Emphasis on Stoichiometry and Structure by Rex T. Morrison
I teach both a first-year and a second-year course in Chemistrv a t West Essex Senior High School in North Caldwell, N.J..W~ receive students fromiour elementary sending districts in North Caldwell. Roseland, Essex Fells, and Fairfield, N.J. About 15% of our students go on to future education with about 60% going on to four-year colleges. I t is a good suburban high-school which offers much to those who desire much, hut like all comprehensive high schools, we have many students who desire little and achieve their desires. We cannot give our sense of values to all students; we can only teach with enthusiasm and desire to prepare students for further study and the ability to function in modern society. In a sense, my backpround is unusual in that 1,taught 6 years and earned 4 years of college credit in the first 7 years after I graduated from Stockport High School in Stockport, Iowa as the fust in a class of nineteen. We had no chemistry or biology course, but I did have a good physics course and the 2'2 yeam of mathematim offered at the high school. I went to college one year at Carthage College which was then in Carthage, Illinois, and I tmk my first chemistry course there and enjoyed it very much. 1 obtained a teacher's certificate for teaching in a rural school by examination a t the end of my first yearif college, and then heean mv teachina career teaching a t a rural school near stock&& where I hadkight students in sin grades. The two oldest had to be prepared for the County Examinations in order to enter highschool, so accountability isn't that new a concept. From there I went to teaching 7th and 8th grade students for the balance of my undergraduate days. Then after graduation from Parsons College which used to be a t Fairfield, Iowa, I started teaching junior high school science at Fort Madison, Iowa. From there I spent two years in the U.S. Navy in the Pacific Theatre emerging as a Lt. (ig) U.S.N.R. This was followed by teaching in high school science, including general science. ~~.hiolow. ".. ohvsics . . and some mathematics. I started on a Master's Degree propam in science education at Drake University and derided to take a degree in Pharmacy as a possihle alternative tu teaching, but after three years in the Drake hiversitv Colleee of Pharmacv. where I waselected Rho Chi, Omicron ~ e l g ~and a received ~ ~ a the Lehn and Fmk Gold Medal. I staved with mv first love, t e a c k . In fad, I taught compounding and ~isp&ing and ~harmaceutical Calculations for two vears at Drake while completing my Master's Degree in science Education. ~
~
~~~
Rex Morrison received a Bachelor's degree from Parsons College, and a Master's degree in science education at Drake University. His extensive teaching experience has taken him from the midwest to the east and south. For over thirteen years he has been a faculty member at West Essex Senior High School in North Caldwell, N.J. In 1974 he received the Manufacturing Chemists Association eastern regional award. ' Rex T. Morrison West . Essex -. Senior Hiah School North Caldwell. New Jersey
-
I
I
Personal PhilosoDh~ . . Because I preferred teaching basic chemistry to its applications in Pharmacy, and also for mercenary reasons, I shifted back w high-school teaching as a chemistry teacher for eight years at Roosevelt High School in Des Moines, Iowa. While there I taught an honors course in chemistry and became the evaluation teacher for Chem-Study in Iowa. I've taught a modified Chem-Study course ever since to my first-year students. From Des Moines, I moved to West Essex Senior High Schwl about 1.7 years ago. I alsosewed as laboratory director for an Advanced Placement Institute at Loyola University in New Orleans for two vears~and as a ~Liaison A.P. Teacher for ~ ~ ~ ~ a week each summer For 3 years a t the Hope College Institute for Advanced Placement Chemistry. My students have won two firsts and two seconds in the State Science Day Competition in Chemistry in New Jersey in the last twelve years as well as participating very successfully in the New Jersey Chemistry League, hut this says more for the quality of studenta i t has been my pleasure to work with than it does about my skill in teaching; although I'm sure i t contributed to my selection as an outstanding chemistry teacher as an MCA regional winner in 1974. Thus, what I'm going t o say about what I do is hased on a broad and lengthy experience from which I hope you can choose something that may be helpful to you. I'm sure that I've been more eclectic than innovative, but I have combined and chosen those ideas and experiments in mv courses that I have found worked well for me and seemed to serve my students well. I admit I'm probably more intent on helping my students who plan to use chemistry in their life work than I am in making it possible for all studentsto achieve success in Chemistry. 1just do not believe you can he all things to all people and at the same time do an excellent job for those who will need what you have to offer. I also believe that no one does his best unless asked to do more than he can do easily, and that self-discipline and good study habits should be an outcome of education. These ideas may be an anachroninm, but they have served me and my students, who have adoptd them, well, and this is, after all, only the view from my classroom. Stolchiometn-Centered lntrodudlon In my first-year course, I stress a topic which seems toembody and integrate more of the basic ideas of chemistry than any other from my point of view-i.e., the ability to perform stoichiometric calculations successfully. In the first place it involves the basic concepts of compounde composed ifatoms of elements and theshorthand description of their interactions to produce certain products under~certainconditions. The halancinr of that equation to arrive at the correct stoichio. metric r~lationshipsis a complex skill and runs the gamut from simple inspection methods to use of the oxidation stau change and ion~electronmethods of balancing, which meanc that students must realize the nature of the species involved in the interaction descrihed bv the balanced equation. Tht concepts of predominant reacting species, complete and nel ionic eauations. are all needed to he readv to do stoichiometrL ~alculakons.I* addition, the ability to'deal with quantitier of eases involved in chemical reactions means that the studenl sh&ld he ahle t o use the general gas law efficiently and ef. fectively. Volume 55, Number 4, April 1978 1 255
-
~
I
Many texts then divide such calculations into pro~ortions with a different algorithm for each type of relationship from weight-to-weight calculations, weight-ta-volume calculations, volume-to-volwe, volume-to-weigbt, etc., ad infiniturn. This seems to me to be more demanding, less effective and missing the simple elegance of a single approach to all such problems. This simple approach involves the mole method and the concepts of unit analysis, the factor-label method or dimensional analysis-or a rose by any other name is still as sweet. My approach to teaching stoichiometry hegins with an experiment in which studentn huild ball-and-stick modeh of the first six members of the alkane series, an alcohol, an ether (whichare isomers), an organic acid, ammonia, chlorine, water, hydrogen and oxygen and concludes with their demonstration of the reactions of two hydrogen molecules with an oxygen molecule to form two molecules of water and of Hz and Clz molecules to form two molecules of HCI. We point out that the oxygen bonding is not as simple as our model might suggest a t this point and that usually light dissociates the chlorine molecule into atoms which interact with hydrogen molecules to form an HC1 molecule and that the hydrogen atom left over then reacts with a chlorine molecule in a chain-wise fashion to form more HCI, but that the equation represents the quantities of hydrogen that react with chlorine to form HC1 in the overall process. This is followed by the laboratory investigation of the interaction of Cu(s) with Ag+@q) and Nos-(aq) ions of a silver nitrate solution. Thus the Idea that the state of the matter is important is introduced, and we never talk about AgN03 molecules which is only a confusing idea that is often introduced a t this point by texts trying to avoid the concept of ions in solution. Then we introduce the mole concept as a number similar to that of the familiar dozen, gross or ream. We compare moles of Cu(s) weiehed out with ihe moles of Ag(s) produced in the reaction, a n l t h e n see that this gives us the mole relations hi^ in the balanced eouation describing this interesting chemical interaction, w e also compare the moles of silver produced ta the moles of AcNO.rlsl ~J The concept "f weighed out to make the A & ~ o . J ( ~solution. a molx mass as the mass in grams uf a molr6.0'2 x 10") of the species represented in the equation is thus developed. This is followed by the concept that mass is conserved in ordinary chemical reactions within the limits of experimental error by reacting the Ag(s) produced with HN03(aq) to convert it back to AgN0&). Dissolving this salt gives a solution containing Ag+(aq) and NOs-(aq) which are reacted with Na+(aq) and C1-(aq), formed in a solution of a weighed quantity of NaCl(s), to form AgCl(s) and a solution of Na+(aq) and NOS-(aq) ions. The solid reactants' mass is compared with the solid products' to show that mass is conserved and that the atomsiresent in the reactants are also present in the products to account for this conservation of the mass of atoms w13 specific atomic masses. Then from an experiment involving Boyle's Law and Charles' Law we develop the combined law and show that PVIT = k for a specified number of moles of gas which can be broken into that number of moles times a gas constant per mole, R. We derive the same relationship from the tenets of the kinetic molecular theory t o show that P V = nRT. This leads also from the dissolving of A&) in HNOs(aq) to the first use of the ion-electron method to balance this equation. Thus the background of concepts is developed upon u,hich the mole method of solvine stoichiometric orohlems is built. For example, in the laboratory preparation of Clz(g),by the oxidation of HCl(aq) with a KMn04(aq) solution, the following procedure would he used to calculate many of the problems associated with this reaction in terms of quantities of reactants and products. We wouldfirst write the readants and products as the species existing a t the time of interaction 256 / Journal of Chemical Education
K+(aq)+ MnOd-(aq) + H+(aq)+ C1-(aq) = CMg) + Mn2+(aq) C1-(aq) + H20(l) This would then be balanced by the ion-electron half reaction method for the net ionic equation 2(5e- + 8Hf(aq) + MnOd-(aq) = Mn2+(aq)+ 4H20(l)) 5(2 CI-(aq) = CMg) + 2e-) 16 H+(aq)+ 2 MnOd-(ad + 10 CI-(aq) = 5 CMg) + 2 Mn2+(aq)+ 8 HzO(l) We make this as simple as the abc's. (a) We halance oxygen with HzO. (h) We balance H atoms with H+(aq) ions. (c) We balance charge with electrons which are always negative and the part of an atom which can move in chemical interactions. Finally, i t is pointed out that electrons can only he lost when another species pulls them off. Therefore electrons lost must equal those gained. The multiplication by 2 and 5, respectively to achieve the least common multiple makes this possible. Then the realization that MnOa- ion must come from the KMnOdaq) solution and H+(aq) and Cl-(aq) from the HCl(aq) solution leads to the complete ionic equation which serves as the basis for the stoichiometry calculation which follows
+
2 K+(aq)+ 2MnOa-(aq) + 16 Hf(aq) + 16 CI-(aq) = 5 Clz(g)+ 2 Mnz+(aq)+ 4 C1-(aq) + 2 K+(aq) + 2 C1-(aq) + 8 HzO(1) which is equivalent to the following formula equation 2 KMnOp(aq)
+ 16 HCl(aq)
= 5 CMg) + 2 MnCMaq) + 2 KCl(aq) + 8 HzO(l) As an example of the calculation used, find the volume of Clz(g) produced a t a partial pressure of 740 torr and a temperature of 27.0°C from 20.0 ml of 12.0 M HCI treated with an excess of KMnOr(aq). The algorithm used is the same as for all stoichiometry problems.
(1) Change what you are given to moles. (2) Change from moles of known to moles of desired using the mole
ratio from the balanced equation. (3) Change moles of desired to units desired by use of unit analysis. The formula equation is used and known and desired species are identified--e.g.: 2 KMnOdaq) 16 HCl(aq) = 5 CMg) (excess) (20.0 ml of 12.0 M) (at 140 torr & 27.O0C) + 2 MnClAaq) + 2 KCl(aq) + 8 HzO(1) Solving:
+
-20.0 ml X
12.0 mol HCl lo3 ml
Step 1 nRT P
step3:v=-=
5 mol Clz 16 mol HC1= 1.50 X 10-2 mol C l k ) Step 2
1.50 X 10W rnol X 62.4 -X 300 K mol K 740 tarr K
Obviously the concept of molm solutions had to be introduced also prior to such a ~roblem.hut molh follows readilv once the mol concept is mastered. Structure-CenteredFollow-up
In my second-year course, an idea which I have found useful is the combination of bonding and organic chemistrv through use of framework molecularhodels~ndrelating tke mod& built to substances they have studied in hiolow without much real understanding ofthe structures involved. This study is directed by having them read the text presentation of bonding and molecular structure followed by the treatment of organic chemistry. Since this is a second-year course, the use of other concepts in the organic chapter that have not been studied in
detail in the second-year coursemakes use of what they have retained from the first year course, constituting a review, which serves them well when they continue a more in-depth nresentation later in the second vear. Following the text ;eading, three duplicated sheets dkect their actiGities. The first explains a simple presentation of IUPAC organic nomenclature and the concepts of alkanes, alkenes, alkynes and several of the functional erouDs occurrine in most of the models to he built. They then-use this material in writing structural formulas for l-propanol, 2-propanol; pentamic acid, propanedioic acid; succinic, fumaric and maleic acids; o d i c acid, Teflon, freon, ethyne, oleic acid and methanal. The cis-trans isomerization of fumaric and maleic acids and their relationship to succinic acid is noted. The cis-9-octadecenoic acid reinforces this concept when they find that is what constitutes oleic acid. The second sheet directs them to build models following a study of the direction book which accompanies the framework molecular model kit. This eives them covalent bond radii for all the elements used, t h e i r k n der Wads' radii, color coding, cluster relationshin to the tvnes of hvhridized orbitals used in bonding the moiecules an2 'the repr&entation of sigma and ni bondini in alkanes. alkvnes and aromatic rine structures. k h u s the Tdeas of a1kanes:alkenes and alkynes presented in the first sheet are utilized and further develo~edin this exercise. The students build one or two models each night and I check them and ask questions about their use and importance as suggested by questions on the sheet directing them to build such models as this
They are asked t o find its common and Geneva name in the "Merck Index," "Chemical Rubber Handbook" or organic texts available in the room and in the library. They build models and answer questions that relate to their biological activity for amphetamine, fructose, glucose, sucrose, mandelic acid, the cis and trans forms of 2-butenoic acid, glyceryl triacetate, propanol, propanal, and propanoic acid, adenosine, 2-propanol, acetone, the Williamson synthesis of diethyl ether, and the dipeptide, glycylalanine. The questioning leads students to discover that they now understand carbohydrates
(glucose and fructose condense to sucrose), fats (long chain fatty acid homologues of glyceryl triacetate), and proteins (polypeptides). The adenosine of the ATP they studied as the energy sink of living systems in biology is now related to structure and component atoms in a way not possible when they studied it in hiology, and the preparation of the surgical anesthetic. diethvl-ether. and the central nervous stimulant. amphetamine, as an example of drug action on living organisms helps to make their chemistry and biology both more meaningful and important to living beings. The third sheet is a review of the important concepts they've been developingand utilizes the information they have studied in the text as well as in answering questions about the structure of three isomeric pentanes, the structure of the octane referred to in motor fuel ratings, the concepts of condensation and addition polymerization and the classification of the polvmer thev saw made from 1.6-diaminohexane and sebacoil chloride. ~ t e a r i cpalmitic, , myristic and lauric acid structures are requested to further fill out the concept of fat structures. ~ a c r o nthe , orange flavor, octylacetate, TNT,and the structure of the u-dichlorobenzene thev had melted in the Chem-Study course helped relate these concepts to previous and future uses of the ideas they were studying. The Krebs cycle of oxidation of foods in hody metabolism was presented in terms of actual structures of the molecules involved, and the transfer of this energy in the formation of ATP which delivers it to the muscles was noted. The nature of soan and the sanonification nrocess. and the structures of the aAd-hr.se indiators they f a d used previously round out the presemation. The models which they had constructed on a scale of 1.54 cm. = 1A had given them a concept of minute molecules maenified bv 254 million times but remarkahlv accurate as toproportions and directions of bonding and thk nature of siema and ni bonding in a much more concrete wav than I havbheenat& to achieve by other modes of tion of this mawrial. I hope these ideas will suggest -- even better ones to those who may read them. I t is obvious that courses taught in this way are demanding on hoth the student and the teacher, hut the-satisfactions are enormous, and my favorite aphorism is that you get out of life what you put into it. The more you give, the more you get, and there is no satisfaction as great as that of accomplishment. I think that the ability to choose not to accomplish in trying to meet the needs of a few of low ability may be one of the reasons for the unhappiness of students and parents with the student's education, but I trust that chemical education is not contributing to this problem.
Volume 55, Number 4, April 1978 / 257
