A Level Chemistry OCR Practice Exam 2025 - Free Chemistry Practice Questions and Study Guide

Hess's law is a principle in thermochemistry that states that the total enthalpy change during a chemical reaction is the same, regardless of the number of steps the reaction takes or the pathway it follows to achieve a given state. This law is based on the concept that enthalpy is a state function, meaning it depends only on the initial and final states of the system, not on the specific path taken to get there.

In practical terms, this means that if you can break down a complex reaction into a series of simple steps, the sum of the enthalpy changes for each step will equal the total enthalpy change for the overall reaction. This allows chemists to calculate the enthalpy change for reactions that might be difficult to measure directly by using known enthalpy changes from related reactions.

This principle is fundamental in thermodynamics and helps in understanding various reactions' energetics, especially when dealing with reactions that do not readily occur in a laboratory setting.

The other options do not encapsulate the essence of Hess's law. For example, while reactions that occur at constant temperature can involve Hess's law, it is not a defining characteristic. Temperature's effect on reaction speed is a separate concept and not related to the ent