D16.6 Calorimetry

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.

Two diagrams are shown and labeled a and b. Each diagram is composed of a rectangular container with a thermometer inserted inside from the top right corner. Both containers are connected by a right-facing arrow. Both containers are full of water, and each container has a square in the middle which represents a metal. In diagram a, the metal is drawn in brown and has three arrows facing away from it. Each arrow has the letter “q” at its end. The thermometer in this diagram has a relatively low reading. In diagram b, the metal is depicted in purple and the thermometer has a relatively high reading.
Figure: Calorimetry. In a simple calorimetry process, (a) there is heat transfer of energy, q, from the hot metal to the cool water, until (b) both are at 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.

qM + qW = 0

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).

Exercise: Energy Transfer in a Calorimeter

The same principles apply when we apply calorimetry to determine the heat transfer of energy involved in chemical reactions:

qreaction = –qsurroundings

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.

Activity: Energy Transfer to a Cold Pack

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,

qreaction = -[qwater bath + qbomb]
A stainless steel cylinder with a screw cap is labeled "bomb". Next to it is a larger brown plastic tub labeled calorimeter. Connected to both is an electrical control box.
Figure: Bomb calorimeter. Reactants are added to the stainless steel “bomb”, which is then placed into a water bath inside the calorimeter. Reaction is initiated by brief electric heating and both the bomb and the surrounding water bath exchange energy with the reaction system. The calorimeter is well insulated so the only energy transfers are between the reaction, the bomb, and the water bath. The heat transfer of energy can be calculated from the temperature change of bomb and water bath.

Exercise: Bomb Calorimetry

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.