What does the coffee cup calorimeter specifically measure?

A coffee cup calorimeter measures the heat transfer that occurs at constant atmospheric pressure. This calorimeter is typically an insulated container, often resembling a coffee cup, which allows for the measurement of the heat exchange that happens when a chemical reaction occurs or when a substance undergoes a physical change, such as dissolving.

The principle behind its functionality relies on the fact that as heat is absorbed or released during a process, it causes a change in the temperature of the contents within the cup. Because the calorimeter is open to the atmosphere, the pressure remains constant, allowing for accurate measurement of enthalpy changes, or the heat of a reaction, in conditions similar to those experienced in real-world scenarios.

By focusing on heat transfer at a constant pressure, the measurements obtained from a coffee cup calorimeter are directly related to the change in enthalpy of the system, which is crucial for understanding various thermodynamic processes. This distinguishes it from other types of calorimeters, such as bomb calorimeters, which operate under constant volume and are therefore used for measuring the heat of combustion specifically.