/mething new from the pwt
Edited by: g; JOSEPHS. SCHMUCKLER#Me: ,,,, Chairman of Science Education +kc-:: -~ ~
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Temple University 345 Riner Hail Philadelphia. PA 19122
Equilibrium
More on Geology and Solution Equilibrium
At the onset of this month's discussion of "Something New From the Past," let me urge you to see the March 1981 issue of THIS JOURNAL. The articles reviewed for this March issue are continuations of the same type.
Two other articles on saturated silica solutions and solution equilibria make for fascinating reading for the teacher's backeronnd for teachine examoles. These articles olus a trio to the nearest "rock-ho;nd" shop will net the teac'her a go& geode and a fossil of a trilobite for use in teaching. Should a "rock-hound" shop not be availahle, most libraries carry the popular "rock and gem" lapidary magazines. Geode specimens and trilobite fossils are readily available even by mail order.
"Determining Ionic Equilibria: The Solution of Problems Involving Ionic Equilibria in Aqueous Solutions," Richard W. Rose and Grover C. Willis, The Science Teacher, 28-32 (April 1968). "All too often, chemistry instructors teach a separate method for each type of problem-one which can be memorized and which requires too little originality and understanding on the part of the student." The authors have worked out their "ideal" method of teaching aqueous equilibrium calculations according to 4 criteria.
"Chemistry of Fossilization," Lawrence Huestis, J. CHEM. EDUC.., 53 ~ 151.270-273. , , , (Mav . " 1975). "How Fossils came to he," Research Reporter, Chemistry, 49, [lo], 17-18, (December 1976).
1) The method should be general.
The method should require the complete solution of each problem, that is, obtaining the final concentration of all species. 3) Part of the method should he the use of a standard clear-to-read format for writing out the solutions. 4) The method should include the writing of a complete set of chemical equations by which equilibrium can be reached. 2).
"The equations come perhaps as close as possible to serving the same purpose as the 'model' in physics."The authors proceed to give 4 problems and their detailed solutions according to their "idea? method. They have used good logic and have given good detail in the use of their method. They describe a S~stepsequence that organizes what is known-starting with a drawing of a container and listing each specie present and its concentration. Only at step 5 are any calculations completed.
"A Dynamic Lecture Demonstration of Dynamic Equilibrium-The B. G. System," Ruben Battino, J. CHEM.EDUC., 52 [I], 55, (January 1975).
One of the most interesting examples of chemical principles involvine chemical kinetics and eauilibrium was written bv Adelle favis. "Chemical Princi~lesExemolified." edited bv Robert C. Plumb, [J. CHEM. EDUC., 425 (June 1973)j was based on Adelle Davis' hook. "Let's Cook it Ripht." Davis maintains that, "A knowledge of the chemical dynamics of the process [of cookinel can helo any amateur equal the best restaurant b a s t sirloin." Davis describes two methods of cooking a roast. One is by the "kinetic"method and the other is the "equilihrium"method. One of the methods produces a delicious juicy, tender meat, which slices beautifully and shrinks very little. Proteins and vitamins are not broken down and minerals are not lost. You will have to read the article to learn which method produces the above delectable dinner!
BGirl + Blgj t G(,, = 4
The LeChatelier principle is demonstrated most aptly while the "system" reaches equilibrium. The author's demonstration looks like fun-and it works! Other K,, values are shown along with theirpertwbations for the class to see the LeChatelier principle in action. "Illustrating Principles of Solubility and the Chemistry of Silica," Konrad B. Krauskopf, J. CHEM.EDUC.,49 [Ill,763, (November 1972). Quartz Geodes
"Have you ever marvelled a t the heauty of quartz geodes in a mineralogy collection-the large delicately colored, sparkling crystals of quartz which have been synthesized by a geochemical process?" ". . . sufficient facts are availahle that a reasonahle mechanism may he proposed, and this mechanism brings out some principles of solubility [equilibrium] that students of chemistry should he aware of." Krauskopf, using known concentration values offers an explanation of solution equilibrium and conditions of supersaturation that could be present in the earth's crust that can lead to the formation of geodes. This article is just what Robert C. Plumb intended it to he, "Chemical Principles Exemplified."
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Solubility Product Constant, K,, tions becameknown and shown. This writer found a reference to one in the literature that is an excellent sequential precipitation of silver salts, each less soluble than the preceding one. He has used it many times where the topic of solubility equilihrium is being taught. The reference is "The Chemistry of Silver: ADemonstration Sequence," J. Rae Schwenck, J. CHEM. EDUC.,36[1], 45, (January 1959). From handhooks of chemistry,the solubility product constants can be obtained. These can then be used as a hasis for excellent classroom Far those readers of these pages, of "Something New From the Past," this demonstration will inevitably become one of your ston-
dords!
"Is the Solubilitv Product Constant? Introductorv Exoeriment in Solubilky Equilibrium," Robert C. ~ o o d m a n a n d Ralph H. Petrucci, J. CHEM.EDUC., 42 121,104105 (February .. 1965). Volume 59
Number 3
March 1982
245
"There is a need in introductory chemistry courses for a simple, director experiment which will illustrate the principles of solubility product. At present, methods which employ the analysis of saturated salt solutions such as silver acetate or one of the lead halides are widelv
yellow precipitate of lead iodide forms hut.that it daes not persist until a certain critical product of concentrations is achieved, until a true solubility equilibrium is established. On a simple quantitative level, the student can gain facility in working with molar concentrations by ealculatine the salubilitv ~ r o d u efor t each titration mixture. Bv an-
silver oxide precipitate may serve further to conIuse the student. Whether silver acetate or lead halides are used in preparing the saturation solution, volumetric analysis is time-consuming and gravimetric techniques are even more tedious." This article describes an experiment which employs the titration of lead nitrate solutions with ootassium iodine solutions as a vivid and
that the w~devariationsin solubility product are a result of interionic forces. If he daes further laboratory work concerning the effects of ionic environment, he may find that although the interionic forces play a part, there are other factors affecting the apparent solubility moduct. This idea can be ~ u r s u e dfurther witha set of titrations de.1..11,.,1 I Illrlll ,Ilnlr.llr ll,? < l I ~ l I l l l..,lll-l 1, 11 1 111. l 1 l I,." T h C ~ ~ ~ l h m . ~,,; & i v I~. ~ I I ..nplit ~~ I .L. pr < d \ ~ rh-ci ~ . ~ t n sample data and their corresponding plots an semi-log paper. "The experiment is open-ended in the sense that it raises more questions and suggests further experiments. One question which is likely to grow out of class discussion is whether all ionic substances have an identical effect in increasing.the values of K,, as the Molar concentration of the ionic environment becomes greater."
246
Journal of Chemical Education
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