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Science Poetry

Coordinating Events

“Reactant and product, by
Way of transition state…
Progress reactive in
Graph summarized:  
Ornate coordinate,
Diagrammatical;
Relative energies,
Here analyzed.”  

To my chagrin, I realized last week that I had accidentally set two essays to post at the exact same time!  That means that I am suddenly a bit less ahead of the writing process here than I like to be.  However, I also cannot help but find it fitting that this misstep happened in the midst of a set of poems devoted to kinetics, given how concepts of timing and (at the molecular level!) collisions are so pertinent to this theme.   

In any event, the next Twitter poem was posted on 21 October 2021, and it described the concept of a reaction energy diagram; this is an efficient way for chemists to communicate information related to both the thermodynamics and kinetics of a chemical reaction of interest.      

“Reactant and product, by /
Way of transition state… /
Progress reactive in /
Graph summarized…”

A reaction energy diagram is a graphical depiction of the relative energies of the distinct species involved in a chemical mechanism.  I’ve highlighted such chemical communication here before, noting another common title of potential energy surface (when such a PES is considered in two dimensions).  

The reaction energy diagram described here cites “reactant and product, by way of transition state.”  This essentially will look like a hill, with the transition state at the peak in the middle.  The height of the hill is called the activation energy, or activation barrier, of the chemical reaction: the higher it is, the greater the barrier that must be overcome, and the longer this process takes.  These concepts are related through the Arrhenius equation, which states:

k  = A * e [-Ea/(RT)]

The rate constant (k) depends on a “pre-exponential factor” (A), which can be dissected into information about the mechanism, multiplied by an exponential term in which the activation energy (represented here as Ea), the gas constant (R), and the temperature (T) are all involved.  

Equations can look complex, but again, the two big ideas communicated here are that: first, the greater the activation energy is, the longer the reaction will take; second, the lower the temperature is, the longer the reaction will take.  The former concept is discussed by this poem.                

“Ornate coordinate, /
Diagrammatical; /
Relative energies, /
Here analyzed.”  

The last lines acknowledge that a reaction energy diagram is sometimes called a reaction coordinate.  This “ornate coordinate” is a diagram that provides an efficient analysis of the relative energies of the reactant, transition state, and product of a chemical reaction.