Hess’ Law Definition

Hess’ Law, also known as the Hess’ Law of Constant Heat Summation, postulates that the total enthalpy change of a chemical reaction is independent of the pathway or number of steps taken to achieve the reaction.
This law is named after Germain Henri Hess, who formulated it in the mid-19th century.

Application of Hess’ Law

Hess’ Law is particularly useful when it’s challenging to measure the enthalpy change (ΔH) of a reaction directly. Instead, the reaction can be broken down into smaller, simpler reactions that are easier to manage.
By adding up the enthalpy changes of these simpler reactions, we can find the enthalpy change of the overall reaction.

Enthalpy is a thermodynamic property of a system. It comprises of the system’s internal energy, plus the product of its pressure and volume.
In a reaction, if the system absorbs heat, this is called an endothermic reaction (ΔH > 0). If the system releases heat, it’s an exothermic reaction (ΔH < 0).
Hess’ law can be observed in the formation of substances; the enthalpy change for the formation of a substance is the same whether it occurs in a single step or a series of steps.

A common usage of Hess’ Law involves enthalpy of formation data. Typically from tables, these values refer to the enthalpy change when 1 mole of a compound is formed from its elements.
By using data for the enthalpy of formation of reactants and products, we can calculate the enthalpy change of a reaction.

Hess’ Law plays a key role in various scientific fields and industries, such as the creation of new materials, pharmaceutical research, energy policy decision making, and environmental science.
For example, in the chemical industry, understanding the enthalpy changes of reactions helps in choosing optimal reaction pathways that save energy or reduce heat output in the environment.