Final answer:
Direct measurement of reaction enthalpy can be difficult due to slow kinetics, complex reaction pathways, and side reactions. Hess's Law provides a method to calculate enthalpy changes indirectly by summing the enthalpies of known steps that add up to the overall reaction. Enthalpy is a state function and depends on the reactants' and products' masses and states.
Step-by-step explanation:
It might be difficult to directly measure the enthalpy change of a reaction for several reasons. One challenge is that some reactions occur very slowly, which makes direct measurements impractical.
Additionally, certain reactions are part of a complex series of reactions, and isolating a single step for measurement can be difficult due to the presence of multiple side reactions.
Also, the energy involved in breaking and forming chemical bonds impacts the enthalpy, complicating the direct measurement of enthalpy changes.
However, thanks to Hess's Law, it is possible to indirectly determine the enthalpy change. This law states that the change in enthalpy for a given reaction can be calculated by summing the enthalpy changes of individual steps consisting of known reactions that combine to the overall reaction.
This is akin to adding the altitude changes over short stretches to get the total altitude change when climbing a mountain.
Enthalpy is an extensive property influenced by the mass of substances and their physical states. Moreover, since enthalpy is a state function, it does not depend on the reaction path but only on the initial and final states of the reactants and products.
Endothermic and exothermic reactions reflect the energy balance between bond breaking and bond formation, revealing whether the reaction absorbs or releases energy.