What Is a Bomb Calorimeter?

The bomb calorimeter is a laboratory device that contains a “bomb,” or combustion chamber — usually constructed of non-reactive stainless steel — in which an organic compound is consumed by burning in oxygen. Included is a Dewar flask holding a specific amount of water in which the bomb is submersed. All of the heat (Q) generated by the combustion passes into the water, whose temperature (T) rises, and is very carefully measured. From the weights, temperatures and apparatus parameters, an accurate heat or “enthalpy” of combustion (ΔH_c) may be determined. That value may be used to evaluate structural properties of the substance consumed.

Volume expansion is prevented by the rigid bomb design, so even though carbon dioxide and water vapor are produced by the combustion, it occurs at constant volume (V). Since dV=0 in the equation dW=P(dV), where work is W, there is no work performed. Also, as heat (Q) neither enters nor leaves — since everything is within the Dewar flask — the process is “adiabatic,” that is, dQ=0. This means ΔH_c=C_vΔT, where C_v is the heat capacity at constant volume. Data adjustment is needed due to the characteristics of the bomb calorimeter itself; there is the heat introduced by the burning of the fuse triggering combustion, and the fact that the bomb calorimeter functions only approximately adiabatically.

The bomb calorimeter has a number of applications, including both technical and industrial uses. Historically, in the laboratory, hydrocarbons and hydrocarbon derivatives have been burned in a bomb calorimeter with the goal of assigning bond energies. The device has also been used to derive theoretical stabilization energies, such as that of the pi-bond in aromatic compounds. The procedure may be demonstrated to — if not practiced by — students, as part of their undergraduate college instruction. Industrially, the bomb calorimeter is used in the testing of propellants and explosives, in the study of foods and metabolism, and in the evaluation of incineration and greenhouse gases.

Considering the example of one aromatic solvent, benzene (C₆H₆), there are six equivalent carbon-carbon bonds and six equivalent carbon-hydrogen bonds in each molecule. Without the concept of resonance, the carbon-carbon bonds in benzene should seemingly be different — there should be three double bonds and three single bonds. Benzene should be well represented by the fictitious chemical 1, 3, 5-cyclohexatriene. Through the use of a bomb calorimeter, however, the actual energy of the six uniform bonds gives an energy difference for benzene compared to the triene, of 36 kcal/mol or 151 kj/mol. This energy difference is benzene’s resonance stabilization energy.