Making a Constant Temperature Phase-Change Medium Coffee Mug: A Real-World Application of Enthalpy of Crystallization

The experiment presented relates the concept of enthalpy of crystallization to the real-world application of temperature-regulated coffee mugs. Students first measure the enthalpy of crystallization of a phase change material (PCM). Observing a temperature increase for a “freezing” process emphasizes that the formation of bonds is an exothermic process. In the case of a phase change, these bonds are intermolecular. In addition, knowing the value of the enthalpy of crystallization allows students to calculate the amount of heat that can be absorbed or released per gram of PCM during the phase change. Then, students insert PCM into vacuum mugs to make constant-temperature coffee mugs that mimic those that are commercially available. Students place hot water (coffee) into their user made PCM mugs and measure the temperature versus time. The PCM absorbs heat quickly and lowers the temperature of coffee to the melting point of the PCM, which is near the desired drinking temperature of the coffee. As the PCM resolidifies, heat is released and the temperature of their coffee is held near the melting point of the PCM. Students compare the performance of their PCM coffee mugs with commercially available mugs. If desired, the procedure can be modified to be a guided inquiry-based experiment. The experiment is designed to be used in either a first-semester freshman chemistry course at a university or a high school chemistry course. The experiment is inexpensive to implement since PCM can be reused. No chemical waste is generated, and student engagement has been positive. The content of the lab corresponds to the thermochemistry content of most textbooks.