Question

Bomb calorimeter used for determining change in internal energy at constant _____

Answer 1

Bomb Calorimeter

Figure 7-11. Bomb Calorimeter

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However, many chemical reactions, such as combustion reactions, do not take place in a solution. To more efficiently measure the heat released by combustion reactions, chemists use a bomb calorimeter, which is a sealed vessel that contains a smaller container called a "bomb." The bomb (not related to a military weapon) is a container designed to withstand high pressure, and is equipped with valves for adding gases and electrical contacts for initiating combustion reactions. (Figure 7-11)

The basic principle is the same: A chemical reaction heats a quantity of water in an insulated container. In this case, however, the reaction takes place inside a sealed container, or bomb. The bomb contains the chemical to be analyzed and enough oxygen to make sure the sample burns completely. The bomb sits submerged in a container of water, and ignition wires start the combustion. Because the reaction takes place in a rigid, sealed container, no pressure-volume work is done by the reaction; all the energy will be released as heat, and none as work. In other words, a bomb calorimeter always measures the heat that is released by a reaction, but in this case the heat represents not the change in enthalpy (ΔH), but the change in internal energy (ΔU). Conveniently, there are simple calculations that can be done to convert the internal energy change into the enthalpy change chemists need.