edited by GEORGE L. GILBERT Denison University Granville. Ohio 43023
Change in Concentration with Time S U B M BY ~ Jean 8. Umland Univars~ol Houston-Downtown Houston, TX 77002 Danlel E. Ryan Denlson Unlrerdly Granrill., OH 43025 This very simple lecture experiment shows how the rate of areaction is fast a t first and then gradually decreases to zero when one reactant has been used up. The demonstration is useful as an introduction t o kinetics. Materials 250-ml beaker 100 ml deionized water 1drop saturated NaCL glass stirring rod or magnetic stirrer with stirring bar pH meter, preferably digital glass dropper 5 drops 2-chloro-2-methylpropane(tert-butyl chloride) timepiece that indicates seconds Procedure Place 100 ml of deionized water in the beaker. Add 1drop saturated NaCl solution. Stir and record the pH. Prepare a table in which to record time and pH. Assign one student to record data in table, one to read the pH meter, and one to read the timepiece. At a signal from the student reading the clock, to the stirred solution add, all at once, a volume of tert- butvl chloride eouivalent to five drons. The time of addition should he r&orded as r&o time. Record pH a t 15-r intervnk e down, [hen at longer intervals until until the rate of d e c r r ~ slows the pH hecomes constant. The infinity reading can be recorded after about 10 min. Comments The saturated NaCl solution is added to provide ions at the beginning so that the pH meter will give a steady reading. T o save time and allow students to concentrate on kinetics, the tahle of data from a orevious run is orovided. The concentration of tert-butyl'chloride was calculated from the difference between the initial and final concentrations of hydrogen ion, assuming that all the tert-hutyl chloride initiallv Dresent was converted to hvdrocloric acid accordina to .HCI. the equation, (CH3)3CCI+ ~ ~ 0 (CH&COH
+
Time pH 0 15
5.85 4.42
A graph of concentrations of hydrogen ion and tert-butvl chlorideas functions of time can be uied to show how to calculate rate of reaction from the slope of the tangent and to
show that the half-life for this reaction is constant (over the first 75%of the reaction). Comparison of graphs of concentration of tert-hutyl chloride, log concentration of tert-butyl chloride, and the reciprocal of the concentration of t-hutyl chloride as functions of time for the first 120 s (>85%reaction) illt~srr~tes one method of finding the order of a reaction. The effect of lowering the remperature on reaction rate can be shown by using a foamed plastic ice bucket to make a constant temperature bath. The effect of solvent polarity on the rate of this reaction can be shown by substituting mixtures of alcohol or acetone and water for the deionized water making the demonstration a good starting point for a discussion of reaction mechanism.
Invisible Water: A Gas Density Demonstration SUBMI-
BY
Richard P. Maciel LlncolhSudbvry Regional Hlgh School 390 LlncOln Road Sudbury, MA 02038 CHECKEDBY
Fred Juergens Unlverslfy 01 Wlrconsln-MadIran Madison, WI 53706 In an eyecatching demonstration of gas densities, we use the ethane, TTE. Its movapor of 1,1,2-trichloro-1.2,2-trifluoro lecular weight of 187 gives it adensity about six times that of air. We use a CHEM Study approach to molecular weights. After introducing the gas laws to show how all gases are alike, we measure some gas densities to show that gases differ. We then try to reinforce this difference in the next class by demonstration. The materials used include several lighted candles, an aquarium with a black back, a 1-1heaker, and a homemade double-walled hottle. Also hidden nearby are a container of bubhle-blowing soap and a huhhle wand. Just hefore class about 10 mL of T T E are poured into each the hottle and the aquarium. Our a ~ ~ r o a is c hto start the demonstration as a side show while leicuring on the apparently, at first, unrelated topic of atomic svmbols and molecular formulae. When the students are seated, we complain about the leaking roof (no matter what the weather is doing). Setting the bottle under an imaginary leak in the ceiling, we start the lecture. After 2 or 3 min we look a t the bottle and say it is about t o overflow. So we pour the "stuff' that leaked from the ceiling into the liter beaker. Of course the students cannot see anything happening. After resetting the hottle under the leak, we decide to put out a candle by pouring the "stuff' from the apparently empty beaker over it. Manv are surorised when the candle flame is obviously swamped and extinguished. Returning to the lecture for a couple of minutes allows time for the vapor to re-form in the bottle. The second time, we use the vapor similarly, except we pour it down a trough of folded paper to extinguish the flame. Again the hottle is allowed to refill under the leak. However, this time and one or two more times, the vapor is poured into the aquarium. Only then do we begin to notice that it is hard to see the surface of the stuff in the aquarium. Somehow the proposal is made to float "something" on the surface of the stuff. From a desk drawer Volume 62
Number 2
February 1985
153
a r e obtained t h e bubble blowing soap and t h e bubble wand. When t h e wand is waved s o t h a t bubbles settle into t h e aquarium, t h e y float, sometimes even bounce, on t h e surface of t h e vapor. (The bubbles should n o t b e blown into t h e aquarium since you can blow o u t a major portion of t h e TTE vapor.) T h e lecture topic now changes smoothly t o gas densities. An impressive way t o finish t h e class is t o ignite liehter-than-air methane bubbles a s described b y Snipp, ~ a t t s o na, n d Ha1dy.l T h e double-walled bottle was m a d e from two bottles available i n t h e supply room. T h e outside, wide-mouthed, plastic bottle originally held 5 lbs of mossy zinc, and t h e interior o n e held two perch specimens. T h e bottom was c u t off t h e smaller bottle, which, when inverted, gave a tight fit when
' Snipp, R., Mattson, 6.. and Hardy, W., J. CHEM.EWC., 58, 354
(1981).
inserted into t h e neck of t h e larger. After being secured in place with several short bolts, t h e joint a t t h e n e r k wasmade liquid-proof by torang a bead of aquarium cement around it. Black spray paint completed the jol~.T h i s bottle allows you to pour apparently unlimited quantities of the vapor without disrlusing its rource.'l'he liquid which has not yet vaporized is trapped between the walls when t h e vapor is poured. If you teach physics, there i s a Inmusin thisdemonstration. When t h e buhhles a r e floating on t h e vapor, careful rxamination will show them to be initially cnlorless, then rolored i ~ y swirls and bands of various colors, and finally (if they last long enough) they hecomr colorless again. Our physics associates tell us almut auarter-wave and half-wave reflectit)ns which develop then-disappear as t h e bubble wall gets thinner through e v a ~ o r a t i o n . he o v e r d effect of the demonstration is "magical" enough t h a t , after using t h e demonstration at Parent's Night, o n e o f our teachers was asked if h e performed a t birthday parties.
Water Electrolysis-A Surprising Experiment We chemistry teachers (and many of our students) know that water can be decomposed by passing direct current through an aqueous solution of nonreaeting salt (e.g., sodium sulfate), and that exactly 2 moles of hydrogen gas (Hz) evolve at the cathode for every mole of oxygen (02) produced at the anode. The measured volumes of evolved gases nicely illustrate several imoortant rclationshios: stoichiometrv. combinine volumes (Gav-Lussac), the Avoeadro hvwthesis. Faradav's laws. etc.. nnh it is n trulv sheltekd student whihas not seen this as a elaskmm de&onstrati& at least once .~ Atier the& important principl-s are well in hand, the mrdent might be challenged and stimulated by the following exprrimmt. l'sing thr u s ~ ~demonitration al apparatus (inverted huret and a central filling rube connerted by a s h m rube, with rnrk-sealed dcrtrodw y l a d at the base uf each invrmd hurrt), suhtitutc an aluminum wire rbr the cathode (normally platinum) usually used, and run the electrolysis in the customary manner. Ask students to measure the relative volumes of hydrogen and oxygen as the reaction occurs. I t will quickly become apparent that "too much" hydrogen is being produced; volume ratios more nearly 311 Hz102 will form (the exact proportion will depend on the current density), and after assuring themselves that noleaksare present in the oxygen buret, students should heasked to explain how they could"botch"such a nimole ~. exoeriment! ~~.~ Anne of t h more ~ ohsewant may note that hydrogen luhblrs cmtinur to form at the cathodeafter puwer isdisnmnect~d, ~ , h r ~o yr g~e sn evolution Ceases immodiawlv. Others may recall the renrtiviry of alkali and alkaline parth metnlr with water. and the bimllarity in chemical pmprrfies between iwyllium and allrminum proclaimed by thrir text. Once they realize that electrolysis removed aluminum's protective coating of oxide (they have heard about that, too) to expose a most reactive metal, the puzzle is practically solved. If you are blessed with a few bright lights who finish this all too soon, and they have written and balanced equations for all reactions occurring, they might try answering questions such as: 1) When (if ever) will the aluminum quit reacting with water? 2) Is the reaction helped or hindered by the applied voltage (cathodic protection?); by acid or base that might he added f.*n the ~. t. h.n-d-emmnartment? ....c..... ~ ~ . . 3) If experiments at several different currents are performed,hydrogen evolution per ampere-hour increases with current density. Why? (I am not sure I know the answer to this one myself.) 4) Suggest some practical applications or consequences of this effect. (Anyone who wants to make Hz out of Al commercially should be advised to compare prices of these commodities.) (My first contact with this phenomenon occurred while examining the effectiveness of various electrode materials to hasten settlement of a finelv divided coal sludee .. hv . electronhoretic action. Floatine aluminum cathodes worked better than anything else, lear,ing a transparent water lagern1,ove thr srdimrnt. wherras with other rathodrs, water remained rloudy, itnd -rttlemmt was less rap~d.Ex'identl) the AI"' ion pnducrd nt thr cathudr roagulnted I he negatively charged colluidal .sludge. enhancing both the extent and the rate of s~ttling.Unfmunatelg, the aluminum metal u,ar ronsumrd too rap~dly to make this economically attractive.) I like this experiment for a number of reasons: it illustrates several central principles of chemistry while also demonstrating the vulnerability of a familiar structural metal. I t forces students to integrate chemical information usually found in different sections of their general chemistry text. Most importantly, it should spark some curiosity and enthusiasm in students bored with "experiments" that merely illustrate what is already known. ~
~~
~~
~~~~
~
~
~
~~
~
Dennis J . Kelsh Gonzaga University Spokane, WA 99258
154
Journal of Chemical Education
