Thermodynamics and Chemistry

CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES

11.5 REACTIONCALORIMETRY 336

ignition

wires

platinum resistance

thermometer

sample holder

stirrer

jacket

calorimeter wall

oxygen inlet

air or vacuum

water

bomb vessel

Figure 11.12 Section view of a bomb calorimeter.

Experimental

The common form of combustion bomb calorimeter shown in Fig.11.12consists of a thick-
walled cylindrical metal vessel to contain the reactants of the combustion reaction. It is
called a “bomb” because it is designed to withstand high pressure. The bomb can be sealed
with a gas-tight screw cap. During the reaction, the sealed bomb vessel is immersed in
water in the calorimeter, which is surrounded by a jacket. Conceptually, we take thesystem
to be everything inside the jacket, including the calorimeter walls, water, bomb vessel, and
contents of the bomb vessel.
To prepare the calorimeter for a combustion experiment, a weighed sample of the sub-
stance to be combusted is placed in a metal sample holder. The calculations are simplified
if we can assume all of the sample is initially in a single phase. Thus, a volatile liquid is
usually encapsulated in a bulb of thin glass (which shatters during the ignition) or confined
in the sample holder by cellulose tape of known combustion properties. If one of the com-
bustion products is H 2 O, a small known mass of liquid water is placed in the bottom of the
bomb vessel to saturate the gas space of the bomb vessel with H 2 O. The sample holder and
ignition wires are lowered into the bomb vessel, the cap is screwed on, and oxygen gas is
admitted through a valve in the cap to a total pressure of about 30 bar.
To complete the setup, the sealed bomb vessel is immersed in a known mass of water
in the calorimeter. A precision thermometer and a stirrer are also immersed in the water.
With the stirrer turned on, the temperature is monitored until it is found to change at a slow,
practically-constant rate. This drift is due to heat transfer through the jacket, mechanical
stirring work, and the electrical work needed to measure the temperature. A particular time
is chosen as the initial timet 1. The measured temperature at this time isT 1 , assumed to be
practically uniform throughout the system.
At or soon after timet 1 , the ignition circuit is closed to initiate the combustion reac-
tion in the bomb vessel. If the reaction is exothermic, the measured temperature rapidly