The Foundations of Chemistry

No work is done when a reaction is carried out in a bomb calorimeter, even if gases are
involved, because
V0. Therefore,

Eqv (constant volume)

EXAMPLE 15-14 Bomb Calorimeter

A 1.000-gram sample of ethanol, C 2 H 5 OH, was burned in a bomb calorimeter whose heat
capacity had been determined to be 2.71 kJ/°C. The temperature of 3000 grams of water rose
from 24.284°C to 26.225°C. Determine
Efor the reaction in joules per gram of ethanol, and
then in kilojoules per mole of ethanol. The specific heat of water is 4.184 J/g°C. The combus-
tion reaction is

C 2 H 5 OH(
)3O 2 (g)88n2CO 2 (g)3H 2 O(
)

Plan

The amount of heat given off by the system (in the sealed compartment) raises the tempera-
ture of the calorimeter and its water. The amount of heat absorbed by the water can be calculated
using the specific heat of water; similarly, we use the heat capacity of the calorimeter to find
the amount of heat absorbed by the calorimeter. The sum of these two amounts of heat is the
total amount of heat released by the combustion of 1.000 gram of ethanol. We must then scale
that result to correspond to one mole of ethanol.

Solution

The increase in temperature is

__?°C26.225°C24.284°C1.941°C rise

The amount of heat responsible for this increase in temperature of 3000 grams of water is

Heat to warm water1.941°C

4

g

.1



8

°

4

C

J

3000 g2.436 104 J24.36 kJ

The amount of heat responsible for the warming of the calorimeter is

Heat to warm calorimeter1.941°C

2.7

°

1

C

kJ

5.26 kJ

The total amount of heat absorbed by the calorimeter andby the water is

Total amount of heat24.36 kJ5.26 kJ29.62 kJ

Combustion of one gram of C 2 H 5 OH liberates 29.62 kJ of energy in the form of heat, that is

Eqv29.62 kJ/g ethanol

The negative sign indicates that energy is released by the system to the surroundings. Now we
may evaluate
Ein kJ/mol of ethanol by converting grams of C 2 H 5 OH to moles.

1365 kJ/mol ethanol

E1365 kJ/mol ethanol

This calculation shows that for the combustion of ethanol at constant temperature and constant
volume, the change in internal energy is 1365 kJ/mol ethanol.

You should now work Exercises 64 and 65.

46.07 g C 2 H 5 OH



1 mol C 2 H 5 OH

29.62 kJ



g

__?kJ



mol ethanol

Benzoic acid, C 6 H 5 COOH, is often

used to determine the heat capacity of

a calorimeter. It is a solid that can be

compressed into pellets. Its heat of

combustion is accurately known:

3227 kJ/mol benzoic acid, or 26.46 kJ/g

benzoic acid. Another way to measure

the heat capacity of a calorimeter is to

add a known amount of heat

electrically.

15-10 Changes in Internal Energy, E 617

Ignition Thermometer

wires

Stirrer

Insulated

outer

container

Steel

container

Sample

dish

Steel

bomb

Water

You should look again at the

Saunders Interactive General

Chemistry CD-ROM,Screen 6.18,

Measuring Heats of Reaction:

Calorimetry.

Figure 15-9 A bomb calorimeter

measures qv, the amount of heat

given off or absorbed by a reaction

occurring at constant volume.The

amount of energy introduced via the

ignition wires is measured and taken

into account.