edited by Denlson Unlvenlty Granville. OH 43023
Iron[ll) Oxide SIBM~TEO BY
When the iron(I1) oxide, FeO, is shaken out of the test tube it reacts immediately with the oxygen, Oz, in the air t o form the familiar red oxide of iron known as rust or rouge, 4Fe0 0 2 2Fez03,thus illustrating the instability of FeO in air. Even if air were excluded, the iron(I1) oxide would show itself to he unstable toward disproportionation to iron, Fe, and FesO4. The Fe304 is itself an intimate mixture of iron(I1) oxide, FeO, and iron(II1) oxide, Fez03; 4Fe0 Fe FesO4.
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Cllfford D. Miller Mountah Vlew College 4849 West llllnols Dallas, TX 75211
CHECKED BY
Ronald E. DiStefano Northamptml County Area Communlly College Bethlehem, PA 18017
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This colorful little demonstration illustrates a relatively easy preparation of a less common oxide of iron and demonstrates vividly why this oxide is so rare. Materials SUBMITTED BY
0.2 M FeS04, 10 mL 0.2 M (NH1)2C201,11mL 3 test tubes, 20 X 150 mm 50-mm Buchner funnel filter flask, 125 mL any grade 4.25-cm filter paper graduated cylinder (25 mL) standard as~irator standard &"sen burner standard test rube holder standard ripstula
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A Spectacular Demonstration 2H2 + 02 2H20
James F. Skinner Wllllams College Wllllamstown, MA 01267
CHECKED BY
Mel Mosher Missouri Southern State College Joplln, YO 64801
Procedure The pale green ferrous sulfate, FeS04, solution is mixed with the colorless ammonium oxalate (NH4)2CzOa,forming the yellow precipitate of ferrous oxalate, FeC204. This is vacuum filtered and rinsed with distilled water and suction dried for 5 min. A spatula full of the moist FeC204 is placed in an empty test tube and carefully heated to drive off the moisture and then the water of crystallization. When no more water dronsformon the unner .. Dart . of the test tube. the heating of the fkrrous oxalate can be increased slowly. when the nrecinitate has all chaneed color from the vellow ferrous oxaiate, ~ec204,to the no& black ferrous odide, FeO, the heat is discontinued and the FeO is allowed to cool for 2-3 min. If the iron(I1) oxide, FeO, is poured out of the test tube from some height above the lab bench, which has been covered with plain white paper (or notebook paper), the iron(I1) oxide will glow as i t falls through the air (decreased lighting in the lecture room increases the effect). I t will singe the paper, and after cooling the residue on the paper will appear as the familiar red oxide of iron we call rust (Fez03).
The title reaction has heen the basis for many excellent chemical demonstrations. While there is folklore in some departments about the one described below, i t does not appear to have been published in this Journal. The beauty of this demonstration lies in its simplicity. Its effect results from nothing more than a chemical reaction, with no gimmicks or complicated electronics necessary. The combustion chamber consists of two inverted copper cones (0.023 in. thick, in our case) soldered together around their rims. At the top is a small hole (about 3/32 in.) and a t the bottom is a second hole into a short neck (about 1in. long) of copper tubing (about 5Is-in. i.d.). The overall dimensions are not critical. A smaller cone would result in a less violent
Comments The slight excess of ammonium oxalate.. (NHJ7ClOa. . ..- - .. assures the complete precipitation of the ferrous oxalate, FeCz04,according to the reaction,
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FeSO, +(NH C O a)2 green
FeC20,
* yellow + (NH,),SO,
The thermal decomposition of the iron(I1) oxalate could be shown to be in accordance with the following equation: FeO
FeCz04-
yellow
A
black
+ CO,t +cot
The combustion chamber.
Volume 64 Number 6 June 1967
545
explosion. A college machinist or a commercial tinsmith can fabricate the device. While closing the top hole with a piece of tape or one finger, thoroughly flush the cone through the bottom hole with hydrogen gas, and close the bottom hole with a rubber stopper. Place the cone, large hole down, in a ring or metal tripod. Remove the stopper, and light the hydrogen gas at the small hole a t the top. The demonstration is suitable only in a large room (or out of doors) where one can stand back a t least 20 feet. Within about 15 s, a low whistle will precede a loud explosion. Do not he deceived into thinking that the flame has gone out prior t o the detonation. While the reaction stoichiometry is simple, the mechanism is very complicated.1~I t proceeds through a series of competing free radical steps, some homogeneous and some occurring a t the walls of the vessel. initiation
~ $ 5 . 2 ~ .
+ O,+
ZOH .
+ 0%-OH. 0-
initiation
+0..
branching
+H , d O H . +H .
branching
OH. + H , d H , O
H *unreactive
H.+ O,+HOr
+H .
propagation species
termination
less reactive species
termination
" .
The hranchinesteos are ~articularlvimoortant in that the concentration o r free radiLals increases very quickly. At eertain concentrations of Hz and OZ,the hranching steps will be
546
Journal of Chemical Education
very efficient and the overall reaction will proceed very rapidly. Under other conditions, the termination steps, eliminating H., may predominate and the reaction rate will be slow. A detailed analysisof all thesteps leads toan expression of the following form in the rate law, 1
conatant + k[H,] - k'[OJ where the denominator will vanish a t certain concentrations of Hz and 02. Initially, the HZ is reacting slowly with atmospheric Oz above the small hole a t the top of the cone. The reaction cannot occur inside the cone initially hecause of the absence of 0 2 . With further reaction, Oz (and unreactive Nz) will enter the cone through the tube in the bottom, replacing the Hz burned a t the top. At critical concentrations of Hz and 0 2 inside the cone, the branching reactions will predominate and the rate will increase quickly. I t is interesting to note that while there is a net decrease in the number of moles of gas, the transfer of chemical energy into molecular translational enerw results in an ex~ansion. The den&stration can he used effectively t o start any class "with a bang" or as part of a discussion of kinetics in a more advanced physical chemistry course. While a hydrogen-filled balloon can be exploded for effect in a classroom, the controlled nature of the demonstration descrihed here affords a more pedagogically meaningful discussion. ~
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' Eggers, D. F., Jr.; Gregory. N. W.; Halsey, G. D., Jr.; Rabinovitch,
S. Physical Chemistry, Wiley: New York, 1964: p 475. Nicholas, John. Chemical Kinetics; Halsted: 1976; p 143. 3Pannetier, G.; Souchay, P. Chemical Kinetics; Elsevier: New York, 1967; p211.
El.
