Hess's Law: The Conservation of Energy in Chemical Reactions
Analogy- Consider that you’re navigating a city’s metro system, perhaps Tokyo or London.
- To get from Station A to Station B, you have multiple route options.
- Some may involve transfers, others may be direct, but no matter which path you take, the total distance between A and B remains the same.
- This concept parallels Hess’s Law in chemistry: the enthalpy change of a reaction depends only on the starting and ending points, not the route taken.
What is Hess's Law?
Hess’s Law is an application of the law of conservation of energy, which states that energy cannot be created or destroyed: only transferred or transformed.
Definition
Hess’s Law
The total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction pathway.
- To illustrate, consider the oxidation of sulfur to sulfur trioxide:$$
S(s) + \dfrac{3}{2} O_2(g) \to SO_3(g) \quad \Delta H = -396 \, \text{kJ mol}^{-1}
$$ - This reaction can occur in two steps:
- Step 1: $$S(s) + O_2(g) \to SO_2(g) \quad \Delta H = -296 \, \text{kJ mol}^{-1}$$
- Step 2: $$SO_2(g) + \dfrac{1}{2} O_2(g) \to SO_3(g) \quad \Delta H = -100 \, \text{kJ mol}^{-1}$$
- Adding the enthalpy changes for these steps gives the same total enthalpy change as the overall reaction:
$$
\Delta H = (-296) + (-100) = -396 \, \text{kJ mol}^{-1}
$$
This is the essence of Hess’s Law: the sum of the enthalpy changes for individual steps equals the enthalpy change of the overall reaction.Note
Hess’s Law holds true because enthalpy ($H$) is a state function, meaning its value depends only on the system’s current state (reactants and products), not on the process used to reach that state.
Applying Hess's Law: Calculating Enthalpy Changes
- Hess's Law is particularly useful for calculating the enthalpy change ($\Delta H$) of reactions that are experimentally challenging to measure directly.
- By combining known enthalpy changes of related reactions, we can deduce the enthalpy change for the target reaction.
- There are two common methods for applying Hess’s Law:
- Summation of Equations Method
- Enthalpy Cycle Diagram Method
