DONNA BOGNER Wichita State University Wichita, KS 67208
Editor's Note on The Teachlng of K,'s Beginning Chemistry Courses
In
Equilibrium is one of the mast abstract concepts found in beginning chemistry courses. Most of the demonstrationsand lsboratwy experimen ?t, used to introduce and illustrate equilibrium remain just laboratcry experiences for many beginning students. Many are soinvolvedthat the concept is lost in the intricacies of the procedures. M h e ~ utilize materials that are foreignto the everyday experiences of the,students. The study of eovilibriumshould make lhem aware of the imoatance ~- of -~ the appl camns of me concept n the r i uer-from the way in which tho Wale, sohener work9 to tne very dellcafe rm um, calctbm,and pH eq~dtDriatnal m.SI oe mainta~nedin the human bmy me amhor and designer of the easy and precise laboratory procedure described below has provided students a learning experience that "zeros in" on the concept and has given them a foundation that teachers can help them apply in either the analytical or biological chemistry fields. F~
~
- reservoir
for liquid
4 crn resin
Lglass wool fonsxchange column made fmm a 6-in. or larger disposable pipet, water softener resin, and glass wool. Resin is available from water softener suppliers
K, Determinationol Calcium Sulfate
or from chemical supply harses (Amberlite CG50, or any cation exchange resin with sufficient porosity will do).
Davld Masterman Jackson Hole High School Box 568 Jackson. WY 83001
A very rapid and precise technique has been developed to determine the K,, of a saturated solution of calcium sulfate. While traditional methods require a minimum of two class periods to obtain a single value for the solubility product constant of a solute, the procedure described below can he completed in 10-20 min by the average student. Ion-exchange resins, such as those used in water softeners, can he used to remove quantitatively Group IIA ions froman aqueous solution. In doing so, an ion originally "loaded" onto the resin becomes displaced and enters the solution. While the sodium ion is generally loaded ontoa water softener resin, any Group IA ion will function satisfactorilv. In this proceduie, Ca2+is removed quantitativel; from a saturated aqueous solution of calcium sulfate bv the displacement of H+ from an ion-exchange column. Since each calcium ion displaces two hydrogen ions, a titration of the eluant should allow one to calculate the concentration of the calcium ion in solution. From this, the K,, of calcium sulfate may be readily determined (see Sample Analysis). Equipment and Materials Each student (or pair of students) will need: ion-exchange eulumn (packed befvre use by teacher), made of: disposable pipet (6 in. or longer) water softener resin (approximately 2 mL, hydrated) glass wool (from aquarium filter material) Pasteur pipets (2 each) 10-mLgraduated cylinder (or pipet) 100-mL beaker (2 each) buret utility clamp wash bottle (with distilled water) Solutions: 5 ml. 1 M HCI 50 mL 0.01 M NaOH drops phuncdphthalcio 5 mL saturated calcium sulfate
408
Journal o f Chemical Education
Exchange Column Assembly The ion-exchange column may be assembled rapidly and inexpensively. Hydrate about 2 mL resin per student overnight in distilled water. The resin may he obtained from your local Culligan man (one jarful will last a lifetime!) or from any distributor of cation-exchange resins. Place a small amount of glass wool in the bottom of a disposable pipet and pack the column with resin (see figure). A pipet with the tip broken off may help in transferring resin to the column. Just before use, pass 5 mL of 1M HCI through the column to load it with H+. Excess acid must be washed through by adding about 5 mL distilled water to the column. The pH of the wash should be neutral before conttnuing. Procedure 1. Filter a small amount of saturated CaSOn into a clean, dry beaker. 2. Pass 5 mL filtered CaSOl solution through the column, using a Pasteur pipet to transfer the liquid. Collect the eluant into a clean, dry beaker. 3. Rinse thecolumn with 10mLdistilled water. Add this rinse to the eluant collected in steo 2. 4. Add several drops of dhenolphthalein to the eluantand titrate it with 0.01 M NaOH. 5. Calculate the following values: [Ht], [Ca2+],[S012-],Kzp. Cleanup Pass 5 mL 1 M HCI through the column to recharge it with hydrogen ions. Follow this with a 5-mL wash of distilled water. Sample Analysis The following data and calculations are typical: Data Volume CaSOl (filtered) Volume NaOH at end point of titration Temperature of saturated solution
5.0 mL 17.2 mL 21.5 "C
Calculations moles of NaOH 0.01 mol 1000 mL
x
mol
[NaOH] = =17.2 mL
moles of H+ at equivalence,mol Ht
= mol OH= 1.72 X
mol
moles of Ca2+,S042mol Cazt
= mol SO:-
- 8.60 X
0.05 L
mol --x mol 1L
simply give a table of a number of acids or polyatomic anions and indicate that these names should be memorized. One text (3) even goes so far as to state that "no easily adoptable systematic way of naming these has been suggested. Consequently, the composition and names of these common ions must be memorized." Robson (4) has given a useful flow chart for naminga large number of inorganic compounds but even that requires the memorization of the names and formulas of the common wolvatomic ions. Our students have f&n-d the following scheme for naming the oxyacids and their salts to he simplifying and useful in avoiding unnecessary memorization. We stress that the principles are not original. Indeed, different textbooks use the same principles in various forms. What seems to help students is the organization of the material. I t gives them a place to start, an overall organizing scheme or a"roadmap" similar to the scheme for working stoichiometry problems (5-7)which so many instructors and students have found useful over the years. The key to this system (presented in the accompanying table) is the five representative -ic acids, one for each of the
System for Naming Oxyacids and meir Salts Representativei c Acids in Each Gmup of the Periodic Table:
Discussion
The average student should be able to complete two experiments in one 50-min lahoratorv weriod. wrovided he or she is familiar with titration techniqLes. ~hiee-significantfigure reproducibility is common. The sample analvsis above w& taken directly from a student's labor~toryreport. The values reported are very close to those listed in the Handbooh of Physics and Chemistry (CRC: K,, = 0.00019). The column as described holds a maxlmum of about 150 2 mL) should he pmol calcium ion. At least 30 ~ m o (about l used for three-significant-figure accuracy. The column mav he reused at least 30 times without anv significant change in performance, even though the repeated acid washes will change the color of the resin. This procedure is extremely rapid and precise, offering the student a high chance of success. I t is far superior to other K., determination labs previously used in our school. Several computer aids have been developed by the author to facilitate this laboratory exercise, including a Visicalc template for data analysis and a pH titration program that assists students in performing and analyzing the titration. For further information on these programs, please contact the author.
111
iv
va
VI
VII
HsBO, boric acid
HzCOS carbonic acid
HKb phosphoric acid
H ~ O I sulfuric acid
HCIOJ chloric acid
Rules for Naming Other Oxyacids (1) (2) (3) (4)
Addition of 1 oxygen lo the acid: per . . . -ic acid Subtractionof 1 oxygen from me acld: . . . -0"s acld Subtractionof 2 oxygens from the acid: hypo . . . -0"s acid All Other analogous acids in the same group of the periodic table are named similarly.
Rules f& Naming Anions of Oxyacids
(1) (2) (3) (4)
Acids ending with "-lc" are associated with anions ending wlth "-ate". Acids ending with ''+us" are assaclated with anrons ending wlth "-ne". The anion is obtained by removing ail the hydrogen ions. Anions in which one or more but not ell the hydrogen Ions have been removed are named with the number of remaining hydrogens indicated before the name of the anion as determined using rules 1-3. If one of two hydrogen ions are removed theanion is commonly referred to using the bi- orefix.
'HNO,
is an e~ception10 lhe other acids in Group V and is wiled nitric acid.
Remnve all
ACID
ANION
Hydrogen ions
per-
Making Sense of the Nomenclature of the Oxyacids and Their Salts Glen E. Rodgers, Harold M. State, and Richard L. Blvens Alegheny College Meadville, PA 16335
The nomenclature of inorganic compounds, particularly the oxyacids and their salts, is often a source of frustration for freshmen chemistry students. While some introductory textbooks (I) do present some methodology for mastering the names of these compounds, a surprising number do little or nothing to help students make sense out of what appears to them to be an "arbitrary procedure" (2). Many texts
-ic
-----t
t+ror -ic (Starting Point)
4
-
per-
-ate
-ate
-
-PI
-0"s
-ite
1-101
hypo- -0"s
hypu- -ite
Nomenclature "roadmap". Removing less than ail hydrogens results In ions covered in Rule 4 in the table.
Volume 84 Number 5 May 1987
409
groups IIIA through VIIA. We stress to our students that these are the only formulas that must be memorized (with the exception of the formula for nitric acid) and that all other common oxyacids and their salts can be named using these and the accompanying rules. In most textbooks, many of these rules are listed in narrative form and often seem only loosely connected to one another. In our system, all of the rules appear in tabular form, and our figure or roadmap shows how the various acids and salts are related. The only remaining difficulty is the identification of the root of the element to he used. That is, students do not remember if it is silicic or siliconic, carhic or carbonic, sulfic or sulfuric acid. After a few in-class examples that illustrate some of the different roots and demonstrate how to use the roadmap, students seem to he able to name just about any common (and some uncommon) oxyacid or salt derived from these acids. For example, if one wants to name the BrOzion, one starts with the representative acid in group VIIA and follows the roadmap as shown in the equation below.
and most important of the exceptions. Another exception is telluric acid, H6TeOs. We sometimes point out that these exceptions differ from the standard scheme only by one or more units of water. Other examples are just unusual variations in the root, such as arsenious acid, HsAs03 (as opposed to arsenous acid) (8). Of course the system does not include subtleties of nomenclature such as ortho-, meta-, di-, pyro-, or thio- acids and their salts, but these are not generally covered in an introductorv course. The advantages of the scheme seem to outweigh any loss due to oversimplification. Students, particularly those who have studied inorganic nomenclature in high school without the benefit of such an oraanizina scheme. find that i t is easv to use and that it simclifies what the; had viewed as frustrating and arbitrary exercise. We as instructors have found that the scheme accounts for the names of a large number of oxyacids and their salts. Moreover, it is a system that can be introduced very early in a course and is independent of such concepts as bonding theories or oxidation states. We invite you to give i t a try:
a
Literature Cited R. E.; Whitten, K. D.;Gailey, K. W. P~inriplesa/ Chemistry: Saunders: New York, 19%.
1. Davis.
2. Lingren, W. E. Inorgonie Nommelofura: A Pmgrommed Appmoch; PrentiecHall:
Chloric Acid
Chlorous Acid
Bromous Acid
Bromite
Like any simple scheme, one must he aware of exceptions and the danaer of overextending the svstem. We have already mentioned nitric acid, H N O ~as , one the most obvious
410
Journal of Chemical Education
Englewaod Cliffs. NJ, 1980. 3. Miller, F. M. Chemistry Struelure ond Dynamics; MoGraw-HiU: New York, 1984: p
8. IUPAC.J. Amsr Chrm. Soc.
1960,82,'5523
