April 2019 Limerick Project

Hess's Law

“A puzzle-like problem: expunction
Of like terms from reactions’ adjunction
Towards a target process…
Keep in mind: Law of Hess! 
Find solution since H is state function.”  

The April 17 limerick is the third of three limericks focused on enthalpy.  So far we’ve seen both enthalpy’s empirical implications (what does it mean for a chemical reaction run in the lab?) and its mathematical relationships.  This third limerick examines one of the most common applications of enthalpy-related concepts: a calculation called Hess’s Law. It is named after chemist Germain Hess, who was the first to publish this mathematical approach in 1840.  

“A puzzle-like problem: expunction/
Of like terms from reactions’ adjunction/

Towards a target process…/”
The essence of Hess’s Law is that if we want to know the Delta H (𝛥H, or change in enthalpy) for a reaction for which that quantity isn’t yet known, we can manipulate other, related reactions for which the Delta H values ARE known and then add those values to obtain the Delta H of the target reaction (“target process”).  

Much more detailed explanations are available, but the paragraph above gives the gist of the limerick’s wording: when we add up the manipulated, related reactions (and thus examine their “adjunction”), we end up “expunging” terms that are identical on either side of the reaction arrow until we reach the target reaction.       

I tend to present this type of problem as a puzzle when I lecture on it: we know the picture on the front of the jigsaw puzzle box (the target reaction) and we fit together the pieces (the given reactions) to get there.    

“Keep in mind: Law of Hess!/
Find solution since H is state function.”   
The last two lines reveal the name of this specific type of calculation and point to the mathematical properties of enthalpy (H) as key to facilitating this type of solution.  

Since enthalpy is a state function, all that matters is the final state of the reaction (having the correct products) and the initial state (having the correct reactants).  Once we have them, we can determine the Delta H of the target reaction. It doesn’t matter how we got to that target (that is, the order in which we put the puzzle pieces together is irrelevant), because enthalpy is a state function.