

Bomb

Stirrer

Stirring

motor

Outer

case

Ignition

terminals

Figure 2.12 A Bomb Calorimeter.(Courtesy of the Parr Instrument Co.)

not ideal. We want to find∆H◦for an isothermal reaction, but the final temperature

is not equal to the initial temperature. For ordinary work, these differences are negli-

gible compared with the experimental errors, but corrections can be made for highly

accurate work. A wire fuse is used to ignite the sample, and the heat of combustion of

the wire and the electrical energy needed to ignite the sample can be included in the

calculation.

EXAMPLE2.34

A pellet of naphthalene of mass 1.234 g is burned in a bomb calorimeter withCcal

14225 J K−^1. If the initial temperature is 298.150 K and the final temperature is 301.634 K,

find∆Uand∆Hfor 1 mol of reaction, with liquid water as one of the products. Neglect the

heat of combustion of the wire fuse.

Solution

The balanced reaction equation is

C 10 H 8 (s)+12O 2 (g)−→10CO 2 (g)+4H 2 O(l)

q∆U−qcal−(14225 J K−^1 )(3.484 K)− 4. 956 × 104 J