Hess’s Law is a fundamental principle in thermodynamics that describes how the change in enthalpy of a chemical reaction is independent of the path taken from the reactants to the products. In other words, Hess’s Law states that the overall enthalpy change in a reaction is the same whether the reaction takes place in one step or through a series of intermediate steps. This principle is named after Germain Hess, a chemist who first formulated it.

Enthalpy (H) is a thermodynamic property that measures the heat absorbed or released in a chemical reaction at constant pressure. It is expressed in units of Joules per mole (J/mol). The change in enthalpy of a reaction is denoted by ΔH and is equal to the difference between the enthalpy of the products and the enthalpy of the reactants:

ΔH = H(products) – H(reactants)

Hess’s Law states that the overall enthalpy change of a reaction is independent of the reaction pathway, and depends only on the initial and final states of the reactants and products. This means that the enthalpy change of a reaction can be calculated by adding up the enthalpy changes of a series of simpler reactions that take place along a different pathway. This is known as the law of constant heat summation, which is another way of expressing Hess’s Law.

For example, let us consider the reaction that takes place when carbon reacts with oxygen to form carbon dioxide:

C(s) + O₂(g) $\to$ CO₂(g)

The enthalpy change for this reaction, ΔH, can be measured experimentally. However, we can also determine ΔH indirectly by using Hess’s Law. We can break the reaction down into two simpler reactions:

$C(s) + \frac{1}{2}{O_2}(g) \to CO(g)$ ΔH₁
$CO(g) + \frac{1}{2}{O_2}(g) \to C{O_2}(g)$ ΔH₂

The enthalpy changes for these two reactions, ΔH₁ and ΔH₂, can also be measured experimentally. By adding the enthalpy changes of these two reactions, we can obtain the overall enthalpy change of the original reaction:

ΔH = ΔH₁ + ΔH₂

This demonstrates the applicability of Hess’s Law in determining the overall enthalpy change of a reaction.

Hess’s Law is important in many areas of chemistry, including chemical engineering, combustion, and biochemistry. It enables chemists to determine the enthalpy changes of reactions that are difficult or impossible to measure experimentally. It also provides a way to predict the enthalpy changes of reactions that have not been studied before, which is useful in designing chemical processes and predicting the behavior of chemical systems.

In conclusion, Hess’s Law is a fundamental principle in thermodynamics that describes how the enthalpy change of a chemical reaction is independent of the reaction pathway. It allows chemists to calculate the overall enthalpy change of a reaction by adding up the enthalpy changes of simpler reactions that take place along a different pathway. This law is a powerful tool in understanding and predicting the behavior of chemical systems.