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April 2019 Limerick Project

Potential Energy Surfaces

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

“Endo/exoergicity/
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.    

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April 2019 Limerick Project

Gas Laws

“The gas laws according to Boyle,
Avogadro, and Charles embroil
P, V, n, and T– traits
Of the phase that inflates–
In equations o’er which students toil.”

I will backtrack a day with this entry, as I inadvertently skipped posting my essay on the 8 April 2019 limerick!  This poem addresses another common introductory chemistry topic: the phases of matter. General Chemistry courses typically examine properties of solids, liquids, and gases. This poem references the development of some key equations via which the gas phase, specifically, is described.         

“The gas laws according to Boyle,/
Avogadro, and Charles embroil/
P, V, n, and T– traits/ Of the phase that inflates–”

As a student, I found the names associated with introductory chemistry to often be intriguingly distracting: the history of chemistry is mostly confined to sidebars in General Chemistry textbooks, but the stories are fascinating.  

Several equations are named for scientists who worked on examining how physical properties of a gas sample are related; these are (Robert) Boyle’s Law, (Jacques) Charles’s Law, and (Amedeo) Avogadro’s Law.  The spark for this particular poem was a variation on the rules-of-cards phrase “according to Hoyle” with respect to Boyle’s name, specifically.           

Boyle’s Law states that as the pressure on a gas sample increases, its volume decreases, if amount and temperature are held constant.  Charles’s Law states that as the temperature on a gas sample increases, its volume increases, if pressure and amount are held constant.  Avogadro’s Law states that as the amount of a gas increases, its volume increases, if pressure and temperature are held constant.  

Pressure is represented with the variable P; volume, with V; amount, with n; and temperature, with T.  When linked together (“embroiled”), they comprehensively describe the properties of a gas, periphrastically described here as “the phase that inflates.” 

“In equations o’er which students toil.”  
The named laws listed above are typically combined into the Ideal Gas Law: pV = nRT, an equation that quantitatively (exactly) relates all four of a gas sample’s variables via the gas constant R.  

The last line of the limerick is simply a rueful acknowledgement that, despite the elegance of any equations involved, truly learning chemistry– or any discipline– is difficult work!      

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April 2019 Limerick Project

Organic Chemistry

“ The topics in classrooms organic
Can sometimes seem roadblocks titanic.
My advice?  Simply, I’d
Heed the Hitchhiker’s Guide…
First and foremost, remember: ‘Don’t panic.’ ”   

The April 9 limerick takes a detour from General Chemistry learning objectives, taking an overall look at the traditional second-year chemistry courses through the lens of a science fiction classic.

“The topics in classrooms organic/
Can sometimes seem roadblocks titanic.”
Organic Chemistry 1 and 2 together constitute an undergraduate course sequence that can inspire significant foreboding.  “O-Chem” often is the last chemistry coursework a non-major has to complete; it is a common hurdle across many pre-professional curricula (pre-med, pre-dentistry, pre-vet, etc.); it involves the mastery of a tremendous amount of material in a two-semester lecture course; it often involves a significant lab component.  All of these facts can loom large to a student, as can the substance of the course material itself!

“My advice?  Simply, I’d/ Heed the Hitchhiker’s Guide…/
First and foremost, remember: ‘Don’t panic.’ ”   
From my own experience, I can say that Organic Chemistry can be a fun and inspiring challenge.  Where General Chemistry involves a broad range of topics; Organic Chemistry is more consistent: it’s highly visual, involving an emphasis on spatial reasoning, and it involves mastering some key techniques and rules, then applying them to several types of interesting molecules and reactions.  However, most students enter the classroom already aware of the challenges discussed above and thus more than a bit apprehensive.  

I’ve thus always thought the textbooks would benefit from a treatment similar to The Hitchhiker’s Guide to the Galaxy, the central text from Douglas Adams’s 1979 novel of the same name.  Adams describes the interstellar encyclopedia: “It looked insanely complicated, and this was one of the reasons why the snug plastic cover it fitted into had the words DON’T PANIC printed on it in large friendly letters.” 

Most textbooks have molecular structures or chemistry-related pictures on the front; while certainly not intentionally alienating, they are not as reassuring as Adams’s famous motto!     

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April 2019 Limerick Project

Redox Reactions

“The ‘oil rig,’ a helpful mnemonic
For redox’s challenges chronic.
Mind errors, potential.
This note is essential:
View of loss/gain must be electron-ic.”

The 7 April 2019 limerick addresses another reaction classification topic, this time looking at “reduction-oxidation” chemistry. Reduction and oxidation are themselves names that correspond to specific processes; they always happen in tandem, so the chemical shorthand becomes “Red-Ox,” or “redox.” Redox is a term that can apply to a wide range of subclasses of reactions; combustion (from the 6 April 2019 limerick) is one of these.  

In particular, the poem clarifies the use of a common memory trick for describing redox processes. The discussion focuses on the most obvious type of redox reaction, a displacement reaction, to keep the discussion as straightforward as possible.

“The “oil rig,” a helpful mnemonic/
For redox’s challenges chronic.”
Redox reactions involve electron movement.  Because electrons are negatively charged, the elements to which and from which electrons flow experience a change in their own charges over the course of the reaction.  These can be challenging reactions to consider, as redox concepts can manifest themselves in several ways.    

In the simplified reaction below, the notation used for the reactants (left of the arrow) shows us that Element A starts out as a neutral metal and Element B starts out with a positive charge, in their reactant forms.  In their product forms (right of the arrow), Element A has a positive charge and Element B is a neutral metal. 

This is also called a displacement reaction because A “displaces” B in terms of forming a compound with C.  

A + BC → B + AC   

The “mnemonic” in question links the movement of electrons to the chemical vocabulary: “Oxidation Is Loss; Reduction Is Gain.”  This statement is abbreviated as “OIL RIG.”

In the reaction above, Element A is oxidized, losing electrons to go from neutral to positively charged; Element B is reduced; gaining electrons to go from positively charged to neutral.   

“Mind errors, potential./ This note is essential:/
View of loss/gain must be electron-ic.”
A common error with these reactions is viewing “loss” and “gain” in terms of the values of the charges on the elements, neglecting the fact that electrons are negatively charged.  (In the example above, the ERROR would be saying: A’s charge becomes more positive; thus, it “gains”; thus, it is reduced.)

The application of the “oil rig” mnemonic relies on considering loss/gain in terms of electrons.  

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April 2019 Limerick Project

Combustion Reactions

“A process denoted combustion
Results in methodic production:
H2O, CO2;
Common products ensue
From a fuel hydrocarbon’s consumption.”

As with the April 5 limerick, the 6 April 2019 limerick addresses another reaction class and how to easily identify it. This specific poem examines combustion reactions and the chemical formulas used to represent specific compounds involved therein.    

“A process denoted combustion/ Results in methodic production:”
This limerick outlines the class of reaction of interest, pointing out that we’ll be able to classify a reaction as a combustion reaction by looking at its characteristic reactants and products (its “methodic production”). The remainder of the poem defines these species more directly.      

“H2O, CO2;/ Common products ensue/
From a fuel hydrocarbon’s consumption.”  
This is the first limerick in my project to exploit chemical notation to obey the rhythmic rules of the poetic form! For the syllables to work here, the third line is read as “H two O, C O two.”  These abbreviations have specific meanings for chemists.

Notably, “H2O” and “CO2” are not formatted correctly, due to Twitter constraints (or at least my lack of knowledge of how to format subscripts and superscripts via that medium!); they should be properly written as H2O and CO2.  These are the chemical formulas for water and carbon dioxide, respectively. The formula for water tells us that each H2O molecule contains two hydrogen atoms and one oxygen atom; the formula for carbon dioxide tells us that each CO2 molecule contains one carbon atom and two oxygen atoms. 

Water and carbon dioxide are the characteristic products when a hydrocarbon fuel [a molecule consisting only of carbon and hydrogen, such as butane (C4H10) or propane (C3H8)] reacts with oxygen to undergo combustion.        

The overall pattern can be seen in the balanced reaction shown below, which represents the complete combustion of propane. 
C3H8 + 5 O2 →  3 CO2 + 4 H2O

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April 2019 Limerick Project

Precipitation Reactions

“Reactions with solid formation,
We classify precipitation:
Mix solutions (aq),
And the (s) formed anew
Will crash out to observer’s elation.”

The next few limericks address specific classes of chemical reactions and how to identify and interpret them: again, a common theme of many General Chemistry courses.  The first, from 5 April 2019, is a reaction type that figures heavily in both introductory chem courses and my interdisciplinary course, Chemistry in Art.   

“Reactions with solid formation,/ We classify precipitation:” 
Much like balancing reactions, another intro-level skill is identifying types of reactions; chemical reactions often have tell-tale reactants or products that allow their classification.  Reactions in different classes follow set patterns, so once we’ve done our classification, we can explore more interesting aspects of the pertinent chemistry.

For instance, a precipitation reaction involves the formation of a solid product called a precipitate; this product “falls” out of solution (parallelling the everyday definition of precipitation).  

“Mix solutions (aq),/ And the (s) formed anew/
Will crash out to observer’s elation.”
To identify a precipitation reaction, we look for a process with two identifying characteristics.  First, the reactants are aqueous solutions (compounds dissolved in water); they are designated as such by the (aq) abbreviation after their chemical formulas.  Second, one product is a solid, which is designated by the (s) abbreviation after its chemical formula. The final line of the poem notes that precipitation reactions are fun to watch, as the solid product “crashes out” of the solution.

Here’s a sample reaction, in which aqueous solutions of potassium chloride (KCl) and silver nitrate (AgNO3) yield a precipitate of silver chloride (AgCl) and a side product of aqueous potassium nitrate (KNO3); we can see the pattern described in lines 3-5 of this limerick:  
KCl (aq) + AgNO3 (aq) → AgCl (s) + KNO3 (aq)   

Precipitation reactions have implications for the interdisciplinary overlap of chemistry and art.  Silver chloride itself is light-sensitive and participates in reactions associated with black-and-white photography.  Some solid precipitates formed in other precipitation reactions are brightly colored and can be used as pigments in mixing and using paints.       

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April 2019 Limerick Project

Stoichiometry

“The task of calcs stoichiometric
Is central to chem’s dialectic.
For reactions, find yield
When this knowledge you wield,
As you monitor products eclectic.”

Another teaching-centered limerick here on Day 4 of this April 2019 project… this one seeks to answer the questions: Why do the concepts of balancing reactions loom so large in so many former chem students’ minds?  Why do we spend so much time balancing reactions in the first place? (What are the disciplinary applications of that skill?)      

“The task of calcs stoichiometric/
Is central to Chem’s dialectic.”
Once a reaction is balanced, we can do a variety of interesting calculations with it; this is a central theme of introductory chemistry.  Learning how to manipulate and use a balanced chemical reaction is the substance of stoichiometry, a word that comes from the Greek for “element” plus “measure.”  A balanced reaction gives rise to the proportions of the reactants and products involved.    

The 1991 version of Father of the Bride, with Steve Martin’s grocery store tantrum, indirectly provides an introduction to these concepts!  In this scene, Martin’s character George has a breakdown when he cannot buy hot dogs and hot dog buns in the same quantities; he dismantles packages of the latter to achieve equivalent amounts of the two. 

Here, George’s “balanced reaction” is:
1 Hot Dog + 1 Bun → 1 Hot-Dog-In-Bun.

He erupts when he cannot purchase his “reagents” (ingredients) in the necessary “stoichiometric ratio” (here, one-to-one).     

“For reactions, find yield/ When this knowledge you wield/
As you monitor products eclectic.”  

One oft-taught application of stoichiometry is predicting the yield of a chemical reaction: how much of a desired product can we obtain, given the starting amounts?  (To return to the cinematic scene cited above, since George has eight hot dogs and twelve buns, his maximum “yield” would be eight hot-dogs-in-buns… to his obvious frustration.)  If a balanced reaction has multiple reactants and/or products, we can apply stoichiometric principles to any of them.

This opens the door to many valuable calculations: the combination of a balanced reaction and an understanding of the periodic table is a particularly powerful tool to “wield.”

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April 2019 Limerick Project

Balancing Reactions

“To balance a given reaction,
Complete an established transaction.
‘Cross the arrow you must
Coefficients adjust:
Conserve mass and ensure satisfaction.”

Many of the April 2019 limericks were written with a potential teaching objective in mind, and the April 3 limerick is one of them.  As I’ve taught the same topics more often, I’ve started to hear more of a rhythm and rhyme when I do so.  

“To balance a given reaction,/
Complete an established transaction.”  
I have had occasion to collaborate with generous teaching colleagues outside of chemistry in the past decade, and thus to benefit from their expertise while hearing their external perspectives on my discipline.  One of the most interesting themes we’ve discussed is that, for non-specialists, learning to “balance reactions” is often one of the most vivid memories of their high school chemistry courses. Balancing a reaction means confirming that the reaction has equal numbers of elements as reactants (on the left side of the reaction arrow) and products (on the right side).

In building on this conversation, it has been illustrative to contrast the skills of chemistry with the discipline of chemistry itself.  In my experience, it’s rare that someone enjoys learning the concrete skill of balancing reactions, but mastering that skill opens the door to many interesting disciplinary applications. The introductory experience is similar to learning piano scales: the skill can open many wonderful doors, but the skill itself (at least for this once-aspiring pianist!) isn’t necessarily the fun part.       

“‘Cross the arrow you must/ Coefficients adjust:/
Conserve mass and ensure satisfaction.”  

Chief among the rules in balancing reaction equations: the only numbers that can change to achieve said balance are the numbers in front of each chemical formula– the coefficients– rather than the subscripts on the chemical formulas themselves.  (Changing the subscripts changes the identity of the chemical species.) 

Further, it’s easy to overlook as one is mastering the skill, but a balanced reaction is an elegant contextualization of the law of conservation of mass; a chemical reaction does not create or destroy matter: it simply rearranges it. 

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April 2019 Limerick Project

Atomic Structure

“Gold foil and a question cerebral
Caused the plum pudding model’s upheaval.
Protons, neutrons— contents
Of the nucleus dense—
Proved a fly in the atom’s cathedral.”

The April 2 limerick summarizes Ernest Rutherford’s “gold-foil experiment,” completed in the early 1900s at the University of Manchester.  The Rutherford Group’s experiment was a crucial step towards the modern understanding of how positively charged protons, neutral neutrons, and negatively charged electrons are arranged in the structure of an atom.  (This has always been one of my favorite “science stories,” especially due to the poetic language Rutherford and his colleagues employed in recounting the experiment.)   

“Gold foil and a question cerebral/ Caused the plum pudding model’s upheaval.” 
Prior to the gold-foil experiment, one theory of atomic structure was called the “plum pudding” model: an atom was predicted to exist with negatively charged electrons scattered through the uniform, positively-charged volume of the atom, as raisins are scattered throughout a plum pudding.  

Ernest Rutherford’s research group investigated atomic structure further by devising an experiment: shooting particles through a thin piece of gold foil, then examining where these particles landed.  If the “plum pudding” model were accurate, the particles would travel directly through the foil, essentially in a straight line. However, the data defied this prediction: some particles were deflected sharply, at random angles, from running into something dense within the atoms!  These data rendered the plum pudding model obsolete.      

“Protons, neutrons– contents/ Of the nucleus dense–/
Proved a fly in the atom’s cathedral.”  

Rutherford described the finding:
“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

He and his group ultimately determined that the “something” that some particles were hitting was the atom’s nucleus, a minute-but-massive volume wherein the atom’s protons and neutrons were gathered.  In Rutherford’s nuclear model of the atom, the dense nucleus accounts for nearly all of the mass of the atom, as well as all of the atom’s positive charge; the negatively charged electrons surround the tiny nucleus in a cloud of mostly empty space. Rutherford characterized the size of the nucleus compared to the atom as “a gnat in Royal Albert Hall”; others pursuing similar investigations restated this metaphor as “a fly in the cathedral.”

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April 2019 Limerick Project

Periodic Law

“The table we call periodic
Took Chem from a set anecdotic
To an orderly art
In which elements chart
Their behaviors and traits episodic.”

 My first poem for the April 2019 project focused on the Periodic Table of the Elements (PTE).  2019 was the International Year of the Periodic Table, marking 150 years since Dmitri Mendeleev’s publication of the first version of the modern periodic table in 1869.  The April 1 limerick highlights why the PTE is central to chemistry as a discipline. 

As discussed in the previous entry, in these brief discussions, I will attempt to provide the minimum context for a poem to make sense to a general audience, within a 280-word count (starting with the first line of the poem itself!). These brief essays are not intended to be exhaustive: any General Chemistry textbook would have far more detail, as well as more precise language. Some pertinent links are provided below to resources with more comprehensive explanations.

“The table we call periodic/ Took Chem from a set anecdotic/ To an orderly art”
Prior to the development of periodic law in the late 1800s, many chemical elements had been studied, but the data regarding these individual elements were relatively random: more anecdotal than systematic.  While different stories recount Mendeleev’s motivation differently, one common theme is that he had recently begun work as a professor, and he was interested in organizing the disciplinary information of chemistry more clearly for his students.  His version of the PTE presented information about chemical elements in a comprehensive, logical manner. (Additionally, the third line nods towards Mendeleev’s work on the visually distinctive table that adorns science classroom walls everywhere: perhaps STEM’s most universal artwork!)       

“In which elements chart/ Their behaviors and traits episodic.” 
In the modern PTE, elements are arranged by atomic number (the number of protons of an atom of each element) into a set of rows and columns.  Each row is called a period; elements in increasingly higher-number periods have increasingly higher atomic numbers and atomic weights. Each column is referred to as a group or family; within each family, elements have similar physical and chemical properties. Thus, overall, the elements’ behaviors repeat predictably, or episodically.  This repetition facilitated the construction of the PTE in the first place, and it allowed for Mendeleev’s prediction of “still-to-be-discovered” elements that would be isolated in the years past 1869, bolstering the PTE’s popularity through its predictive capability.     

The story of how the PTE was organized is compelling, involving far more than one scientist and deserving far more than 280 words. I’ll return to this topic, though, which lets me keep this initial discussion perfunctory.