Introduction/Theory
A bomb calorimeter is a sealed container capable of holding several atmospheres of gas pressure.
A weighed sample of substance is placed in contact with an ignition wire inside the bomb. The
bomb is filled to about 20 atm of pressure with O2, sealed, and placed in a known amount of
water. An electric current is passed through a wire to ignite the mixture. As the combustion
takes place, the heat evolved raises the temperature of the calorimeter and its surrounding water,
as measured by a thermometer. In order to prevent heat loss from the calorimeter system, it is
surrounded by a second water bath, whose temperature is continuously adjusted (by the
experimenter) to match that of the calorimeter. Thus, the heat transfer between the system
(calorimeter and contents) and the surroundings (the water jacket, primarily) is zero, making the
process adiabatic: q = 0. Some calorimeters use good thermal insulation instead of a second
water bath to prevent loss of heat to the surroundings.
Since the combustion takes place in a sealed container with constant volume, the work done on
or by the system is also zero, so that DU = q + w = 0 for the overall process shown below.
The initial temperature of the system is T1 and the final temperature is T2 after combustion of
reactants to products and heating of the calorimeter. Since U is a state function, the pathway
from initial to final state does not affect the value of DU. This allows us to separate the two
processes (combustion and heating), although, in reality, they occur simultaneously.