I
Fred Basolo Northwestern University Evanston, IL 60201
Robert W. Parry University of Utah Salt Lake City, UT 841 12
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An Approach to Teaching Systematic Inorganic Reaction Chemistry in Beginning Chemistry Courses
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Chemistm as taueht in (American), hieh schools and colleees has had an interesting history. Initially chemical processes occupied the time of students and practicing chemists. Since theory was rudimentary, people carried out reactions and made correlations. Periodic relationships evolved. The correlations saved much burden on the memory, hut some details were lost. With the advent of electronic models of the atom, periodic relationships began to he less important in chemical pedagogy and atomic theory became the correlating roncept. 'Chose details of chemistry which did not follow directly and obvit,usly from the rlertrunic model of the atom were banished from the classroom or delayed till later courses. Most courses introduced atomic structure very early in the year, and it was knuwingly assumed that once the student ~tnderstoodthe origin of the I.yman, Paschen, and Bnlmer seri1.s lines in the swctrum of hvdn)een, [he rwt of'chemistrv was a &mil i;ho$e reactions and pr&esses which supfollowed niceiy. ported model were presented; those which did not uwere reserved for higher courses,,, The of the oxygen mofecule was gleefully presented to lower levehtudents, h u t the fact that magnesium metal hurned in oxygen to give MgO while sodium hurned to give Na202 and potnsrium burned to give KO2 frequently was not mmtioned. Most of us knew from years of indoctrination that an atom with a nohle gaseloctronk configuration was much toostable to have any chemistry. This idyllic scene was brilliantly overturned when Neil Bnrtlett exploded the nohle g n myth in 1962. ALw the l'.S. was rudely jarrcd when theSoviet Union olared Soutnik into orbit in 1958. For the first time in its reLent history the U S . became seriously concerned about its ranking in science and about the ways in which science was taught. Various governmentally sponsored programs in chemistry, physia, mathematics, and biology were formulated by teams of experts. The CHEM Study program was one of those Droerams which was destined to have a areat i m ~ a con t the t ~ c h & of chemistry in Amerim Its hist&y is re&ling. When the first team of experts developed a proposed outline for the courJe, it was natural to prt:sent electronic srrurture first. Apprupriate reactinn details were then suggested fur the course. An outline of such a proposed program was submitted in the planning stage M Prnfeswr Kenneth Pitzer of the IJni\,ersity of California-Berkeley for review. His brilliant
772 1 Journal of Chemical Education
insight was to remake the outline and define the philosophy of the CHEM Study program. He wrote (in part) on April 20,
1960 I was very much interested to see the tentative detailed outline for the high school course as prepared at the preliminary contributors meeting April 2-5, 1960. While, I noted many interesting and good individual ideas and realize that many other features might well be improved upon more thorough consideration, nevertheless I find myself so seriously in disagreement with the general philosophy which seems tounderlie part of the outline that I want to indicate this diffrrrnrr in opinivn very clrarly at this time. I'wsil,ly I can best illustrate my pmnr by first stating somegmeral nhiloiwhv. I brlievr vrrv stronelv thnt chernistrv is an cxoerlmenral scienceand should be sdnresenied to the stud&. ~t theieme time. . ~. chemistry rs not merely nn accumulatiun of experimental facts hut has n very general and beautifulstructure which also should bepresented ns soon as pmsible even if all oithr experimental ha% is not yet available to the student. Some compromise is required between one policy in which one would accumulate the complete experimental basis for a given generalization before presenting the theory, and the other extreme in which the theory is presented first as authority "from high3,and the experimentalhasis is later provided to whatever ,,tentis feasible. Even in rhr area of eaprriment one must distinwish berwem those cxpt.rimenrs which are nrcrsvible to the student ar rhis stare or his work and experimentswhich are so complex or whose interpretation involves such advanced theory that they are not accessible to the student at this stage. The student will readily accept the idea that many other experiments, similar to the ones he does in the laboratory, have been performed and the results are as stated. If he doubts any of these results.. it is . ~ossihlefar him to eo " into the laboratorv and reneat the exneriment itself. Thus it is hv nomeansnecessarvtode~~
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student could do in his laboratory and those which are possible only at a much more sophisticated level. Again a compromise is needed since some of these inaccessible experiments are nevertheless so dramatically important in establishing atomic theory, molecular structure, etc., that they should he included even though not in the accessible cateeorv. .. , Theories are hkewise suhjeet to some classification uf this same typr.'l'hrre are those theories which may hc developed rrgorously in terms of mathematin availahle to the high srl~oolstudenr, and there are also theories which can only be developed and understood at a much more ~ophisticatedmathematical level and therefore can be presented to the high school student only by authority.
Our tentative outline as pro@ April 5,1960, contains in the first semester's work three general sections.The first one, "Introduction," seems to me quite satisfactory. The next two sections, "Atomic Structure and the Periodic Table" and "Molecules and Chemical Bonds," are based almost entirely upon this type of experimentally inaccessible material and also to a very large degree upon theoretically inaccessible material. I fail to see any significant number of high school chemistry experiments which can be directly tied to this material, and I would he very reluctant to see practically the entire first semester pass before a chemical reaction is related to the main thread of classroom instruction. Note that the first section which does associate experiments quite satisfactorily does not primarily involve chemical changes. I would recommend that the outline he altered by moving into position 2 some real experimental chemical reactions and the study of this basic chemical reaction information for a somewhat larger variety of the most common elements and compounds. This should include the elements of quantitative stoiehiometry,atomic weights, etc.
recent years may well he delayed until later courses, and that some systematic reaction chemistry can be used as vehicle to illustrate in an interesting fashion how these concepts permeate chemistry. One sometimes hears the comment that "we cannot exoect students to memorize hundreds of specific reactions." 'We believe that most things in excess including memorization of reactions can be bad, hut we also believe that memorization of reaction types and of systematizing trends are no worse than memorization of equations such as mvap
log1oP1- bloP1 = - (2,30)(1.99) xo-- kt log10 -
x
[A
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1
2.30
AGO = -2.30RT logto K We appeal for balance. We feel that modern students have been shortchanged in that they have rarely been exposed to examples of systematic reaction chemistry, but have been given excessive amounts of detailed baby physiral chemistry. We think it is best to wait and teach most physical chemistry correctly in advanced courses, after the students know some experimental facts. While the periodic table is seen on the wall of nearly every chemistrv cl&sroom. few teachers present even a fraction of the fascinating information which it implies. This is true even though the information is nnt hard to master. Some of the things which a student will find useful inrlude the names and svmhols of selected elements. the rlassification of elements into main group elements, transition metals, inner transition metals. and noble eases, the maximum and minimum oxidation n"mhers of thk main group elements, and some general trends in the chemical properties of the main group and transition elements. The latter goal can usually be developed through a systematic examination of reaction types. Although many schemes for classifying reactions are available we find an old and time-tested systematization1 useful. Others may prefer their own scheme. Reactions can be listed as:
From these logical and brilliantly stated ideas emerged the CHEM Studv motto "Chemistry-An Experimental Science." In the development of the now newly formulated CHEMS program, teams of chemists with strong physical chemistry backgrounds developed chemistry as an experimental science. I t was not surprising that the resulting program was strong on ohysical-chemical principles and light on systematic react&chemistry. Aft&Professor Pitzer's comment it was also no surprise that concepts of atomic structure were used as a tool, not as a revealed truth from which all else flowed. His view was clearly and expertly incorporated into the finished oroeram. The emohasis on chemistrv as an experimental science was the re& pedagogical novelty. 1f indeed, this point is acceoted. it should he possible to present an experimental rourse'hased on system&ir reaction chemistry irom which rhemical principles ran i ~ eevolved and expanded. In our judgment this type of course can wplll~cmore trartable and interesting l'ur wrtain groups of students and tearhem. Some tearhers have indeed expressed the opinion that systematic I. Combination 11. Decomposition reaction chemistry should be returned to our beginning 111. Replacement ehemistrv This ~~- ~courses: ~ ~ ~ was ~ the messaee of a panel session IV. Metathesis entitled " ~ e s c r i ~ t i vChemistry, e ~hen,-whereand How," in V. Neutralization which one of us (FB) participated during the summer of 1977. The meeting was sponsored by the New England Association The student will soon learn that each group contains thouof Chemistrv teachers. Similarlv, an international meeting in sands of reactions and examples can be chosen to illustrate which we bdth participated wh$h was arranged a t ~ c ~ a & r many subgroupings in each category. Let us consider the Universitv in Canada hv Professors Ronald Gillesoie and possibilities and start with the category listed as I. CombiDavid ~ i m p h r e yexpressed support for this view. nation. This mouoine mav be subdivided into (1) elements Suooort for the idea is one thine. How the task is carried plus oxygen, (2) elements plus other non-metal's,(3) oxides out iisomethiig quite different. In this paper we will consider plus water, (4) metal oxides plus non-metal oxides. Examples some concrete examples for the presentation of "systematic are easy to find and may he selected to illustrate important descriptive inorganic chemistry." By descriptiue chemistry trends in the periodic table. Consider the Combination catewe mean chemical reactions, syntheses, and commercial gory in more detail. processes. It is the chemistry that students can often see, hear, I. Combination and/or smell in lecture demonstrations or laboratory experiments. I t is the chemistry that students may remember all of 'Elementsplus Oxygen their lives. I t is the explosion which results when a balloon This is a useful sub-category, since oxygen is involved in filled with oxveen and hvdroeen ienites. It is the ouneent odor . many chemical processes. of h u r n i n g s i k u , the p&k color which appears when Milk of Metals with Oxygen. The reaction of Mg with 0 2 is easily Maenesia is added to an acid solution containina phenoland dramatically demonstrated and is widely used in photoph&alein. In our view, i t will frequently he the Ehemistry graphic flash bulbs. The reaction of iron with 0 2 (even though which fascinates the observer. We are not suggesting a return complex) is familiar to any student who has watched metal to the chemistry of 50 years ago when isolated facts were ohiects corrode into scrao. Further. the resistance of P t and presented along with the occurrence, preparation, reactions, Au to corrosion explains their use in jewelry and scientific A d uses of eac6 of several elements. &her we are suggesting instruments. Gold-olatine makes sense. The followine- reacthat a study of reaction types and the periodic table may help tions can then he written and demonstrated as illustrative about what chemical the student to make int&igent process or processes will occur in a given system. We are not iuggestingihat important r o n r e p t ~ s u c h ~ equilibrium, s elementary thermorhemistry, oxidation-reduction, atomic Seese, W. S., "In Preparation for College Chemistry," Prenticestructure. and reaction rates he omitted. Rather we are Hall, Inc., Euglewood Cliffs, New Jersey, 1974,pp. 113-125; Kolh, D., J. CHEM. EDUC., 55,184 (1978). suggesting that the more physically oriented treatments of 2 Storck, W. J., Chem, and Eng. News, 56.32 (1978). these topics which have appeared in elementary texthooks in ~~~~
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Volume 57. Number 11, November 1980 1 773
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4Li + O2 2Mg + 0 2 4Fe + O1 Au
+0 2
Pt + 0
2
2Li20 2Mg0 2Fez03 N.R. (no reaction) N.R.
(1) (2)
(3) (4)
The student can then see that when oxygen is limited CO will form, hut when oxygen is present in excess COZis the more commonly expected product. (As blacksmiths taught us, a bellows makes the fire hurn hotter.) I t is then convenient to make the point that the observation about the nature of the product obtained is a more general one. If an excess of oxygen is used, the highest oxidation state for the group is usually found. For example,
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Pl + 502(,,e~~l PKho (8) If oxygen is in short supply, the resultimg oxidation state is two less than the maximum.
The point can be made that this difference of two in oxidation state applies equally well for other elements of groups IVA, VA, VIA, and VIIA. If the electronic structure of the atoms has heen discussed, then this hehavior can he explained on the basis of the s 2 "inert" electron pair. Still certain anomalies exist. Some are very important. Because the combination reaction of SO2 with 0 2 is very slow under normal conditions, sulfur burns even in excess air to give mainly Son. The very important SO3 (used to make HzS04, the numher one industrial chemical) can he formed only through the use of a catalyst along with excess oxygen. Further it is convenient to point out here that N2 does not combine readily with 0 2 because of the very strong N=N triple bond. The student knows that N2 and 0 2 exist in the air without reaction. One can however apply Le Chatelier's principle to chemical equilibria and explairi why small amounts of polluting NO or NO, arise in an automobile engine operating a t very high temperatures with high compression ratios. Finally, the reluctance of the halogens to react with oxygen can be illustrated by noting that 2
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N.R.
(10)
Halogen oxides are not formed by direct comhination. Elements plus Nonmetals Oxygen, because of its wide occurrence and importance in qature provided the basis for a subcategory. I t is now convenient to expand the analysis to consider the reactions of the elements with other nonmetals. Again the metal, nonmetal scheme is useful for reactions with other nonmetals. Repre774 1 Journal of Chemical Education
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2Cs I2 2CsI 3Mg + N2 Mg3N2 Zn+S-ZnS
(5)
We include lithium since i t is the only member of the alkali metals which behaves as expected when it hums in oxygen. We would point out to the student that the reaction of sodium with oxygen gives Na202 and that with potassium it gives KOz. We would then have to honestly confess that gross generalizations from the periodic tahle are not absolute. Although thev mav he valid 90% of the time. nature still reserves the rigit to be demur. More detailed analysis of these trends in terms of lattice enereies - from ion sizes mav well he delayed to more advanced courses. Nonmetals with Oxygen. Some of these processes are familiar to the student and can he conveniently considered. For example, most students know that coal (or better coke) will hurn to give both CO2 and the toxic CO. Cautions on the burning of fuels in a closed space are constantly given. This knowledge can he summarized by the combination reactions
Cla + 0
sentative reactions which should he related to the periodic tahle are Metals (11) (12)
(13)
Nonmetals
These reactions indicate some further points: an oxidizing agent such as chlorine cannot take sulfur to its maximum oxidation state but fluorine, a much stronger oxidizing agent, can take sulfur to its maximum state of +VI. Further, fluorine is the only element capable of undergoing a direct comhination reaction with noble gases such as Xe. Oxides plus Water Because of the important role played hy water in our lives and because of the importance of the concept of acids and bases, this subcategory is a suitable one. I t is possihle to show easily that when metal oxides dissolve in water a base is formed. Often the oxide is not soluhle. We can then write metal hydroxide
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Metal oxides + water oN'
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Reaction
MgO + Hz0 Mg(0H)z (19) Liz0 + Hz0 2LiOH (20) Altos + Hz0 N.R. (21) Fez03 + HzO N.R. (22) Within the students' experience, soluble hydroxides arise from the elements of groups IA and IIA. Most other metal oxides are not appreciably soluble in water. Nonmetal Oxides. One of the features of the periodic tahle is its easy identification of nonmetals. The students then can he shown that these form oxides which readily react with water to yield acids.
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SO2 + Hz0 PlOm + 6H20
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HzSOs 4H3P04
(23) (24) 2N02 + Hz0 HN0s + HNOl (25) The last reaction is worthy of special comment. Even the average student will notice that two acids are formed from one oxide. Whv? The role of oxidation number of the nonmetal in generating an acid of thesameoxidation number or of two acids of different oxidation numbers is eajy to develop. ~
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Metal Oxide plus Nonmetal Oxide It will not he hard to show the student that these are special types of neutralization reactions which are very common in the high temperature chemistry of glass making and modern metal refining. Salts are the products. We can write: CaO + SiOz 6Li20 + PIOlo
--
CaSiOs 4Li3POI
126) (27)
II. Decomposition
Decomposition reactions are the opposite of comhination. I t is possible for the student to develop a general feeling for the kinds of compounds which decompose, what the products are, and which compounds decompose most easily. I t is convenient to sub-classify processes either hy products or by types
of compounds which decompose when heated or electrolyzed. For example, we ran write the fdlowing equations in which oxygen is a product. Formation of Oxygen
The gold reaction is used to put gold trim and designs on chinaware. Decomposition of Certain Sait Hydrates
The decomposition of hydrates to give water and anhydrous salt is an easy and obvious process. Loss of water I t is easy to point out that these processes have long been used as laboratory processes for preparing oxygen. Other subgroups involving types of compounds decomposed can be written Decomposition of Metai Carbonates
NiC03 $ NiO + Con A
NazC03 -+ N.R.
(32) (33)
Several methods of considering this set of equations are possible. One can point out that the driving force for the reactions is the formation of stahle metal oxides and the loss of C01. One can note empirically that all carbonates except the maingroup IA elements undereo this nrocess. Alternativelv. the teacher out that metals with small ionic might find it u s e f z t o radius and hieh nositive charee tend to lose C01 most easily, whereas metal carbonates wkh ions of large size and low charge, resist decomposition. Thus A l ~ ( c 0decomposes ~)~ at temperatures well below room temperature to give A1203 and C O s whereas Na2C03 can be melted a t 851 "C without COz loss. ~~~~~
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Decomposition of Ammonium Compounds
Two types of decomposition are common here. In one case an ammonium salt can be decomposed into an acid and gaseous ammonia.
In the second type of process a redox reaction occurs. The process can he dramatic and, in some cases, dangerous. Thus we can write
This decomposition reaction (36) is probably the major reaction in ammonium nitrate explosion processes. Thus NH4N03, besides being used as a fertilizer, is auseful explosive. Years ago an accidental explosion of a barge filled with NHdN03 in the harbor a t Texas City almost demolished the city. A more controlled redox process which can be used to give a s~ectarularvolcano-like lerture demonstration is the reaction (NH4)2Cr207$ N ~+ Crz03 + 4H20
On the other hand the hydrolysis process which gives metal oxide and a volatile acid is a somewhat more complicated reaction. Hydrolysis (Metal Oxide and Acid Formation)
(38)
The point can be made that such decompositions are common for compounds where one element is in a low and one in a high oxidation state. Students should be able to guess what happens when NHlMnOa is heated, and they should recognize that the reaction is potentially dangerous. Decomposition of Noble Metai Compounds
Since noble metals do not react directly with oxygen to give the noble metal oxides, it is not too hard to accept the fact that noble metal compounds decompose on heating to give the stahle metals.
Cursory examination of such decomposition reactions suggests a simple empirical "rule." Salts of M+ and M2+ metals will usually lose water on beating, while salts of M3+ and M4+ metals will usually undergo hydrolysis. For M3+ and M4+salt hydrates, the M-0 bond is stronger in the
systems and it is easier to break the O-H bonds to give metal oxides than to break M-0 and fonn water. A somewhat more detailed alternative treatment can be based anain - on sizecharge relationships for the metal cations. Cations whirh are large and of low charge put little strain on the water molerule. of these b;dr&es liberates water. Thus, Na~S0410H20 loses water on heating. By contrast, a small cation of large charge distorts the electronic configuration of the water molecule profoundly and hydrolysis is to be expected. This is true if the free acid is volatile. Thus, AlC13.6H20 undergoes hydrolysis to give A1203 and HCI. However, LaCl+3H20, where La3+ is larger than A13+,does not undergo such marked hydrolysis even though it is a +3 ion.
mo tin^
Ill. ReDlacement Reactions Several subcategories are ~ ~ s e fin u lconsidering replacement processes. Hecause water and acids are so much a part of our knvironment, it is convenient to first consider replacement reactions involving hydrogen displacement. Hydrogen Displacement
It is very easy and lots of fun to demonstrate that the metals of Grouns IA and 11A will displace hvdroaen from water. (Mg reacts siowly with water because of the insoluble magnesium carbonate coating. The reaction goes a t higher temperatures.) This is one subgroup. A slightly less reactive group displaces hydrogen from aqueous acid solution, and i t includes Al, Zn, Fe, Ni, and Sn. A third group will not react with acid solution and it includes Cu, Hg, Ag, Au, and Pt. These generalizations allow us to write 2Na + Hz0 2NaOH + HZ (45) Ca + 2HzO Ca(0Hh + Hz (46) Ni Hz0 N.R. (47) Ni + 2HCl- NiC12 + Hp (48) Cu + HCI N.R. (49) Our embryonic metals activity series based on these reactions would show Group IA, Group IIA > Al, Zn, Fe, Ni, Su > H > Cu, Hg, Ag, Au, Pt.
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Volume 57, Number 11, November 1980 1 775
Metal Replacement In the subgroups Al-Sn and Cu-Pt shown above, metals can he arranged within each group using metal displacement reactions as a guide. Thus we write: Zn + FeSO1- ZnS04 + Fe Ni + ZnSOl N.R. Cu + 2AgN03 Au + AgN03
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+
Cu(N03)~ 2Ag N.R.
uses the reaction
(52) (53)
V. Neutralization Reactions
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2A1+ Fen03 2Fe + A1203 + heat (54) Halogen Replacements Nonmetals, as well as metals, have an activity series. Displacement reactions such as
2KF + Clz establish the reactivity order
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iV. Metathesis Reactions
These reactions can he developed as douljle replacements which take place only if (1)a precipitate forms, (2) a gas is liberated, andlor (3) a slightly ionized compound such as water forms (handled under neutralization). Precipitation Reactions For handling precipitation reactions in water, the student should learn the following solubility rules: 1) All metal nitrates and acetates are soluble. 2, 1\11metal chlorrdes are wluhle except AgC1, PbC12,and HaCI*. 3) All metal rultater are aluhleerrept rhr s~~lfatpsuf Ca2', Sr2*,B& and Ph2' .
4) Most Group IA and NHI+ salts are soluble. 5) AU metal oxides and hydroxides are insoluble except for the Group IA and IIA elements. 6) All sulfides are insoluble except for Group IA and IIA elements and NH4+. I) All carbonates and phosphates are insoluble except for Group IA elements and NH4+.
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The application of the rules is then easily illustrated.
+
Ca(N03)z+ Na2C03 2NaN03 CaC031 (58) Z~CIP + NazS LZnS + 2NaCI (59) Gas Formation For handling gas formation reactions, students ere given the experimental "rule" here that all metal carbonates. sulfites, A d sulfides react with acid to form the gases CO~,'SO~, and H2S, respectively. Also metal chlorides react with concentrated HzSOa to give HCI gas, further metal nitrides may react with water to give gaseous NHB.This information permits us to write many reactions of the type NazC03 + 2HC1- 2NaCl+ COz + Hz0
+
CaS03 2HBr
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CaBrn + SO2 + Hz0
776 / Journal of Chemical Education
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NaCI + H2S01
(50) (51)
While the above treatment involves reactions generating metal ions in solution by oxidation, the basic fact that metals are generated from their compounds by a reduction process is not hard to develop. This idea can then he extended to consider the generation of metals in nonaqueous systems using the cheapest reducing agent, carbon, to obtain the desired metal. This is true in the manufacture of steel and permits an easy link between the steel and coal industries. Another spectacular nonaqueous reduction process, the thermite reaction, will be rememhered by students for years if it is conducted as a lecture demonstration.
and
MgS + 2HC1- HoS + MgCll (62) One commercial method for the preparation of HCI involves the reaction NaHS04 + HCI
(63)
A convenient lahoratory method for the preparation of ND3 MgsN2 + 6D20
+
3Mg(OD)z 2ND3
(64)
As noted ahove the formation of a nonionized product, such as water, from strongly ionized reactants provides the drivina force for the neutralization metathesis reaction. Thus acid: ~~-~ - - - - ~. -.--. and bases are most simply defined in terms of H+ and OHin water, and it is noted that the formation of HzO provides the driving force for the process. ~
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NaOH + HCI NaCl + H20 ~ . ~ ) + H20 KOH + H z S O ~ ( ~ ~KHSOl
(65) (66) (67) 2KOH(,,.,,) + HsS04- KzS04 2H20 NiO + 2HN03 Ni(NO& + H20 (68) 2NaOH + SO2 NazS03 + H20 (69) Representative reactions are given ahove t o illustrate the five reaction types propmed. Students should memorize none of these specific reactions. They should grasp the significance of the different reaction types, and learn the rationale which has been given. With this and some knowledge of the periodic table, students should he able to make reasonable messes as to hundreds of different specific reactions of the &pes discussed. T h e lahoratory work accompanying a course based on reaction types has been alluded to in the development just given. Some specific laboratory exercises, described by Taylor and Weave+ of Northwestern University include specific examples of each of the five categories. After the experiments have been completed the student may be asked to complete twenty new equations which follow from the laboratory experiments, the lecture material, and the periodic table. In addition to the use of reaction types as an approach to teaching descriptive chemistry, other general methods can he applied. For example, i t is possible to teach general methods for the preparation of elements from their compounds on the earth's crust. Furthermore students can apply simple solubility rules (or their knowledge of qualitative analysis, where i t is still heing taught) to understand the forms in which common elements occur on the earth's crust. A convenient classification is the following:
--
(1)
+
Soluble Saltr-these would have been washed into rheweanr
or ocean deposi~~, and are the sourceof ions such as Nar, Myit, K*, (:I-. nr-. and I-.
(2) Insoluble Sulfides-these would persist on the earth's crust as minerals of the type HgS, FeS, CuS, CuzS, and PbS. Many of these are important metal ores. (3) Insoluble Carbonates and Sulfates-the more important of these are the Group IIA compounds such as CaC03and BaSOl, hut
other systems could occur such as NiCOz and PhS04. (4) Metal Oxrdes-Often these are found for metals with oxidation states greater than two, e.g., Fezor A1203, Sn02, and Ti02. Due to the hydrolysis type reaction mentioned earlier,weathering of these metal ions on the earth's crust is expected to generate the hydrated metal oxides. Again these constitute important metal ores. Next it is possible to generalize that elements are made from their compounds by redox reactions. It is then possihle to classify the reactions involved as follows: ( 1 ) Oxidation of Element or Its Ions
a ) Chemieol lONaCl + 2KMnOa + 8HdOa
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5NanS01 -
(60) (61)
Taylor, H. V., and Weaver, T. R., "Laboratory Experiments for General Chemistry A02," Northwestern University, Evanston, IL 60201,1977, pp. 3144.
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b) Eleetrolyticol 2NaC1+ 2H20 2KHF2
2NaOH + Hz + CL 2KF + Hz.+ F2
(71) (72)
(2) Reduction of an Element or Its Ions a ) Chemical
Fe2O3+ 2C Tic4 + 2Mg b) Eleetrolytical
2Fe + CO f CO2 Ti + 2MgC12
--
2NaCI 2Na + C4 2AI2O3 4A1+ 302
(73) (74)
(75) (76)
(3) Thermal Redox Reaction
These sample reactions, and others, can be used as a basis for making some general comments on the production of metals and nonmetals from their compounds or minerals. I t is also necessary to discuss the concentration of low grade ores and the purification of metals, e.g., the electrolytic refining of copper. The formation of Clz from NaCl nicely illustrates that the laboratory method and the commercial procedure need not be the same. In the laboratory, the method of choice is that which is most convenient without undue regard t o cost. The commercial process has to be the most economical, and i t has to be adaptable to large scale production. Unlike Clz, the preparation of Fz has to he electrolytic because there is no oxidizine went which is sufficientlv powerful t o do this iob. sodium i s made electrolytically because it is difficult to have a strone enough reducing aeent to do i t chemically, whereas aluminum is produced eikc&olytically for economic reasons. Such a Drocess might be changed as prices of electrical energy change: The therkal redox method of making elementsis seldom used, and is added for historical reasons and for completeness of classification. More examples of workable approaches to teaching descriptive inorganic chemistry could be devised; for example, one could consider several different types of reactions used to prepare hydrogen compounds. However, we have perhaps already treated more types of reactions than can be covered adeauatelv in anv. beeinnine course. In anv event. if students " are expuscd to much of this descriptive chemistry, we can hope that the\ would have Aome feeling for chemical reactions and industrial processes. This would apply to those students who do not take additional chemistry, as well as our chemistry majors and even our PhD candidates. No longer would we have responses by chemistry majors such as "AgCl is a pale
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green gas."dMaybe PhD candidates would be able to handle annrooriate auestions on descri~tivechemistrv during thesis defense. ~ h & people e are extremely intelligent andvhave a good knowledee of some of the more sophisticated aspects of chemistry, h i t they often know littie ordinary reaction chemistrv. Chemists must accept the fact that lavmen will expect &em to know how pure copper or steel is made from their ores. and how bakiw soda and other common substances e expect an art hiscorian to are manufactured. ~ u r e l i w would be able to tell us, without havinr to look it up, who painted the Mona Lisa and during what p&od.5 What needs to be done in order that more descriptive chemistry be included in beginning courses? There is no easy answer to this question. Some topics would have to be omitted, such as detailed physical chemistry and atomic physics. Authors will have to write textbooks which devote more space to systematic reaction chemistry, and introduce concepts t o explain reactions. Teachers should develop a feeling of ureencv - . in their discussion of descri~tivechemistrv, because if they are nor enthusiastic ahout this the srudents will detect it and tend to he"turned off." Also it isextremely. important . that persons on commitwes to prepare national examinations insist that more reaction chem~stryquestions be asked. This addition would prompt teachers to-cover more descriptive chemistry in order that their students do well on these examinations. It is clear that this article is directed toward thme who share our views that chemical reactions are the heart of chemistry and who believe that this material is stimulating to students. We can only report that in our combined sixty years of teaching i t has been possible t o teach descriptive chemistry in a manner that is appreciated and perhaps even enioved . . bv students. Even afteitwenty years hsve passed, former nonchemistry majors often remark that all they remember from their chemistry course is the thermite lecture demonstration and the rotten e m odor in the laboratory. I t is true that students recall bestthe chemistry that they can hear, see, and/or smell. If done properly, this can also go a long way toward motivating students in the direction of chemistry. We hope that chemical educators soon come to realize this, and will do something about i t in the very near future.
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Acknowledgment We wish to thank our general chemistry students who have told us, even some twenty years after having had our course, that what they enjoyed and remembered most about the course were some of the chemical reactions. Because of many such remarks, we were prompted to write this article which we dedicate to those students who have volunteered their feelings to us about their course in general chemistry. Davenport, D. A., J. CHEM. EDUC., 47,271 (1970). Baaolo, F., J. CHEM. EDUC., 54,267 (1977).
Volume 57, Number 11. November 1980 / 777
