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Fractional Bond Order Wai-Kee Li
The Chinese University of Hong Kong Shatin. N.T., Hong Kong Molecules with fractional bond orders are often of interest t o students as well as to teachers. With the aid of a recently studied molecule, the following problem is designed to test the student's understanding of multicenter bonding and elementary molecular orbital theory. Question
The ionic compound [S&2+][(AsFg-)z] has recently been prepared.' The SzId2+ cation has been found to have Cz symmetry, and its structure is shown in the figure. From the bond lengths determined, it has been concluded that 1) the S-S
bond order (b.0.) is 2% (S-S distances in S2(b.0. = 2) and Szt (b.0. = 2l/d are 1.892and 1.825A, respectively), and 2) the I.-Ib bond order is 1%(1-1 distancesin 1%(b.0. = 1)and lzt (b.0. = ll/z) are 2.662 and 2.557 A, respectively.) The fractional bond orders can he obtained from conventional MO theory. Show how you can arrive at this result.
are formed by the two mutually perpendicular a* orbitals on the Sz unit and the a * orbitals on each of the 12+units I2+
SZ
(axy)4(az)2(ar*)1(rj*)2 $ four-center two-electron (4c-2e)bond (~zy)4(a~)2(~x*)'(rYY)1 4e-2e bond
Acceptable Solutlon
The SzIa2+cation has a distorted right triangular prismatic structure, composed of one Sx and two formally positively charged Iz+ units joined by weak sulfur-iodine bonds. The value (-90")of the I.-S-Ib angle suggests that the S-I bonds
Izt
'
(rxy)4(az)2(r,a)2(r,*)1
Passmore, J., Sutherland, G.. Chem. Soc. Chem. 289 (1980).
Volume 62
Whidden. T., and White, P.
Number 7
July 1985
SX
605
If it is assumed that the four electrons of the two 4c-2e honds are equally shared by the three participating units, b.o.(S-S) = 3 - k = 2%
y = 0.0833 M = molarity of KOH after mixing
2) Calculate the molarity of the KOH before mixing.
. . . -- v nneCnnal v mual . . . c inltae~
b.o.(I-I) = 3 - 1 - % = Ilk,
Thus the formation of S-I bonds results in a net transfer of electron density from Sz to the Iz+ units. Also, it is noted that the rr*-a' interaction takes place in an asymmetric manner, giving rise to preferential binding between one sulfur and one iodine.
KOH, 100 rnl X yM = 400 ml
X 0.0833
y = 0.333 M (molarity of KOH before mixing)
rnolarity X valence = normality
Normality of KOH solution before mixing = 0.333 N
Learner-Generated Questions To Buffer or Not to Buffer, That is the Question A. Mancott QueensboroughCommunity College Bayside, NY 11364 Buffer solutions are solutions that can sequester relatively large concentrations of hydrogen or hydroxide ions with a minimal change in the pH of the huffer solution. These solutions are prepared by adding a specific amount of a weak acid or a weak hase and the salt of that weak acid or weak hase. A buffer solution may also he prepared by a chemical reaction. The following problem involves the preparation of a huffer solution by a chemical reaction hut does not have the resultant solution act as a huffer. This problem is appropriate for second-semester general chemistry students and requires cognitive skill at the application level. Question 100 ml of a KOH solution is added to 300 ml of a 4.00 M solution of HCIO. It was found that the pH of the resultant solution was 2.50 units greater than the original acid solution. Calculate the normality of the KOH solution before it was added to the acid solution. The K , for HClO is 3.20 X 1W8. Acceptable Solution 1) Solve for the [H+]of the 4.00 M HClO solution.
pH of the 4.00 M HClO solution = 3.45 pH of the mixture = 3.45 + 2.50 = 5.95
[Ht] of the mixture = 1.12 X 1 0 F Calculate the molarity of the HClO after mixing with the KOH solution. Viniti.Ciniti.~= VensCanal HCIO, 300 rnl X 4.00 M
= 400 ml
X yM
y = 3.00 M (molarity of the HClO after mixing)
React HClO with KOH and calculate final concentrations. HC10 + KOH = KC10 + H20 3.00 - y = final concentration of HClO
y = concentration of KOH after mixing
Robert G. Landolt Texas Wesleyan College Fort Wodh. TX 76105 How many instructors have heard students react to an examination with "Why didn't you ask questions on the material that I studied?" Or, perhaps as frequently, dis. a professor . cwers that student. respond with answt.rs t o a question that he ur she did nut ask (or, at least, did nt,t intend 1oi. la it nut obvious that someone must know material rather well in order to phrase a good question about it? A strategy in which students compose and answer a question may be used to identify subject matter actually studied as well as to determine the degree of thoroughness of understanding. Evaluating the response to this strategy is highly subjective, requiring considerable attention to individual student responses. Question Help write this quiz (or exam)! Compose (ask, phrase, construct, etc.) a good question about (on, concerning, etc.) . . . and provide a good answer in the space provided. Comments In several years of use in organic chemistry, the most informative responses for learner-generated questions have come when the subject of the question has been a fairly detailed topic such as "the structure of benzene" and "nucleophilic substitution." Application at all levels, including general chemistry, is possible. To establish the value of a particular question-response, it is helpful to consider categories of questions in a hierarchical order such as suggested by The hierarchy includes memory, translation (words to symbols, etc.), interpretation, application, analysis (identifying logical steps in a thinking process), synthesis, and evaluation (value judgement) questions. Thus, a query calling for interpretation of facts or concepts would receive more credit than one merely requiring recall of information. Experience dictates that students he asked to confine their responses to specified spatial limits. However, liberal allotment of time for refining ideas is highly appreciated. After all, does not the quality of an instructor-generated examination question reflect the time devoted by the professor to its composition? The optimum frequency of use of learner-generated questions on quizzes or exams is yet to he determined. However, it seems likely that students who prepare for classroom evaluation by phrasing and answering their own questions as a study tactic profit from it as a reinforcing, summarizing function.
y = final concentration of CIO-
'
Solve for y.
606
Bloom, S. 0.. "Taxonomy of Educational Objectives," Longmans. Green and Co.. New York. 1956. Baldridge, K. P., "Seven Reading Strategies." Baldridge Reading Materials, Inc., Greenwich, CT, 1977, pp. 65-67.
Journal of Chemical Education
