April 2019 Limerick Project

Potential Energy Surfaces

“An energy surface, potential,
For reaction info: essential. 
Is shown in simplicity
Through diagram self-referential.”

The April 10 limerick summarizes a common visual shorthand used in several chemistry applications: a potential energy surface.  Potential energy surfaces are graphical representations that show the energy of a chemical compound or reaction as a function of some independent variable.  Different types of surfaces can be drawn for different chemistry-related scenarios. Some can be quite complex, but the surface described in this limerick is a simple two-dimensional graph.     

“An energy surface, potential,/
For reaction info: essential.” 

For a chemical reaction, as described in this limerick, the potential energy surface is a two-dimensional graph that represents the energy involved as a function of “reaction coordinate,” which can generally be interpreted as “reaction progress.”  Organic chemists often use these diagrams in representing chemical mechanisms (step-by-step depictions of how molecules react) to show how molecules encounter one another and absorb or release energy over the course of a reaction step. A potential energy surface is a reliable, essential tool for communicating basic information about a chemical reaction.

Is shown in simplicity/
Through diagram self-referential.”

An endoergic reaction (also called an endergonic reaction) is a reaction that requires an input of energy (as heat, light, etc.) to proceed.  It would be shown on a potential energy diagram with the reactants at lower energy than the products.  An exoergic reaction (also called an exergonic reaction) is a reaction that releases energy as it proceeds.  It would be shown on a potential energy surface with the reactants at higher energy than the products. 

In both cases, one would interpret the endoergicity or exoergicity of the reaction by looking at the placement of these products and reactants relative to one another: the energy diagram is self-referential. Furthermore, once a reader is fluent in the diagrams’ conventions and terminology, the potential energy surfaces are visually rich and relatively simple, compared to many other ways of presenting quantitative data.