Calorimetry is the study of heat exchange between a system and its surroundings. A calorimeter is a device used to measure the heat exchanged between a system and its surroundings. Calorimeters are often used to measure the net heat released or absorbed by a chemical reaction at a constant pressure, called the heat of reaction (). This is also called enthalpy of reaction. If the substances absorb heat from the surroundings, the reaction is endothermic and is positive. If the substances release heat to their surroundings, the reaction is exothermic and is negative.
A bomb calorimeter is a constant-volume calorimeter used to measure the heat of a reaction for combustion reactions. The calorimeter contains a bomb chamber into which the reactants are placed, and it is wired to an electric heater that ignites the reactants and causes the combustion to occur. The rest of the calorimeter is filled with water and pressurized oxygen. A stirrer keeps the water mixed so that the heat is evenly distributed, and a thermometer records the temperature of the water.
Combustion is exothermic, so the heat released by the combustion is absorbed by the water inside the calorimeter and causes the temperature of the water to rise. In calorimetry experiments, the heat capacity of the calorimeter, Ccal—not the specific heat—is used. Each calorimeter has a unique Ccal value that must be determined experimentally. The heat capacity of the calorimeter is the heat required to raise the calorimeter by one degree Celsius.An important caveat is that the entire bomb calorimeter should not lose any heat to its surroundings. Calorimeters are well insulated against heat transfer. A system that does not exchange heat or matter with the surroundings is an isolated system. Thus, a bomb calorimeter is built as an isolated system to ensure that no heat is released to the environment around the device and all of the energy released by the combustion is used to change the temperature inside the calorimeter. Because a bomb calorimeter is an isolated system the sum of the energies of the reactants and the calorimeter must be zero,
Bomb calorimeters are useful in industrial laboratories, but the more common device in a general chemistry lab is the simple calorimeter, sometimes called a "coffee cup calorimeter," which is a constant-pressure calorimeter. A simple calorimeter works on the same principle as a bomb calorimeter but is usually made of two nested foam cups. A simple calorimeter is typically used to measure the heats of reaction for other types of processes besides combustion reactions. To measure , the reactants are placed inside the inner cup, usually in an aqueous solution, and a stirrer and thermometer are inserted through a cover. The stirrer ensures the heat is distributed throughout the reaction system, and the thermometer measures the temperature of the solution.
Because foam is a good insulator, there is essentially no heat exchange with the surroundings of the calorimeter. Thus, any temperature change of the solution is a result of energy either being absorbed or released by the chemical reaction.In a coffee cup calorimeter,