What is Hess's Law in the context of thermochemistry?

Hess's Law is a fundamental principle in thermochemistry that states the total enthalpy change of a chemical reaction is the same whether the reaction occurs in one step or multiple steps. This means that the enthalpy change can be calculated by summing the enthalpy changes of individual steps in a reaction pathway, allowing chemists to determine the enthalpy change even if the reaction mechanism is unknown or occurs in several stages. This principle relies on the concept that enthalpy is a state function, meaning that its value depends only on the initial and final states of the system, not on how it gets there.

In practice, Hess's Law is particularly useful because it enables the calculation of the heat released or absorbed in reactions that are difficult to measure directly. By using known enthalpy changes from related reactions, one can infer the enthalpy change for the reaction of interest. This is pivotal in various applications, from industrial processes to energy calculations in biochemical systems.