Squaric Acid-A Workshop for High School Chemistry Teachers H. D. Gesser, W. G. Baldwin, and G. G. Hickllng University of Manitoba. Winnipeg, ME, R3T 2N2 Canada
Communications between high school and university chemistry instructors are at best sporadic and often nonexistent. A vehicle sometimes used to improve these communications is a workshop, which can be useful to both particiants and their students. It is our view that the greatest kalue from such a workshop is obtained when the workshop has a ~racticalfocus, with participating teachers experiencing bdth an experimental challenge, such as they often put to their pupils, and an opportunity to upgrade their knowledge of modern laboratory procedures. To be a true representation of a student's challenge, it is important that any compounds presented be ones that the teachers are unlikely to have encountered previously. Recently we organized a one-day program based on the structural determination of squaric acid (3,4-dihydroxy-3~yclobutene-1,2-dione).~ The structure is interesting, containing groups that can be deduced from functional group analysis (Fig. 1).
Figwe 1. Squaric acid.
In the hope that our experience may encourage others to investigate similar forms of communication, using other compounds, we present a brief report of the workshop.
388
Journal of Chemical Education
After registration and a brief welcome, an introductory lecture on the representation and importance of chemical structures set guidelinesfor the day's activity. The 55 participants were sectioned into two laboratories and divided into teams of about five teachers. The challenge to eachteam was to identify the characteristics of, and propose a structure for, an unknown substance. Each team was given the elemental comnosition (56.1%0. 42.1% C. 1.8% H)and told that the compound decomposed above 300 O C . while no specilic lahoratory instructions were distributed, we didgivea few hints along with specific equipment and chemicals that could be found in a tvoical . . meter, con.. hieh school laboratorv-DH ductivity probe, glassware, balances, litmus paper, cyclohexme, aqueous solutions of alcoholic silver nitrate, dibromine, iron(II1) chloride, potassium iodide, and standardized (approximately 0.05 M) solutions of hydrochloric acid and sodium hydroxide. Although initially there was some confusion as the teachers floundered with no explicit procedures to follow, they soon began to think about the problem and to suggest various tests. Several teachers later confessed that this experience served to remind them of how their students must feel as thev are ex~osedto. and finallv cope with, many new ideas and techniques. ~lt&atelymuch constru&ive activity ensued. A summaw of the qualitative observations and of the inferences drawn from them, which several groups used to ~~~
' Schmidt. A. H. Synthesis lS80, 961.
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volume NaOH titrant (mL) Figure 2. pH tibation curve far squaric add. 0.122 g of squaric acid titrated with 0.0498 M sodim hydroxidesolution. Apparent molar mass calculated as 57 g mol-'.
volume KOH titrant
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Fioure 3. Conductlometrictibation curve of sauaric acid. 0.0548 a of souarb a& in 80 ml 01 w t e r tibated mm 0 0958 M pmaa urn hydroxide soknon COnd~Ctlviryis reportedasread out-this represents 1110me value in ppmTOS COnduCtivlwdata correned tar dilution Apparent molar mass calcu atm
A Summary o l the Obaervalionr and lnlerences Reported by Teachers Observation elememal composition: 56.1% 0, 42.1% C. 1.8%H the solid was soluble to a limlted extent In water but not in cyclohexane blue libnus turned red in the aque o m soiution me conductivity of me aqueous M)lution was quire high bromine in water was decolwlzed potassium permangsnate solution was decoloriz~a potassium iodide solution was not obviously changed a yellow-green precipitate formed in the alcoholic silver nibate sol!+ tion lron(lll) chloride solution turned purole
inference simplest formula: HC202
me compound is probably ionic a highly polar the compound is more acidic man water the compound is a strong elecholyie, readily producing ions the compound is unsaturated n readily oxidized the compound is unsaturateda readily oxidized no yellowing of solutlon indicates mat the compound is not an oxidizing agent (no oxidation of 1- to ).I a reaction occurs to form a silver salt a phenoliciike group is present
predict a structure for the compound, is given in the table. The quantitative titration on a weighed sample using the standardized sodium hydroxide solution was performed by several groups, using a pH meter to establish the titration curve. A sample titration curve is illustrated in Figure 2. From the titration curve the teachers determined that the apparent molar mass of the compound was 57 g mol-1. Further, they deduced from the shape of the curve that the unknown acid was neither a strong, nor a classical weak, acid. Because of the slight inflection before the equivalence point, some participants speculated that the acid was diprotic and further suggested that it could have a large fimt dissociation constant and smaller second dissociation constant. While neither a simple acid-base nor a thermometric
titration could prove this hypothesis, a condurtiometric titration can show the formula to be C*OIHI.(The thermometricand conductiometric titrations were not doneduring the workshop. However, we had done the conductiometric titration using an Omega CDH-1 conductivity probe; a typical conductivity curve is illustrated in Figure 3.) At this stage the hands-on aspect ofthe project was terminated. Over lunch speculation and interaction continued. Instruction in some modern asoects of structural determination followed. The mass spectrum, infrared absorption spectrum and NMR sDectrum of sauarir acid were distributed. and teachers were given an opportunity to examine the instrumentation used and have a staff member answer any questions. Certainly the single peak in the 13CNMR needed explanation. The IR, which showed the presence of C=O and C=C a t 1820 and 1640 cm-' and broad absorptions a t 2325 and 3450 ~ m - ' ,did ~ not indicate clearly the presence of a hydroxyl or carbonyl group commonly found for most organic acids. Overall, the message about the importance of structure to chemistry, and the idea that even simple experiments can he used to gain a great deal of information, seemed to get through in an enjoyable manner. While we will continue to talk and ~ r o v i d eresenta at ions a t teachers' ~rofessionaldevelopmeit days, ihe invitation to visit us &d work with us proves to be even more beneficial. The evaluation of the day's activity was very favorable and encourages us to plan additional workshops. We are grateful to the University of Manitoba Outreach p r o m and the Province of Manitoba Department of Education for their support and enrouragemeni. We particularly appreciate the assistance of colleagues in the Department of Chemistry for helping with the workshop. Cohen, S.; Lacher, J. R.: Park. J. D. J. Am. Chem. Soc. 1959,81,
3480.
Volume 67
Number 5
May 1990
389
