Which law states that the total energy change in a thermodynamic process is the sum of the energy changes of the individual steps?

Hess's law is a fundamental principle in thermochemistry which asserts that the total enthalpy change during a chemical reaction is the same regardless of whether the reaction occurs in one step or multiple steps. This law relies on the concept that enthalpy is a state function, meaning that it depends only on the initial and final states of the system and not on the specific pathway taken to get there.

This principle is particularly useful in calculating the enthalpy changes of reactions that are difficult to measure directly. By breaking down complex reactions into individual steps, each with measurable enthalpy changes, one can add these changes together to find the total enthalpy change for the overall reaction.

The other concepts mentioned, such as the first and second laws of thermodynamics, and the law of conservation of energy, while also critical to understanding energy changes in thermodynamic processes, do not specifically state that the total energy change is simply the sum of the energy changes of individual steps. Instead, they address broader principles of energy conservation and transformations. This specificity is what makes Hess's law the correct choice in this context.