DOUGLAS D. SMITH Guillord High School Rocklord. lllinos 6111 1
Rates of Reaction-Analogies From the Australian Science Teacher's Journal' comes the following on chemical kinetics (see also, this Journal, 52, 379 (1975) and 53,195 (1976)). Zero-Order Analogy A zero-order chemical reaction is one in which the rate of a reaction is independent of the concentration of the reacting materials, H = h . [CIu.An example occurs when a gas reacts with the surface of a solid. A plot of volume versus time will produce a straight line with the slope equal to "k." Use about 8 1 of water in a hottle which has an exit a t the bottom. Adjust the flow so that it takes four seconds for 10 ml to flow into a graduated cylinder. Measure the volume a t various times, such as the numher of seconds to reach the 10, 20,. . . 80 ml. A straight line should result when total volume is plotted versus time. First-Order Reaction A first-order reaction is one in which the rate of the reaction is dependent upon the concentration of a reactant, R = h[XjL. An example would be the decay rate of a radioactive nucle-
A 50-ml huret is clamped upright with a piece of hose bending up from the bottom so that the water level in the huret will not en below the 451.111 mark. A clamo is attached ta the how and adjusted 5,s t hat \\ hen the bnret'ii Cilltdvitl~ water w d allc.wrd ro tlmv. n o more than 1 ml runs out in 10 s. The suggested data to be graphed is the total elapsed time as the volume chances in 2-ml increments, uw to about 20 ml. Let the apparatus run for about a n o t h e r t i e n t y minutes so that the water will he a t equilihrium (no more is running out). Call the equilihrium reading V,. A plot of lot (V, - V) versus time will be a straight line. The clamp controlling the flow of water should not he changed during a lab session or it will affect the rate constant. Also outlined are the instructions for using the iodination of acetone (first-order with respect to acetone, and zero-order in iodine) and a hydrochloric acid-thiosulfate reaction.
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Fogliani, C. L., and Townsend, I. T., Austral. Sci. Teachers J.. 28, 82, (1977).
US.
Buffer Effect Demonstration on the Overhead Projector Submitted by Paul G. Hobe, Jr. 7170 Paris Avenue Louisoille, Ohio 44641 ,111 interei~ingand colort'ul d e m o n s t r ~ ~ t ihai ~ ~ nbeen de\,elrmrd w h ~ r hillnstratt.i that one d r w ot dilute acid or hnse will change the pH of water by a large amount while larger amounts can be added to a buffer solution before any pH change is noted. It follows: Prepare a buffer solution. Fifteen grams of HAc and 20.6 g of NaAc in 250 ml of solntion is a good choice. This solution is 1 M in each solute and has a p H of 4.7. Arrange four Petri dishes on the projector stage. A slide directly under the dishes can be used to label the dishes and to show other information. Under one dish write "water" and place in it a small amount of water and 2 drops of methyl orange. If necessary adjust the pH so that the yellow color is visible. Beside the first dish place another dish labeled "huffer" or "solution-X. Place some of the huffer solution and 2 drops of methyl orange in this dish also.
In a similar manner set the second pair of dishes, one with water and phenolphthalein and the other with the huffer and phenolphthalein, on the projector stage. The solution in these last two dishes should be colorless. Show the class that one drop of 1N acid to the first dish will turn the methyl orange to red while many drops are needed to do the same to the huffer solution. To dispel the notion that you simply performed a titration in the second dish, add 1N NaOH base to the second pair of dishes. As expected the water will turn pink upon addition of one drop of NaOH while again many drops of NaOH are needed to turn the huffer solution pink. I use this demonstration in a laree erouv lecture situation as an introduction to buffers by showing this effect and asking whv a solution can neutralize both acids and bases. A discussion on buffers then follows. An alternate demonstration for small groups involves replacing the Petri dishes with beakers with enough volume to be visible to the entire class. In this way, the pH can he followed with a pH meter as well as with dye indicators.
Lab ~ i n t l Rubber eye-dropper hulhscan be used to protect the glass ends of equipment. They can he slipped over the end of: (a) glass tubing so the sharp edges are not exposed. (h) a pipet or buret tip to prevent chipping. (c) a thermometer to protect the hulh.
In a pinch they could he useful as a rubber policeman on the end of a stirring rod.
' Markowitz, M. W., and Boryta, D. A,, J. CHEM. EDUC., 40,482,
(1963).
Volume 56, Number 1. January 1979 i 47
Zubmitted by Zobert E. darkion Fairview High School '515 Greenbriar Boulevard 3oulder, Colorado 80303 The Chemathon is a year-end activity in general chemistry ~ n dis hased on the student's laboratory skills. This activity s natterned after the decathalon of track and field. The student competes in a variety of events utilizing labo,atom techniaues and a knowledee of laboraton,-nrocedures. . 'ointa are awarded based on accuracy and correctness and on lumber of events com~leted.I have used the Chemathon on In optional, extra-credit basis. Extra credit is awarded ac:ordine to the rank or lace of the student's finish. & ~ e f the k he math on, a list of events is provided, thus the .tuden~mardocmsiderable preparation beforehand. During he contest,they are permitted &e of their laboratmy manunl ~ n ddata h w k . T h r contest favors those who have developed :ood laboratory techniques, kept good data books, and pre)ared well. Student comments are eenerallv favorable. Some reallv njoy the contest, and in some classes a real spirit of compe.ition develoos. I Dresent the names of winners to all classes md there is'mueh interest in who won. Others have comnented on the contest and feel that it is a real test of what thev lave learned and a fine way of reviewing. The events are listed below. Shown is the maximum ~ o i n t otal and criteria for receiving maximum points. I. Experimental Determinations A. Use of a balance (10 ~ o i n t s ) Accuracy of weighing (+0.05 g) and correct recording of value. B. Titrotion (50points) Liquid-liquid acid-base titration Accuracy of normality (+I%),clear recording of data and problem set up with answer. C. Qualitatiue Analysis (5Opoints) Silver group- - - - -Ag+,HgsZ+and Ph2+ Correct identification of the three ions as being present or absent. D. pH (15points) Correct value of pH (+0.5 pH) by useofindicators along with explanation of indicators used. E. Temperature (5 points) Correct recording of a temperature value. F. Heat of reaction (5Opoints) Potassium Hydroxide dissolving in water. Correctness of value (*0.5 kcal), clear recording of data and oroblem set un for answer. r~ (30 points) 11. Quiz on ~ a k o r a t o l'rocrdures A. Prerisr idmriliration of common laboratory apparatus. H. Use oi a handbook to look up crrtain sperilied information. C. Use of a calculator or a slide rule. D. Solving a problem to correct units and correct number of significant figures. E. Laboratory procedures 1. How to use litmus and other test papers correetlv. 2. How to deiermine if a sample is dry. 3. How to decide if a titration value is correct (two runs must agree). 4. How to weigh out a given mass of sample. 5. How to heat test tubes safely. 6. General Safety Procedures.
for instructor's notes. Unknown s a m ~ l e are s prepared in advanre and arc dispensed by rode n&her. Several setups are provided for those determinations that take a longer period bf time. Students are given an hour to complete their work. These arc the basic ideas of the Chemathon. I am certain others will have different ideas and emnhases thev will want to make. Should the readers want a cop; of the "eitry blank" that I have used, I would be happy to send a copy.
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Points shown above are maximum noints. Lesser noints Ire awarded for lesser accuracy, unclear;ecording of da&, and .ncorrect math solutions. Points are deducted for extra sam?les, poor laboratory techniques, misue of equipment, no lafetv elasses. or anv other unsafe nractices. SO^ usefk mechanics include ireparation of an "entry blank" with places for students to record their data plus space 18 1 Journal of Chemical Education
Producing Flame Spectra Armstrong' reports on a method of producing spectra when it is not possible to produce a spectra by heating the desired salt in a Bunsen burner flame. The method consists of adding a salt into canned heat and thoroughly mixing. Pieces of the mixture about the size of a pea or larger can be burned. A shield should be used to prevent splattering; Pyrex tubing of two inches by six inches is suggested. I t is important to note the chemical activity of the ions as compounds like cupric chloride will corrode the sides of the can. Plastic containers can then be used. Armstrong, H. L., The Physics Teacher, 10,529, (1972).
