Hess's law

Hess's Law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy change for the reaction is the same, regardless of the number of steps the reaction is carried out in. This principle is a manifestation of the conservation of energy principle and is pivotal in the field of chemical thermodynamics. It allows for the calculation of the enthalpy change (ΔH) of a reaction that is difficult to measure directly by using the enthalpy changes of multiple intermediary reactions that sum up to the overall reaction.

Overview

Hess's Law implies that the enthalpy change of a chemical process is independent of the pathway taken from the initial to the final state. This is because enthalpy, represented as H, is a state function. A state function's value is determined solely by the current state of the system, not the path the system took to reach that state. Therefore, if a chemical reaction can occur by more than one route, the total enthalpy change is the same, regardless of the route by which the chemical reaction occurs, as long as the initial and final conditions are the same.

Application

The application of Hess's Law is particularly useful in calculating the enthalpy changes of reactions that are too slow, too rapid, or otherwise difficult to measure directly. It is often employed in the calculation of standard enthalpy changes of formation, combustion, and reaction. By using Hess's Law, these enthalpy changes can be determined indirectly through the summation of the enthalpy changes of a series of simpler reactions that lead to the desired reaction.

Calculating Enthalpy Changes

To calculate the enthalpy change of a reaction using Hess's Law, one must: 1. Write the equation for the overall reaction and break it down into a series of steps for which the enthalpy changes are known. 2. Ensure that the sum of the steps represents the overall reaction. 3. Apply the law of conservation of mass and the law of conservation of energy to the system. 4. Sum up the enthalpy changes of the individual steps to find the overall enthalpy change of the reaction.

Examples

A classic example of the application of Hess's Law is the determination of the enthalpy change of the formation of water from its elements, hydrogen and oxygen. Since the direct measurement of this reaction is difficult, Hess's Law can be applied using known enthalpy changes of combustion of hydrogen and oxygen to indirectly calculate the desired value.

Limitations

While Hess's Law is a powerful tool in thermodynamics, it assumes that the enthalpy change is independent of the temperature at which the reaction takes place. However, in reality, the enthalpy of a reaction can vary with temperature. Corrections for temperature dependence can be made using Kirchhoff's law of thermochemistry.

Conclusion

Hess's Law is a fundamental principle in chemical thermodynamics that provides a method for calculating the enthalpy change of a reaction through the use of enthalpy changes of a series of intermediary reactions. Its application is crucial in fields such as chemistry, physics, and engineering, where understanding the energy changes in chemical reactions is essential.