Hess's law states that the total enthalpy change during a chemical reaction is the same regardless of the pathway or number of steps taken to achieve the final products. This means enthalpy is a state function—it depends only on the initial and final states of the system, not the process or steps used to get there. The law is based on the first law of thermodynamics, which emphasizes conservation of energy. In practical terms, Hess's law allows calculation of heat changes for complex reactions by adding up the enthalpy changes of individual steps that sum to the overall reaction. It is widely used in thermochemistry to calculate heats of reaction, formation, or combustion when direct measurement is difficult.
Summary of Hess's Law
- Total enthalpy change (ΔH) is path-independent.
- Enthalpy change for a reaction equals the sum of enthalpy changes in individual steps.
- Enables calculation of reaction enthalpy via alternative reaction routes.
- Rooted in the first law of thermodynamics (energy conservation).
- Useful in determining heat changes in various chemical processes.
This fundamental principle helps chemists understand and quantify energy changes in reactions without needing to measure every step experimentally.
