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 tmt. 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 iludge. 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. ~
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Dennis J . Kelsh Gonzaga University Spokane, WA 99258
154
Journal of Chemical Education
