GEORGE L. GILBERT Denison Unlversly Granville, Ohlo 43023
The Precipitation of Ferrous Hydroxide: A Lecture Demonstration Submitted by: Checked by:
0. W. L a u Chinese University of Hong Kong Shatin, N.T.,Hong Koug Erwin Boschmann I U P U a t Indianapolis Indianapolis, I N
Dissolve ahout 0.2 g uf ft!rroui nmmonium sulfate in H test tuhe in ahout 5 ml of distilled water and then add four drum of 2 M sodium hydroxide solution. An instantaneous prec&itation of the white to verv pale green ferrous hydroxide occurs. After one minute or;; a riddish-brown precipitate of ferric hydroxide (best described as hydrous ferric oxide) appears on the sides of the test tube and near the surface of the solution. However, the bulk of the precipitate sinks to the bottom of the test tuhe and the color gets darker green and some turns blue. All the precipitate turns greenish-blue on shaking the test tuhe. The precipitate settles to the bottom again in about 15 min and it now appears dark hlue. This persists for a few weeks. Ferrous ammonium sulfate should he wrd instead of ferrous sulhte because the former is more stal~leand lrss suswptible tu nir oxirlarion. Remarks Ferrous hydroxide, like other transition metal hydroxides, is insoluble in water. The insolubility of these hydroxides arise primarily from the very unfavorable entropy terms since the entropiesof solution of polyvalent ions are quite negative and the entropy of solution of hydroxide itself is also negative (1).
That ferrous hydroxide will be oxidized by air can be predicted from the electrode potential of the Fe(III)iFe(II) couple in basic medium (2).The experiment also shows that the rate of uxidation of the ft.rn,ui ion in l~asicmedium is (:onsirlerahle. hvdroxide isu,hite, hut in theoresenccofnir In fart. irun (111 . . it quickly darkens owing to oxidation (3). The awearance of the hlue precipitate is a hit unexpected to the students and this compbund-is, in fact, an example of a class of inorganic compounds which contain ions of the same element in two different formal oxidation states (4). The most obvious and striking feature of many of these so-called "mixed valence" compounds is the presence of intense absorption in the visible region of the spectrum, not present in compounds containing either valence state alone. Thus, whereas the colors of almost all Fe(II1) and Fe(I1) compounds are colorless, pale green, the color of almost all mixed valence orange, or pale . . systems simultaneouily containing FetIIl) and FetIl) ia deep hlue tu Mark, of which I'russian hlue is anuther well-known example. The experiment described is indeed a verv simple one: however, it can serve as a lecture demonstration for suih topi& as solubility of ionic compounds, electrode potentials of transition elements and their modification by formation of 474 1 Journal of Chemical Education
either an insoluble compound or a complex ion, and mixed valence compounds. Literature Cited (1) Phillips, C. S. G.,and WiUlams,R. J. P., "lnarganic Chemistry,"OIford University Plea, Oxford. 1965. Yol. 1,p. 259. (21 Day, Jr. M.C.,andSelbin, J.,"TheanticalhorganieChemistry,'.2nd &.,Van Nostrand, Reinhold Co.. New York, 1969.p. 358. (31 Catton,F.A.,a"d Wi1kin~on.C.. "Advanced lnorganieChemiJtry?3rd ed.,Interreience . Publishers, New York, 1972. p. 860. 141 EmelLur. H. J., and Shave. A. G.,(Editoral, "Advances in Inorganic Chemistry and Kadiochemistry." Academic P r o s Inc, New York, 1967, Vol. 10, p. 253.
The Variation of Vapor Pressure with Temperature Submitted by:
Checked by:
Derek A. Davenport a n d Viswanathan Srinivasan P u r d u e University West Lafayette, I N 47907 Madeline P. Goodstem Central Connect~cutState College New Britain, CT
Demonstration Provide three cans of butane lighter fluid whose nozzles have been adapted to fit inside each of three identical bicycle tire gauges.' Place one can in ice, another in a beaker of water maintained at about 50°C, the third being left a t room temperature. When temperature equilibrium has been achieved (-5 min with occasional shaking) take the pressure of each can. Lay the three tire gauges on a temperature-calibrated base-line drawn on an overhead projector. A shadowgraph (in a double sense) of the non-linear increase of vapor pressure with temperature will appear on the screen. A single butane can and a single tire gauge can also be used with the readings heing marked one by one in the overhead projection, hut this procedure is more time consuming and less dramatic. Remarks Since warehouses frequently reach close to 50°C there would seem to he no danger in heating butane cylinders to this temperature. Only at much higher temperatures do the lapped seams peel apart.' As a matter of principle a safety shield should be used. Davenport, Derek A., J. CHEM. EDUC.,53,306 (1976). Tested Demonstrations is a monthly feature designed to present lecture demonstrations and experiments in a format convenient for elassroom use. Readers interested in either submitting or checking demonstrations should contact the column editor. An outline of format requirements was given s on page 166 of the March 1976 issue of t h ~ Journal.
