SOME ILLUSTRATIONS of REACTIONS between SOLIDS H. S. VAN KLOOSTER Rensselaer Polytechnic Institute, Tmy, New York
T
HE old adage that solids do not react, as embodied in Aristotle's statement, "Corpora non agunt nisi ,fluids," has hampered progress in the chemistry of solids till well into the twentieth ceutury. The systematic study of reactims between solids goes back to the w x k of Spring' on the effect of pressure on chemical reactions and that of Sir Roberts-Austen2 on the diffusion of gold in lead a t different temperatures. An intensive inquiry into the behavior of metals and alloys, inaugurated around the turn of the
' 2. phyrik. Chem., 15, 65 (1894).
Pvor. Roy.Soc. (London), 59,283 (1896); ibid., 67,101 (1900).
century, showed that reactions in sslid alloys are of ,~ quite common occurrence. In 1909 M a ~ i n g working in Tammann's laboratory, found that compressed metal filings reacted at temperatures below those of "eutectic" mixtures. I t was soon recognized that pressure as such merely serves to bring the reacting substances in close contact. Research on solid non-metallic substances started around 1910 with the work of Cobb4 who heated mixtures of calcium carbonate and silica, calcium sulfate
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$ 2 .anorg. C h . ,62, 265 (1909).
' 3. Soc. Ckem. Ind. (London), 29,69,250,335,399 (1910).
and silica, and calcium carbonate and alumina a t tem- and pestle, one may heat the dry mixture gently over a peratures between 800' and 1000°. These substances small flame. A number of other reactions which may interacted without the formation of liquid phases. be classed as double decompositions and are suitable Cobb assumed, however, that the solid particles were for demonstration purposes, can readily be performed. surrounded by what he called "quasi-vaporous" films In order to follow the course of the reaction one may which facilitated effective molecular contact with neigh- take a pronounced change in color as proof that the boring particle^.^ Recently Taradoire6 carried out reaction is proceeding a t a perceptible rate. As a some low-temperature experiments in closed evacuated guiding principle for choosing the proper combinatubes and concluded "that a necessary but insufficient tions, we may look up the heats of formation of the condition for reactions between solids is obtained when reactants and of the resultants. If the sum of the one of the reactants has a vapor pressure." Although former is less than the sum of the latter, the reaction is it is true that the materials that Cobb used decomposed likely to proceed. This is, of course, not a very reliwith the escape of gases, H e d ~ a l lin , ~ 1912 and subse- able criterion, but may serve for want of a better one. quent years, has demonstrated that nickel oxide and Chemical thermodynamics teaches us that we should cobalt oxide react with alumina, magnesia, and stan- compare the free energies (AF) of the substances innic oxide, forming either solid solutions or compounds volved in the reaction and not the heats of formation a t temperatures which exclude the existence of appreci- (-AH). There are, however, vety few data for A F able vapor pressures. In the industries reactions in the of solids available, and in case they can be found, we solid state have been carried out for years without at- frequently do not know what their values are a t higher tracting much attention on the part of scientists. temperatures. Ordinarily the values for A F and A H Metallurgists have long been familiar with the process reported in the literature for anhydrous solids (at 25') of "malleablizing" in which cementite (FesC) is con- are not far apart, as is seen from the following 1ist.l' verted into iron and graphite. Another process, known as case-hardening, in which carbon dissolves TABLE 1 in y-iron with the subsequent formation on cooling, A P i n col. A H in cot. A F i n ad. A H in cal. of cementite and a-iron, was the subject of a recent AgCL -26.220 -30,300 Znc11 -88,260 -99,280 AgBr -22,900 -23,810 ZnBn -74,090 -78,230 investigation by Tammann and Schonert.* There are -15.170 ZnI, -49.930 -50,505 AgI -15.810 also indications that a number of ceramic fireproof pigCdBn -70,270 -75,390 ments used by the ancients were obtained by the dry method. As an example, let us consider the reaction: The whole subject of reactivity in the solid state has been reviewed recently by a Swedish author HedvalLq PhS (black) + CdSO, (white) = PbSO, (white) + CdS (yellow) + 218.0 < - '216.2 33.9 22.2 who has devoted over thirty years to this branch of chemistry. It was a matter of surprise to the writer From the values of the heats of formation1' we may to find amone over five hundred names in the index of ~ ~ d ~ book a l scarcely l ~ ~ three dozen ~ ~ ~ and l i infer ~ that h the reaction will proceed from left to right, a ~ ~authors. ~ ~~~t of r the work i on solids ~ ~ which is sustained by experimental evidence. has ~ conclusion lead sulfide "suau~ contains a small evidently been carried out by continental workers, amount of sulfur which is expeued by heating. The among whom, besides Hedvall; may be quoted Pischcadmium sulfate is obtained by heating the hybeck, Huttig, Jander, Jost, Seitb: Tammann and IP..L...,I~ drated salt CdSOP/3H20. The lead sulfide is daced Nearly all the work mentioned in the literature bas a t the bottom of a test-tube and compressed to layer been done on the interaction of metallic oxides with of about one inch deep. On top of this is placed an other oxides or with salts (halides, sulfides, silicates, intimate mixture of lead sulfide and cadmium sulfate and so forth). There are some examples of double (dull gray in appearance) -and this layer in turn is decomposition of salts, e. g., the formation of mercuric covered with a one-inch layer of cadmium sulfate. On iodide from potassium iodide and mercuric chloride. revolving the test-tube over the flame of a Bunsen This is usually shown by pulverizing the dry mixture burner the middle layer will assume a yellowish hue, in a mortar and is sometimes erroneously represented due to the formation of cadmium sulfide. For comas showing the effect of pressure on the speed of chemi- parison, a tube containing dried lead sulfate and cadcal reactions.1° Likewise, potassium iodide will r e a d mium sulfide with an intermediate mixture of the two easily with lead chloride. Instead of using a mortar undergoes no change except for a darkening of the cadmium sulfide on heating. . On cooling - it resumes its light color. 1.SOC. C h m . Ind. (London), 29,336 (1910). The following reactions, all performed in a similar Bull. Soc. Chim., 6 , 866, 1249 (1939). 7 Bn., 45, 2095 (1912); Z. anorg. allgem. Chem., 86, 201, 296 manner, may likewise be used for demonstration pur(1914). poses Z . anorg. allgem. Chem., 122,27 (1922). HEDVALL, "Reaktionsfihigkeit fester Stoffe," J. A. Barth, -
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a
Leipzig, 1938. lo Cf., VAN KLOOSTER, "Lecture demonstrations," 2nd ed., Easton. Pa., 1925, p. 58.
" BATES, 1.Am.
Chem. Soc.,61,1040(1939). In kcal., Cf., LANCE,"Handbook of Chemistry," 3rd ed.. Handbook Publishers, Sandusky, Ohio, 1939.
K B (grayish) 87.1 KnS 87.1
+ 218.0 CdS04 = CdS + K d 0 4 + 33.9 + 338.6 + Ag,S04
