Enthalpy is a fundamental concept, as it allows for the quantification of energy changes that can occur in chemical reactions. Whereas pressure and volume are easily measurable, the total enthalpy of a system cannot be measured directly, as the internal energy generally contains unknown or very difficult-to-determine components. In most everyday situations, the change in enthalpy occurs at constant pressure, so any enthalpy changes are linked to the heat absorbed or produced by the reaction. This type of calorimetry is called isobaric calorimetry. Suppose the cup we use in this experiment is very insulating, like in the case of a Styrofoam cup. In this case, the type of calorimetry we are using is adiabatic calorimetry. This means that the reactor (in our case, the cup) keeps all the heat released or absorbed by the reaction, allowing us to measure it.
So, if there are changes in the enthalpy but no changes in the volume, then it would mean that all the enthalpy changes will be linked to the heat released or produced during the reaction. As the reaction progresses, it will either generate heat, increasing the temperature in its surroundings (exothermic reaction), but it can also be the case that the energy absorbs heat (endothermic). The heat can be easily measured using the following equation:
q = mwater x Cwater x (Tf-T0)
In this equation, m is the mass of water inside of the cup, Cwater is the thermal capacity of water and Tf – T0 is the final temperature minus the initial temperature. In this case, we can assume that all the heat will be transferred into the water, which is why we use the thermal capacity of water, and therefore the water temperature will change.
Having our Styrofoam cup equip ped with a thermometer and knowing the amount of water we have put in our cup, we can calculate the heat exchange and understand how much the enthalpy has changed.