Does the Path of Reaction Matter?
This post will introduce the idea of Hess’s law and reaction pathways for Prelim Chemistry.
Topic: Enthalpy and Hess’s Law
- Investigate Hess’s Law in quantifying the enthalpy change for a stepped reaction using standard enthalpy change data and bond energy data, for example:
- Carbon reacting with oxygen to form carbon dioxide via carbon monoxide
Hess’s Law
If a reaction A + B -> C took the pathway D + E -> B and then A + B -> C, would the total enthalpy of the reaction change? No! The reaction pathway does not impact the total enthalpy of the reaction. That’s like travelling through 5 roundabouts and 3 freeways to get to a destination that is simply next door (this analogy is slightly exaggerated), the destination does not change and nor does the distance between your initial and final locations. So likewise, regardless of the reaction pathway of a reaction (the steps that are taken to get to the reaction), the enthalpy will remain the same.
This is conveyed by Hess’s Law which quantitatively shows us that the total enthalpy of a reaction will equal to the sum of all of the reaction step enthalpies in stoichiometric ratio.
This video goes through Hess’s Law in a bit more detail.
This video has several example problems that may benefit your understanding.
