AUFBAU1: A TEACHING RESOURCE FOR 14-16 YEAR OLD CHEMISTRY STUDENTS
[Dr. Roger Peters: firstname.lastname@example.org]
AUFBAU1: A TEACHING RESOURCE FOR 14-16 YEAR OLD CHEMISTRY STUDENTS
In principle, Chemistry is the subject par excellence for developing a student's cognitive, literacy, mathematical, and practical skills: if, and only if, a course reflects a balance of the various strands; i.e., biological, industrial, inorganic, organic, physical, and practical. Indeed, the subject's principal raison d'๊tre may be its suitability as a vehicle for development of such skills; certainly, the majority do not continue with its study after an introductory course (... and the benefits in the medium to long term are debatable).
Be that as it may, at least three major problems have bedevilled this subject (apart from curriculum development). First, as J. E. Huheey has written (Inorganic Chemistry, Harper Row, New York, 1983): "In attempts to simplify and generalize, implicit half-truths and an occasional falsehood are allowed to creep in. [And] One of the challenges of teaching (or text writing) is the problem of presenting material (especially to a person new to the subject) in a nondistracting way without oversimplifying (or even falsifying) the material for the sake of 'helping' the student." Second, as J. Barrett has written more recently (Understanding Inorganic Chemistry, Ellis Horwood, Chichester, 1991): "At the school level, some earlier theories are put forward as present-day explanations." And third, examinations of introductory syllabuses usually reveal an inherent imbalance because their physical chemistry content is nugatory.
Centring on the content of a GCSE Chemistry syllabus (Midland Examination Group No. 1781), which was approved in 1994 by the statutory authority in the United Kingdom, I have developed the first comprehensive teaching resource for 14-16 year old students that contains no falsehoods, no half-truths, and no non sequiturs whilst ensuring both a near-optimal use of the time allocated to the subject and a near-optimal balance of the various strands (partly by incorporating topics in the GCSE Biology and Physics syllabuses which appear in advanced level Chemistry).
Aufbau1 comprises a student's [auf1stam.zip (Am.); auf1stbr.zip. (Br.)] and a teacher's version [aufbau1.zip], together with these notes. The teacher's version shows a 1:1 correspondence with the student's: ... except, of course, that it contains the (near-exhaustive) answers available, as 100 separate but linked files in British orthography, in one zipped folder of 1.3 MB.
Students are more likely to appreciate the logic of Chemistry if they can see that each topic builds upon knowledge previously acquired. Accordingly, each of the main sections i.e., Reference, Selected Principles, Organic Chemistry, Metals, and Experimental adheres to the Aufbau principle in their presentation of topics [Contents' List]. Furthermore, setting aside the Experimental Section, there are two
optimal routes through the resource; each takes maximum advantage of the Aufbau principle, but the first ensures a greater variety: thus, Reference Section ฎ Selected Principles ฎ Organic Chemistry and Metals (in the ratio of 2:5), or Reference Section ฎ Selected Principles ฎ Organic Chemistry ฎ Metals.
The Experimental Section is complementary to, but entirely independent of, the main theory sections: but all include a rigorous approach to 'the' scientific method (i.e., hypothesis testing).
The Appendix, which includes a playlet in English [Von Ankunft bis Abschied (From Arrival to Farewell)], is similarly independent of the main sections. At what stage, or even whether, this section should be distributed is like every aspect of this resource entirely under the control of the teacher; however, distribution at quite an early stage could be decidedly advantageous because, despite one's very best efforts, there are times within any course when the enthusiasm of one's students wanes.
Finally, at the risk of sounding both pompous and waspish, I conclude this preamble with three caveats. First, the thoroughness of Aufbau1 might lead one to suppose that it is targeted at those students who plan to study Chemistry at an advanced level. However, the resource was written from precisely the opposite perspective: that of students whose interests, quite reasonably, centre on other subjects. Second, one of the underlying premises of Aufbau1 is that a teacher will read the text with their charges, simply because most students have the splendidly brutish habit of 'reading through the punctuation'. Providing this practice is observed, then, apart from the pure biology and physics questions, all the answers to Aufbau1 can be obtained from an admixture of demonstration and individual experiments, common sense, a Periodic Table, and (most commonly) the text itself; there is neither a need nor an advantage in using a chemistry textbook. And third, despite its optimized structure, Aufbau1 is not a course; purposively, each worksheet was designed to be self-contained (in the sense that no explicit reference is made to any other worksheet). In consequence, the format of self-contained worksheets ensures that the resource will be invaluable to any teacher who requires a new source of classwork, homework, or mock examinations either to complement or to supplement their established teaching programmes. # ง
* In Aufbau1, the only r๔le of a teacher which has been reduced, or perhaps even removed, is that of an amnuensis.
# This resource does provide near-exhaustive coverage of Chemistry for Years 10 and 11. However, there is no duplication of the Science topics covered in Years 7 to 9; each student is assumed to have a nodding acquaintance with, but not fluency in, this material.
ง Aside from a few inevitable coincidences, all the questions in this resource are de novo.
The following pages of these teacher's notes, which include details of additional features of this resource, focus on each section; that is: REFERENCE SECTION; SELECTED PRINCIPLES; ORGANIC CHEMISTRY; METALS; EXPERIMENTAL SECTION; and APPENDIX.
REFERENCE SECTION [Contents' List]
This section was designed, and is located, to provide a vehicle for overcoming two inherent (but unrelated) difficulties. First, that of presenting students, at the start of a course, with a panoramic view of Chemistry whilst convincing them that they can and will make genuine progress. And second, that of ensuring students have a decreased tendency to consider a data base as 'out of sight, and so out of mind'.
1. The Introduction focuses on chemical calculations and on chemical equations in an attempt to preempt the possibility of students erecting a 'mental barrier' to these topics.
2. The completion of the worksheets which follow the Introduction, but which should be executed in parallel (to ensure variety), provides each student with his or her personalized data base ... constructed, albeit, under the 'eagle eyes' of the teacher.
3. Unsurprisingly, considering the variation in class sizes and the abilities of students, there are several correct methods of teaching a typical worksheet; that described below, for Formulae and Solubilities of Salts, should be regarded as illustrative and non-prescriptive.
(a) The teacher reads, with clear pauses at the punctuation marks, the brief introduction. [Or, more usefully, this task can be allocated to a student providing the other students are aware that this 'privilege' will fall to each and every one of them in due course.] And then a few demonstration or individual experiments are executed with sodium carbonate, sodium hydrogencarbonate, calcium carbonate, and calcium hydrogencarbonate.
(b) Using the standard 'cross-over' method, the students are shown how the formulae of the chlorides and ammonium salts have been obtained.
(c) Armed with pencil, eraser, and rough paper, each student attempts to determine one line (e.g., the nitrates or the potassium salts).
(d) Those students who have grasped the principles are let loose on the remainder; obviously, the teacher can then focus his or her attention on those who are still uncertain.
(e) The teacher shows that the same principles apply to the more exotic compounds, such as vanadium(V) sulfate, and emphasizes that correct text will always state the oxidation state of the cation. And then students practise with any examples that come to mind though there is considerable merit in using those compounds which are referred to in the Metals Section.
(f) The teacher can conclude this topic with some vivid demonstration experiments on concentrated sulfuric acid as a dehydrating agent; the standard 'sacrifices' are hydrated copper(II) sulfate and sucrose, but tissue paper is more spectacular and can be related more easily to epithelial tissue.
(g) Students should draw upon this complete matrix of chemical formulae throughout the rest of Aufbau1. Nevertheless, there still remains the possibility that the 'little horrors' will view the worksheet as 'out of sight, and so out of mind'. Fortunately, via the photocopier, the teacher can wave a magic wand; i.e., present his or her students with the original as an 'unseen'. (... Now, class. I know you are all keen on films. Well, the main feature next Wednesday will be 'The Return of Formulae'. No, Candy Floss. It is not a horror film ...)
4. Each worksheet in Aufbau1 contains apart from the nominal subject content - one or more mechanisms for a teacher to contribute to the development of one or more of his or her students' cognitive, literacy, mathematical, or practical skills; an exemplar might be useful ... In Abundances, the content is the abundances of elements (derived, importantly, by the application of a principle introduced in Chemical Calculations); the mathematical and practical skills being developed are good practices in the construction of graphs. *
5. Implicitly, each worksheet in Aufbau1 is layered; i.e., it can be pitched at more than one level, depending on the students' abilities and the time available as the following exemplar should illuminate.
(a) At its most superficial, Redox Potentials is merely a vehicle which allows students to become au fait with the geography of the Periodic Table; thus, after brief demonstration experiments with the simplest type of electrochemical cells (e.g., Mg/Cu, Fe/Cu, and Cu/Cu), students complete the second Table (which provides the self-consistent, physical basis for the metal reactivity series). Because neither the obtention nor the use of redox potentials is within the (present) syllabus of an introductory course, there is obviously no need to consider this worksheet in any greater depth; however, during the course ...
(b) The demonstration experiments could be used as a springboard for students to develop hypothesis testing within the context of potential differences. #
(c) The Tables' qualitative and quantitative features could be used to support a variety of (otherwise unsupported) facts and hypotheses. ง (... Now, class. I have just made the assertion that ozone is a better oxidant than difluorine. So, why have none of you checked in your Reference Section to be sure that I have not spoken complete twaddle? No, Fletcher Christian. That does not imply ...)
6. Finally, and perhaps most importantly, students should be advised that they are not expected to understand, much less remember, all (or even most) of the contents of this section at the earliest stages of the course. Thus, using Pollution as an illustrative example, few (if any) students could be expected to understand the footnote on natural selection: but, they will be much more receptive to this mechanism when they come eventually to Biocides and to Copper Compounds (2).
* Good practices in the construction and interpretation of graphs are treated systematically in both the Metals and Experimental Sections. Incidentally, the use of pencil (and eraser) to complete each worksheet has obvious advantages; pen should be reserved for alternative answers and/or additional explanations provided by Mr., Mrs., or Ms. Chips.
# The normal distribution of a typical class will ensure a variety of qualitative and quantitative independent variables (inter alia, types of metals/electrolytes, depth/surface area of electrodes, concentration of electrolyte, and temperature). Some of the students should determine that some of the chosen independent variables have no effect on the dependent variable of potential difference; fortunately, this allows a teacher to point out that determining the absence of a relationship between two variables is useful: if, and only if, the values of the constants and the variables are precisely documented.
ง In principle, students could solve the chemical puzzle when they have completed Electrolyisis (2); and, in essence, Electrochemical Cells provides the solution: but, there is distinct merit in delaying one's encouragement to solve the puzzle until Cadmium (or even ignoring it).
SELECTED PRINCIPLES [Contents' List]
This section was designed to achieve one principal objective: that of providing students with a sound physico-chemical basis for the subject. Strictly speaking, this objective is not realizable if one interprets the Chemistry syllabus literally. Fortunately, however, the companion Physics syllabus is awash with topics which can and, indeed, must be incorporated seamlessly into an introductory course in Chemistry.
1. The Introduction, available as a single document [Am.; Br.], focuses on structure and chemical bonding; the entire section is built up from crude (but intuitive) mental images of atomic and molecular hydrogen. Self-evidently, the presentation of these two topics does not follow long-established conventions for an introductory text, but does ensure compatibility with advanced studies. In addition, and perhaps of much more importance, students should warm to the notion of pivoting their understanding of these topics around energy and energy changes (which, themselves, are the pivots of their Physics course). *
2. The remainder of the section, which introduces new topics in a logical order, continues with these two pivots. Ideally, I would like to have developed the whole of this section around the concepts of free energy and entropy; regrettably, both of these are outside both the Chemistry and Physics syllabuses though students will be blissfully unaware of the limitations imposed by these lacunae. #
3. At first sight, this section in particular (and Aufbau1 in general) might be perceived as being overly mathematical. This would, however, be a false perception because the arithmetic and algebraic concepts never stray outside either the Mathematics or the Physics syllabuses; moreover, regardless of whether or not they plan to study the subject further, students will appreciate the opportunities of consolidating and/or developing their mathematical skills in a complementary context. Teachers will, though, correctly perceive that this section could have had the more cumbersome title 'Physical Chemistry admixed with the Chemistry of Non-Metals': but the use of such a title, on the one hand, would have provided students with a possibility of erecting undesirable mental barriers, and on the other, could have prevented them from (subconsciously) appreciating the application of important principles to nominally diverse parts of the syllabus(es).
4. The section concludes with Electrochemical Cells, whose content purposively dovetails with both of the following Sections (that is, Organic Chemistry and Metals).
5. And finally, teachers will note that none of the main sections formally introduce the topic of molecular geometry. There are two reasons for this lacuna: first, I assume that each student has access to a model building kit (even if this is merely a lump of plasticine and some dead matches secreted in a pencil case); and second, as an activity, the construction of molecular models invariably releases the imaginative qualities of students in a warm(?)-spirited environment. (... No, Fletcher Christian. You are supposed to be constructing a model of propane: not one of Candy Floss with a valency of 30! ...)
* In the UK, the concept of Electronegativity is not introduced until Years 12 & 13. However, the following note may be worth bearing in mind ... The difference in electronegativities of two atoms is a measure of the polarity of the bond between them; and a difference of zero corresponds to pure covalent bonding. This is precisely the case in pure metals (and almost so in alloys). Accordingly, the use of this concept allows a self-consistent presentation of the bonding in, inter alia, (metallic) sodium, difluorine, the H-C bond in methane, hydrogen fluoride, and sodium fluoride.
# Wherever possible, energy changes are considered exclusively in terms of thermicity; however, when such use would be inappropriate (e.g., in considering active transport, biosynthesis, electrolysis, and photosynthesis), the more general idea of ergonicity has been used. (Incidentally, throughout Aufbau1, all numerical values of the energy changes equate to changes in free energy).
ORGANIC CHEMISTRY [Contents' List]
I suspect that the vast majority of students, having completed an introductory course, perceive Organic Chemistry and the Petrochemical Industry to be synonymous. This false perception, which has persisted for at least three decades, has been engendered by the far too literal interpretation of any given syllabus; in particular, the focus on: three alkanes (methane / 2-methylpropane / butane) and their combustion reactions; two alkenes (ethene / propene) and their bromination and polymerization reactions; and one alcohol (ethanol), and its synthesis, combustion, and susceptibility to oxidation. However, such a narrow focus cannot possibly hope to illuminate either the importance or the breathtaking diversity of organic chemistry. Fortunately, or otherwise, a deeper and more balanced perspective can be achieved by interpreting a syllabus in the correct spirit.
1. Sotto voce, students will have been introduced to organic chemistry in Selected Principles, but this Introduction is self-contained. *
2. The three worksheets which follow the Introduction, (i.e., Alkanes, Alkenes, and Alcohols), complete the presentation of the conventionally accepted 'core content' in the context of a desirable balance between the industrial and biological aspects of organic chemistry.
3. The worksheets from Carboxylic Acids through to Detergents, whilst maintaining the balance stated above, attempt to provide students with strong hints as to the richness and diversity of organic chemistry: but, nevertheless, ensure continuity because one way or t'other each worksheet implicitly refers back to the Reference and Selected Principles Sections, or to the core content.
4. The section concludes with Male Contraceptives, which is a rare example of a fragment of genuine research that can be understood in terms of the defined syllabus. # (... Yes, Candy Floss. I do appreciate that the word vasectomy does come to mind when you see Fletcher Christian: but I am not at all sure that renaming this worksheet is a particularly valuable exercise. ...)
5. And finally, should teachers wish to complete this Section, before undertaking Metals, they can do so confident in the certain knowledge that their students will be repeatedly exposed to organic chemistry in Metals (starting with Calcium & Magnesium). ง
* Teachers may be puzzled by the complete absence of any reference to oil pollution in this resource. Admittedly, the short-term effects on vertebrates are spectacular: but, aside from the malevolent potential of the carcinogens present in crude oil, there appears to be no medium- to long-term effects on either aquatic or terrestrial food webs.
# Obviously, this research is presented in a highly truncated form; in particular, the synthetic route shown is, of necessity, an adaption of that actually executed (e.g., I used tetramethylguanadinium azide for the nucleophilic substitution step).
ง Broadly speaking, four topics are usually omitted from introductory courses: namely, the reactions of (a wide range of) functional groups, reaction mechanisms, line formulae, and aromatic chemistry. Whereas the omission of the first two topics appears reasonable on the grounds of cost, safety, and the need to limit the contents of a syllabus, I do not see any justification for the omission of the latter two. In order, on the one hand, to ensure that the integrity of the GCSE syllabus was maintained, but on the other, to support my convictions, these topics have been excluded from the main text but are included in the Appendix (specifically, points 4 to 7; i.e., over 'half-term' in the playlet).
METALS [Contents' List]
Three points need to be considered before the overview of this section. First, self-consistency demands that the electrochemical and the reactivity series are one and the same; in particular, to present the highest members of the series as ... K > Na > Li > Ca ... cannot be justified by qualitative experiments which conflate the rates and the energy changes of their reactions with a limited number of reactants. Second, there is a widespread misconception that all metals in Groups 1 and 2 are extracted by the electrolytic reduction of their fused salts: currently, only magnesium, lithium, sodium are obtained by electrolytic reduction (the method of choice for the other metals is thermochemical reduction). Furthermore, the extractive method is not directly related to the metal's position in the reactivity series; nevertheless, I have included questions within Tin & Lead, centring on the heat energy changes involved in the reduction of metal oxides with hydrogen, which allows teachers to superimpose any viewpoint. And third, although the thermal stabilities of compounds do parallel the position of the metals in the reactivity series, it is remarkably difficult to provide data which unambiguously support this assertion and which are meaningful to students; there is but one attempt in Aufbau1 (namely, the decomposition of Group 2 carbonates in Strontium).
1. As with Chemical Energetics (2), the Introduction seeks to minimize the strong possibility of students blurring the important distinction between the rate and the energy change of a reaction.
2. The section follows the electrochemical series from Displacement Reactions (1) through to Silver, but there are four caveats. First, I have not been able to include a theory-worksheet on potassium. This metal is extracted by the thermochemical reduction of potassium chloride with sodium; whilst this extractive method is typical of that used for Group 1 and 2 metals, and does provide an excellent example of an application of Le Chatelier's Principle, its inclusion might confuse students (because sodium is less reactive than potassium). Second, by some nifty footwork, I have side-stepped around the slightly 'inconvenient' position of calcium by incorporating it with magnesium. Third, I have included reaction types at the earliest possible stage; the superficial treatment of ligand-exchange reactions is a direct consequence of keeping to within the defined syllabus. And fourth, there is a near-exhaustive presentation of 'the' scientific method; this includes a series of line graphs from Extraction of Copper to Silver, which introduce the various types in a logical sequence.
3. And finally, the section from Strontium through to Platinum, which neither follows the electrochemical series nor introduces any new principles, but which does keep to within the spirit of the syllabus, was designed to allow students to appreciate the richness of Chemistry.
* Calcium is extracted by the thermochemical reduction of calcium oxide with aluminium. The preferred method of extracting magnesium, by the thermochemical reduction of magnesium oxide with ferrosilicon, has been omitted; its extraction by electrolytic reduction is included in Electrolysis (2).
EXPERIMENTAL SECTION [Contents' List]
In the early 1990s, the statutory authority in the United Kingdom decided to replace a beautifully balanced method of assessment with one which focused exclusively on 'Investigations'. Setting aside the perceived wisdom of this decision, two of its important consequences have been the necessity to reconstruct and to develop practical schedules within the framework of hypothesis testing. This section which is complementary to, but entirely independent of, the main theory sections presents 22 practical schedules, based on worksheets which were warmly received by external examiners in the period 1986 to 1994; these schedules have been lightly edited, partly to extend the Aufbau principle to 'the' scientific method, and partly to provide clearly defined teaching vehicles. *
1. Strictly speaking, the 'recipe' schedules 01 and 02 have no place within a scheme which focuses on investigations: however, I assert that linear (01) and convergent (02) syntheses must be included if students are to obtain a balanced perspective of practical chemistry. #
2. The schedules from 03 through to 22 are clearly investigative in character; furthermore, regardless of what variables or hypotheses have been or are chosen, each is a genuine investigation because no student will find any of the quantitative details in reference sources. 03 through to 10 are investigations in which both the method and the variables are stated explicitly; good practices in the acquisition, presentation, and interpretation of data are introduced systematically. 11 provides the student with his or her first opportunity to design and execute their first complete investigation. 12 through to 16 are investigations in which the variables are stated explicitly, but the method of execution is chosen by the student. 17 through to 22 are investigations in which the student has complete responsibility for both design and execution; purposively, 17, 18, and 19 each have a narrow focus, whereas 20, 21, and 22 each provide a vehicle which allows the student to stretch his or her imagination.
3. And finally, these schedules have been designed to ensure that only modest demands are made on conventional resources; however, there are two caveats. First, there is an presumption that thermometers of a reasonably high quality are available. Thus, a -10 to 110ฐC thermometer is little more than an expensive stirring rod; a short-stem -10 to 50ฐC thermometer is probably not much more accurate, but it is much more precise (at least ฑ 0.2ฐC); and a long-stem -10 to 50ฐC thermometer is even more precise (at least ฑ 0.1ฐC), but its high centre of gravity means that it should really be used with a clamp. And second, based on experience, schedules 20, 21, and 22 should result in a most pleasing conflation of biology, chemistry, and physics resources.
* Teachers might gauge the complementary character of this section by comparing, for example, my novel investigation summarized in Metals: Displacement Reactions (4) with schedules 06, 10, 12, and 17.
# In the late 1970s, I used a pseudo-Simplex method to optimize the yields of free-base porphyrins; however, because the final procedures are unsuitable for students, I redefined safety as the absolute primary variable (with cost and reproducibility as secondary variables).
APPENDIX [Am. (15 pp.) Br.]
The appendix allows students 'to recharge their mental batteries', by considering ten fragmentary points of discussion from a different perspective [1-3 and 8-10 are within the GCSE syllabus, whereas 4-7 are outside (these are presented over 'half-term', and so can be excised)], together with Playlet 1 to leaven these fragments.