Tag: teaching

“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

“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.