On Fridays I have a short quiz on the week's work. The grading of these quizzes was done in my office by 5-6 members of the class. During these grading sessions we discussed the quiz and the week's work. Graders had an opportunity to ask questions, defend their own answers, work problems at the blackboard, and explain concepts to their peers. These exchanees allowed me to assess where difficulties lav and to engage in dialogue with almost every student in the class. Once the initial barrier was breached. students came in more often on their own. This program theiefore served very adequately to increase rapport between teacher and student. Once students were actively involved in the grading process and themselves exoerienced trvina ambiauous an. -to made . swers, assigning values to degrees of correctness, &d intermeting imoossible to read and/or follow answers, they were much iess prone to argue vociferously over a few points:Many students commented that being involved in grading radically changed their outlook on exams. I observed a marked increase in neatly presented, precise answers.
If a molecule or ion contains one element not included explicitly in the rules, its oxidation number is found by difference. For example N is +5 in N03- because each 0 is -2, and the sum for all four atoms is -1. The rules will take care of all situations encountered in general chemistry and more. They will be qualified in higher level courses but serve well at the time they are presented and used.
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Calculators in Freshman Chemistry George Wiger California State University, Dominguez Hills Carson, CA 90747
Elementary Oxidation-Number Rules
A recent article in the Journal [56,526 (1979)] addressed the difficulties arising from the presence of calculators in freshman chemistrv courses. The nrohlems noted are certainlv cause for concern, and the solution suggested is clearly one means of response. I would like to suggest an alternative. I have gradually come to perceive the danger of calculators as a oaDer tieer. Indeed. I now view the handheld calculator as a gekx?"hoon for the teaching of freshman chemistry. In particular, the pocket calculator has placed mathematics in its proper position as a tool in the mastery of chemistry rather than an obstacle in its study. Interpolating with log tables to solve pH problems undoubtedly yields a better feel for logarithms than does the "keystroke" approach, hut does it teach more about acidity or hydrolysis? What about significant figures, clearly a calculator related problem? I would like to suggest that it is not necessarily true that students ten years ago, by not violating significant figure rules, demonstrated an understanding of the restrictions. Given the tools available, YOU could not produce answers with excess significant figures. Here calculators mav.be oerformine a service hv underlinine. the need for a strong emphasis on the significant figure relationshios in the teachine of chemical couceots. There is an additional area where c&ulators are a defkite plus in the classroom. The advent of the calculator has facilitated the coverage and testing of fundamental chemical relationships involving moderately complex calculations. Much more is available to the students, since much less time needs to he devoted to numher crunching. Given the above perspective, the way to proceed is straightforward. Make the calculator related problems obvious to the students. Then proceed in the traditional fashion, with emphasis on the understanding of relationships and their underlvin~concents rather than on numericallv correct answers. '?here is udthing novel in this approach akd that is my basic tenet: we have overreacted to the nresence of calculators. If we simply teach systematic approaches to problem solving, require extensive supporting work to be shown, and explain the analysis of answers in nonquantitative terms, the pocket calculator will play its proper and valuable role.
Eugene M. Holleran Neil D. Jespersen St. John's University Grand Central and Utopia Parkways .Jamaica. NY 11439 A survey of many general chemistry textbooks leads to the conclusion that a less confusing introduction to oxidation numbers would be desirable. When a student first encounters oxidation numhers his theoretical background is minimal, so he is presented with a list of rules to memorize. In most cases these rules are either inadequate or too complex. We present the following list which is short, simple, unequivocal, and easy for the student to learn and to use. Oxidation Number Hierarchy (1) The sum of the oxidation numbers of all the atoms in a molecule or ion must equal the total charge (zero for neutral mole. 1 -1 ....,
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(2) Metallic atoms of groups 1A and 2A have midation numbers of +1 and +2., resoectivelv. (3) Hand F are assigned orGation numbers of +1 and -1, respec-
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tively. 0 is assigned an oxidation number of -2. ( 5 ) Atoms of group 7A are assigned oxidation numbers of -1. (6) In binary compounds, atoms from groups VIA and VA are assigned oxidation numbers of -2 and -3, respectively. First and foremost, i t should be noted that this list is a hierarchy. As a result, if two rules conflict, the rule that occurs first takes precedence. This process gives 0 = -1 in H202, Br = +5 in BrOs-, 0 = +2 in OF*, 0 = -%in Li02, etc. If a conflict occurs within the same erouo. the liehter element follows the rule, so that, for exampie, I is +1 i L 1 ~ 1 . Aoolication of the first rule would eive an oxidation number of +?for Fe in Fe3+, +1for Hg in HlgZ2+,and, in general, an oxidation numher of zero for any uncombined element (as for S in Ss). The subsequent rules then apply to combined elements. (4)
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670 1 Journal of Chemical Education
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