What’s a bomb calorimeter?

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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 which holds a specific amount of water in which the bomb is immersed. All the heat (Q) generated by combustion passes into the water, whose temperature (T) increases, and is measured very carefully. From the weights, temperatures and parameters of the apparatus, a precise heat or “enthalpy” of combustion (ΔHc) can be determined. This value can be used to evaluate the structural properties of the consumed substance.

Volume expansion is prevented by the rigid bomb design, so even though carbon dioxide and water vapor are produced by combustion, it occurs at constant volume (V). Since dV=0 in the equation dW=P(dV), where work is W, no work is performed. Furthermore, since heat (Q) neither enters nor leaves – since everything is inside the Dewar flask – the process is “adiabatic”, ie dQ=0. This means ΔHc=CvΔT, where Cv is the heat capacity at constant volume. Data adjustment is necessary due to the characteristics of the bomb calorimeter itself; there is the heat introduced by the burning of the fuse which ignites the combustion, and the fact that the bomb’s calorimeter only operates approximately adiabatically.

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

Considering the example of an 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 would appear to be different: there should be three double bonds and three single bonds. Benzene should be well represented by the fictional chemical 1, 3, 5-cyclohexatriene. Using a bomb calorimeter, however, the effective energy of the six uniform bonds gives an energy difference for benzene to triene of 36 kcal/mol or 151 kj/mol. This energy difference is the resonance stabilization energy of benzene.

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