Experimental values for heats of formation are not always negative (exothermic). The sign of the heat of formation depends on the convention chosen for the system. However, it is commonly expressed as negative (exothermic) because of the way the heat of formation is defined.
The heat of formation, also known as the standard enthalpy of formation, is the change in enthalpy that occurs when one mole of a substance is formed from its constituent elements in their standard states at a given temperature and pressure. The standard state refers to the most stable form of an element at a specific temperature and pressure.
By convention, when a compound is formed from its constituent elements, and the reaction releases heat (the system loses energy), the heat of formation is assigned a negative value. This negative sign indicates that energy is being released to the surroundings, making the process exothermic. On the other hand, if a reaction requires input energy to form the compound, the heat of formation is assigned a positive value, indicating an endothermic process.
In practice, experimental values for heats of formation are determined using calorimetry techniques. Calorimetry measures the heat exchanged during a chemical reaction or physical process. By conducting carefully controlled experiments and measuring the heat changes, scientists can calculate the heat of formation for a substance.
It's important to note that while experimental values aim to reflect the true heat of formation, there can be some inherent uncertainties and limitations in the measurement process, which can lead to slight variations or discrepancies between different experimental values.