applications and crnaloqies Pictorial Analogies VIII: Types of Formulas and Structural Isomers
edited by RONDELORENZO Middle Georgia College Cochran. GA31014
Formulas and Class Composition 1. Empirical Formula - Simplest Ratio
John J. Fortman Wright State University Dayton, OH 45435
2 males for every 1 female
Although students in general chemistry learn to calculate empirical and molecular formulas, they o h n do not appreciate the relationship between the two, and the still further information given in a structural formula. The empirical formula gives the simplest whole number ratio of atoms in a compound just a s one might know the simplest ratios of gears to springs in a machine (Fig. 1) or of males to females in a classroom (Fig. 2). The molecular formula tells the actual number of atoms of each element in one molecule, analogous to the actual numbers of springs and gears in a machine (Fig. 1) or the actual numbers of males and females in the classroom (Fig. 2). Finally, the structural formula gives the arrangement of the atoms in a molecule, which can be compared to the blueprint of a machine (Fig. 1)or the seating arrangement for the class (Fig. 2). Structural formulas are needed to show the difference in structural isomers. An analogy to this can be demonstrated in the class by having three students assist you. First, the four of you can all join hands to f o m a "straight" chain isomer of butane. I t is good to show a t that time that Part of a presentation at the 195th National ACS Meeting and the 3rd Chemical Congress of North America. Toronto, Canada, June 9, 1988.
Types of Formulas
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1. Empirical Formula Simplest Ratio 3 Gears for every 2 Springs in a Machine
2. Molecular Formula-Actual Numbers 80 males and 40 females
3. Structural Formula - Seating Arrangement
Figure 2. Analogy of class composition to types of formulas. the "straight" chain actually can tie twisted around in many different ways. What makes it unique is the order of connection. Then let loose ofstudent's hand that you are holding, analogous to breaking a bond, and get hold of a foot of the middle student, analogous to making a bond to form a different bonding pattern. I t is obvious that no amount of twisting around by the "straight" chain of four can produce this branched isobutane structure. Figure 3 illustrates these structures and may be shown to the class to summarize the results after doing this activity or be used in place of the activity if you or your students are not comfortable acting it out. Full page copies of these figures will be sent on request to those who wish to make overhead transparencies by photocopier or themofax. Acknowledgment The illustrations are thanks to l3ruce Stivcr of the WSL Media Services and funds from the chemistry department.
2. Molecular Formula-Actual Numbers 24 Gears and 16 Springs in one Machine
Structural Isomers
3. Structural Formula- Blueprint Shows actual arrangement of parts
Figure 1. Analogy to types of formulas.
Figure 3. Analogy to structural isomers of butane. Volume 70 Number 9 Seotember 1993
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