The device works by isolating a reaction inside a heavily insulated container. This insulation prevents heat from escaping into the surrounding laboratory room. A surrounding medium of known mass absorbs the heat released from the reaction.
This medium is usually water because it has a high specific heat capacity. A thermometer measures the resulting temperature change of the surrounding water bath. Chemists then convert this measured temperature change into a specific heat energy value.
They use the thermodynamic equation q = mcΔT to find this total heat. In this formula, the letter q represents the total heat energy transferred. The variable m stands for the mass of the liquid water.
The letter c denotes the specific heat capacity of that water. The symbol ΔT represents the overall temperature change during the experiment. This calculated heat helps determine the total enthalpy change of the chemical reaction.
Enthalpy represents the total heat energy content of a given chemical system. There are two common calorimeter designs used in chemistry laboratories. The constant-pressure coffee-cup calorimeter is used for reactions happening in a liquid solution.
The constant-volume bomb calorimeter is primarily used for highly energetic combustion reactions. These reactions burn samples under high oxygen pressure inside a sealed steel vessel. Students often mistakenly think that a calorimeter measures heat energy directly. It actually measures temperature changes, which researchers then use to calculate the heat.