What is Hess's Law?
Hess's Law of Constant Heat Summation, or simply Hess's Law, states that the total enthalpy change for a chemical reaction is the same regardless of the number of steps the reaction is carried out in. In other words, if a process can be expressed as the sum of several steps, the overall enthalpy change is the sum of the enthalpy changes for each individual step.
Section 2: The Core Principle of State Functions
This law works because enthalpy (H) is a state function. A state function is a property of a system that depends only on its current state (like temperature and pressure), not on the path taken to reach that state. Since the initial reactants and final products are the same, the total change in energy between them must be constant, no matter the intermediate reactions.
Section 3: A Practical Example
Consider the formation of carbon dioxide (CO₂). This can occur in one step: C(s) + O₂(g) → CO₂(g), with an enthalpy change (ΔH) of -393.5 kJ/mol. Alternatively, it can happen in two steps: 1) C(s) + ½O₂(g) → CO(g) (ΔH = -110.5 kJ/mol), and 2) CO(g) + ½O₂(g) → CO₂(g) (ΔH = -283.0 kJ/mol). Adding the enthalpy changes of the two steps (-110.5 + -283.0) gives -393.5 kJ/mol, proving Hess's Law.
Section 4: Why is Hess's Law Important?
Hess's Law is incredibly useful for calculating the enthalpy change of reactions that are difficult or impossible to measure directly in a calorimeter. By combining the known enthalpy changes of other, measurable reactions, chemists can determine the heat of formation for many compounds and predict the energy output of reactions.