Tested Overhead Projection Series Comp~ledby
HUBERT N. ALYEA Pr~nceton Un~vers~ty
6. IONIZATION, ELECTRICITY
6. Production of Electricity (continued) Dem. 3 17-Hydrogen
Electrode
1)r~icerl~veloped by Kenneth Jacknzan, Denver Ulerlj, and Cmsby Rogers. To show: e.m.f. geuerated by a hydrogen electrodc coupled with metal half-cells. Materials: SD-316; also Special Device SD-317, hydrogen electrode, prepared from the following materials. Two 9 X 0.02C-in. P t wires cleaned by rubbing in moist, sea-sand; 5 in. of 6-mm glass tubing, 3 in. of 7-111m plastic tuhiug slit 1 in. a t one crid; H-electrode conriection via rubher tubing to needlc valve on tank of elcrt,rolytir hydrogen. Forming Pt spiral. Draw a taper 2 in. long a t the end of the glass t,ubing. Anchor a P t wire by st,icBing it, red-hot, into the he~t,edtubing near the tip; then wrap 7 in. of the wire closely around the taper; and finally tapc the last 2 in. of mire to the tubing, and anchor it wit,li a 11inj d ; , ns in Figure SD-317.
illaking Pt-black. Completely immerse this spiral in 50 ml H 2 0 containing 1.5 g PtC14.6Hs0 0.015 g P ~ ( O A C ) ~Connect . it in series with a DPDT switch, a 2500 ohm potentiometer, a 6-v de source, and a piece of clean l't wire for the ot,her electrode. Adjusting the potentiometer to moderat,e evolution of gas, pass current for 10 min, switchingpoles every 30 secs. Wash with, and store under, distilled water.
+
Pza'ihing Pt-black (imperative) : Connect the Pt-black spiral to t,he electrodes, adjust the potentiometer to give a sn~oothflow of bubhl~s,and pass current in one direction only, for 15 min. Pl,oceclure: Projert cells SD-316-317. Insert Zn/Zn++ in one culture tubc. Slip the plastic tube over the spiral. Put the H-cell in the other culture tube, add 111 H2SOn,and iusert the ICCI-aq. salt bridge. Adjust tbe needlevalve of the tank to 2 bubbles of hydrogen per second; they flow past the Pt-black spiral and escape through the slits in the plastic tubing. Turn onswitch, record voltage. Repeat, coupling the hydrogen electrode with thc ot,hcr metal half-cells. Observations: Only correct procedure will give reliable results.
Correct Incorrect Couple Cn/H2 Zn/H? Pb/& Zn/H* Zn/H% Pb/Hn Handbook - 0 . 3 4 0.76 0.76 0.76 0.13 10.76 Found -0.30 0.76 0 . 1 6 10.86a 1 . 5 % 0 . 2 0 ~ a-no HZflowing. b-PL el~ctrodenbt cleaned. e-bright Pt. Note: If the needle valve does not permit fine enough adjustment, interpose s. 6-mrn glass T-tube wit,h its vertical arm immersed 20 cnr into wxl,er, nnd adjust a pineheoek on the robber tubing. Ahernate
to
Dem.
317-H-electrode
Generathg
Hs
Own Hz Ezperiment developed by: LeRoy G. Hornbeck. T o show: Electromotive force measurements with halfcells using an H-elertrode which generates its own hydrogen. Materials: SD-3lfi mossy zinc, H-electrode as described below; strips of Zn, Cu, Mg; 1M solutions of ZnSO4, CuS04,MgS04,H2SOn.;sat. I'X-aq. Preparation of H-electrode. XIake a coil of 0.020 in. P t wire t o fit into a half-cell. Drop the coil first into aqua regia, then rinse with distilled water. Connect to the negative pole of 2 dry cells in series and another Pt wire to the positive pole, and electrolyze in 1%platinic chloride 5 min, then in 1 Af H2S04for 5 min. A dense black P t deposit must be obtained. Store under water, do not allow to dry. SD-317.
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Electrodes for emf. Meorurernentr
Journal of Chemical Education
Procedure: I n one half-cell put Zn/Zn++. I n the other put a few pieres of mossy zinc, the Pt coil, and l M
H2SOI. Place both half-cells in SD-312, KC1-moistened salt-bridge, and fill the chamber with water. Measure voltage. Repeat, checking other metal half-cells against this standard H-electrode. Dem. 3 18-Zn-Cu
Coupling
T o show: The coupling effert of Cu on the reaction between Zn and HC1. Materials: Mossy Zn, dil HCI, two 4-in. lengths of To. 24 bare copper wire. Procedure: ( a ) a piece of mossy zinc, (b) a copper wire, (c) a piece of mossy zinc resting on a mushroom-shaped piece of copper wire. Half-fill each with dil. HCI and project. Observations: The zinc reacts slowly, the copper wire not a t all. I n the third cell the Zn/Cu couple gives rapid reaction, the hydrogen gas evolving from the copper wire, not the zinc.
--
+ 2H+ Zn++ + Hz (slowly) Cu/Zn + 2Hi Zn++ + H. (fast) Zn
Procedure: Darken room; turn projert,or off and on, as experiment progresses. ( a ) Project SD-320 with filament lighted by 110 volts a.c., 60-cycle current. Move magnet in to cover it. (b) Repeat, but use 110 volts dc ihrough the filament, and rotate the magnet. Observatzons: ( a ) The filament vibrates whenever the magnet 1s over it. (b) The wire vlbrates at the same rate as themagnet is rotated. Explanations: ( a ) A magnetic field should repel the filament 60 t,imes per minute in phase with the 60-ryde ac current,; this would be too fast to be perceived; fortunately the drag of the heavy filament lags this to an observable rate. (b) The wire is repelled at right angles to t,he magnet,ic field as the magnet revolves. Dem. 3 2 ]-Lead
T o show: Operation of a lead storage cell. 4fateriaL~: Dil. H,SOn, two 4 X '/z inch lead foils inserted as electrodes in SD-305a without hatt,cry; milliammeter; 6-12 volt do. power source. Procedu~e: ( a ) Projert f1111 of dil. H2SOI. Pass !)-\'olts through rell for 5 min. ( b ) Cor~nertelectrodes to mcter and projert. Pour dil. H2S04into t,hat cell.
Dem. 3 19-Overvoltage
Experiment developed by Kenneth V . Jackman. T o show: Effect of overvoltage in the reaction of Zn with HCI. Catalytic effect of Pt or Cu wire. A{aterials: 3-in. lengths of Cu and of P t wire; clil. HC1; Zn amalgam made by cleaning a zinc strip or rod (not mossy zinc) in dil. HCI, then immersing in HgCIS-aq until it is coated with shiny Zn-Hg amalgaoi; C-2. Procedure: Project C-2 with the two cells threoquarters 5 ml. dil. HC1. Drop a Zn-amalgam full of water coated strip into each cell, and then the copper wire into cell 1 and the I't wire into cell 2, touching the amalgam.
Observations: ( a ) The rell charges up, forming Ph01 and Pb. (b) The cell discharges, t,he met,er rerords about 1volt, and PbSOnis formed.
Chemical reactio~is
+
Observations: Zn+Zn++
+ 2e-;
and2H+
+ 2e-
Storage Battery
Hz t
Because of the high overvolt,agr for H, on Hg, the hydrogen bubbles form along the Cu and P t wires, not on the zinc. Reaction is more rapid on the Pt.
Pb+Q
+ Pb + 2HrS0, + 2Pbt2 + 2H20 + 2SOnc' charged
discharged
C. Consumption of Electricity Dem. 322-Flecfrolysis
of NaCl
Kxperinzenl developed by Robert Baynard. Dem. 320-Faroday
Effect
Device developed by Denver Vlery and his pupil Christie Burke, at the U.S. Science Pavilion, Seatlle, Washington, 1962.
T o show: Movement of a lamp filament in a magnetic field. Construction of SD-320. Thc bulb is single filameut, for 110-volts. The horseshoe magnet must (-onic with a hole in it. to support the machinescrew axlc.
T o show: Products from the electrolysis of NaCI-:I+ Jfaterials: E-1 with nligration U-tube and P t electm(l~~s NaC1-aq, solid litmus, hotplate, stirrer, funnel, filter paper, 250 ml beaker, HCI-aq. Preparing concentrated indicalor solution: Boil 1 g litmus powder with 100 1111 water, stirring to dissolve, until a deep colored solution is obtained. I'ilt,ar to give a vlear, deep hlue, solution. Procedure: RIix equal volumes of t,hei~!.dicat,or ~(~luti~~t NaC1-aq, pour int,o t1.e U-tubr, iusert in 1:-1 :~nd projert. Dilute HCI-aq about tenfold with ~ r a t t :LIKI ~, odd one drop of this very dilute HCI t,othe ncg:itivc pole to t,nrn t,he solution in that. am1 re& llisert 1'1 c l c ~ trodes. 'Iurr? on currelrt,.
+
Obseinalions: In a few millutes the solution turns bluo (duc to OH-) around the negat,ivepole. .It thr positive pole CISforms, t,he HCI from it turning the litmus pink, and t,he HC10 bleaching it. Volume 43, Number 8, August 7966
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