Calorimetry is an experimental technique used to quantitatively measure heat transfer of energy. Energy is exchanged with a calorimeter, a device with known heat capacity that therefore can relate ΔT to q. A calorimeter must be insulated thermally so that energy does not transfer beyond its physical boundaries. In calorimetry it is useful to define a system, the substance(s) undergoing the chemical or physical change, and the surroundings, everything else that can exchange energy with the system.
For example, if we place a piece of hot metal (M) into cool water (W), heat transfer of energy occurs from metal to water until the two substances reach the same temperature.
If this occurs in a well insulated calorimeter, this heat transfer would ideally occur only between the two substances. Thus, the magnitude of q is the same for both substances.
The arithmetic sign of q is determined by whether the substance loses or gains energy. In our example, energy is transferred from the metal (qM is negative) to the water (qW is positive).
The same principles apply when we apply calorimetry to determine the heat transfer of energy involved in chemical reactions:
Here, qreaction is defined as the change in energy of all atoms present in reactants and products.
A reaction in which there is heat transfer from the reacting substances to their surroundings (a reaction that heats the surroundings) is an exothermic reaction. For example, the combustion reaction that occurs in the flame of a lit match is exothermic. A reaction in which there is heat transfer from the surroundings to the reacting substances (a reaction that cools the surroundings) is an endothermic reaction. For example, when the substances in a cold pack (water and a salt such as ammonium nitrate) are mixed, the resulting process transfers energy from the surroundings, making the surroundings colder.
If the heat capacity of a calorimeter is too large to neglect or if we require more accurate results, then we must take into account energy transferred to or from the calorimeter as well as energy transfers within the the calorimeter. An example of this is a bomb calorimeter, a type of calorimeter that operates at constant volume. Its name stems from it having a strong steel container that will not explode when an exothermic reaction occurs inside it. Bomb calorimetry is used to measure energy transfers for reactions such as combustion reactions. The reaction occurs inside the “bomb” (see the figure below), which is immersed in a water bath and the temperature change of the water bath is measured. Energy has to be transferred to the bomb so the bomb can heat the water, so you need to account for the heat capacity of the bomb as well as the water. That is,