Which principle states that the enthalpy change of a reaction depends only on the difference between the enthalpy of the products and reactants?

The principle that states the enthalpy change of a reaction depends only on the difference between the enthalpy of the products and reactants is Hess's law. This law is grounded in the idea that the total enthalpy change for a chemical reaction is the same, regardless of whether it occurs in one step or multiple steps. Hess's law is based on the concept of state functions in thermodynamics, where the change in a system's state (such as enthalpy) depends only on its initial and final states, not on the specific path taken to get from one to the other.

For example, if a reaction can be broken down into several steps, the overall enthalpy change can be calculated by summing the enthalpy changes of each individual step. Therefore, any reaction's enthalpy change is consistent and can be determined from the standard enthalpies of formation of the reactants and products involved, which emphasizes the difference in their energies.

Understanding Hess's law is fundamental in thermochemistry because it allows chemists to calculate enthalpy changes for reactions that may be difficult to measure directly, thus providing a valuable tool for predicting how much energy is absorbed or released in various chemical reactions.