Final answer:
Hess's law dictates that the sum of individual heats of reaction corresponds to the total heat of reaction for the summed equations, useful for indirect measurements of enthalpy change in complex reactions.
Step-by-step explanation:
Hess's Law and the Addition of Heats of Reaction
When tackling thermochemical equations and their associated heats of reaction, we rely on a principle known as Hess's law. This law states that when multiple thermochemical equations are added to yield a final reaction, the heats of reaction can also be added to provide the total heat of reaction for the final chemical equation. This approach is particularly useful for complicated reactions that cannot be easily measured in direct experiments.
Enthalpy, a state function, means the enthalpy change for a reaction is only dependent on the initial and final states of the reaction, not the path taken. This is analogous to calculating the altitude change when climbing a mountain, regardless of the path or breaks you take. Therefore, by adding the ΔH (change in enthalpy) values of individual steps that make up the overall reaction, we are able to calculate the overall enthalpy change. This calculation is regardless of whether the steps actually occur in that precise order in a real-world setting.
It's important to note that heats of reaction are recorded in kilojoules, and to accurately calculate the change in enthalpy, the physical states of the reactants and products must be specified in the thermochemical equation. Following Hess's law, if we add multiple equations together, we can effectively sum their enthalpies in the same fashion, allowing us to determine the total heat of reaction.