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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.”

Categories
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.