The Direct Reactions of Solids F. FEIGL, L. I. MIRANDA, and H . A. SUTER Laboratorio Central do ProduqEo, Ministerio do Agricultura, Rio de Janeiro, Brad1 (Translated by Ralph E. Oesper, University of Cincinnati)
I.
T WAS long believed that dry solids do not react both fusible and infusible inorganic reactants will be m t h each other and that a t least traces of water given in this paper. must be present if reaction occurs. However, the Numerous instances of the production of inner classic work of Hedvall (12) and later that of Jander complex salts and of color lakes by gently heating (18) and of Hiittig (17) have proved beyond doubt mixtures of the reactants are described. No previous the occurrence of direct mutual chemical action in reports of the preparation of these materials by this the solid state. These men and their collaborators method have been published. Double decompositions have explored this field systematically and have found by the dry method, involving inner complex salts, that many of these reactions can be formulated stoichio- have been accomplished in our laboratory. Notemetrically. Because of their fundamental significance worthy also are the typical redox reactions reported Hedvall's results have been mentioned in some of the here, and the 61~tcase of catalysis by the dry method.* recent texts on general chemistry. Likewise some The wide variety of reaction types that have alteachers have included a discussion of reactions in ready been accomplished in the solid state shows that the solid state, usually emphasizing the technical these are not exceptions; doubtless they are a few importance of sintering processes, or pointing out the instances drawn from a large area of chemical phegeochemical aspects of this type of reaction. How- nomena. ever, demonstration experiments illustrating this chapter of chemical change were lacking. In 1940, van REACTIONS OF MIXTURES OF SOLIDS, WHICH PROCEED Klooster (23) recommended the formation of colored SIMILARLY BY THE DRY AND WET METHOD sulfides and iodides by heating a mixture of the solid For the following experiments reactions were selected reactants as particularly adapted to performance a t that quickly go to completion when a solid, slightly the lecture table. The writers have been studying reactions in the soluble precipitant is added to a water solution of a solid state, particularly those that can be represented salt. In almost all cases the reactions in the solid by simple chemical equations. Among these are state were carried out by heating or igniting a mixture many whose occurrence can be directly established of the reactants, which by themselves were stable a t without recourse to supplementary physical or chem- high temperatures. The reaction mixtures were stirred ical tests, because a striking characteristic color constantly with a platinum wire throughout the change is seen when the mixed solid reactants are heating. This procedure practically precludes any heated. Obviously these systems are excellent for involvement of water and the ensuing reaction can didactic purposes, since they react quickly and simple rightly be considered as having occurred directly between the reactants in the absence of any solvent equipment is adequate. The clear-cut reactions studied in this laboratory or reaction medium. 1. The action of Magnesium Oxide on Anhydrous conform, without exception, to Hedvall's rule that Iron (Ferric), Manganese, Copper, Nickel, and Cobalt the reactivity of crystalline solids is increased by any loosening of the lattice. The most general method Sulfates. These sulfates lose all their water of of securing this result is to raise the temperature, crystallization when gently ignited. The resulting whereupon the solid expands. This expansion is the anhydrous salts are powders. The first three are visible manifestation of the thermal spreading of the white; nickel sulfate is yellow; cobalt sulfate is violet. ionic or molecular lattice whose rigid spatial arrange- If mixed with ignited magnesium oxide and heated, with ment characterizes the solid state. The first reported stirring, a reaction occurs. This is quite rapid with copcases of reactious between solids accordingly involved per, iron, and mauganous sulfates, and slow with a considerable increase in temperature in order to cobalt and nickel sulfates. The color change is quite secure the necessary activation and consequent re- distinct-to gray-black (CuO, Ni20a, CoeO,, MnOz), activity. The extension of this principle of thermally to brown (FezOa). The reactions correspond to the loosening the lattice of fusible and sublimable materials equations : has led to the successful realization of reactions a t CuSOd + MgO CuO + MgSO, temperatures just below the respective melt in^ or * The successful praduction of metallo-organic inner complex At such temperatures the molesublimation makes it extremely probable that reactions of purely organic cules are highly activated the is tor- salts solids (with low melting points) will likewise be accomplished by respondingly great. Examples of the reactivity of the dry method. 18
-
Fe(SO&
+ 3Mg0
-- + + Fe20a
+ MgO + 0 2Co(or Ni)SO, f 2Mg0 + 0 MnSO,
3MgS01 (14)
Mn02
general reaction is
MgSO'
C%(orNi2)Os
+ 2MgSOb
These changes are qualitatively comparable with the results obtained when magnesium oxide is suspended in water solutions of these sulfates and air drawn through or an oxidizing agent added. Of course, the hydrous oxides or higher oxides are produced under these latter conditions. In contrast, however, and as was to be expected, the thermal reaction is quantitative only if a large excess of magnesium oxide is employed. The reaction of calcium and barium oxides is entirely analogous to that of magnesium oxide, but i t must be noted that it is much more difficult to dehydrate them completely. Zinc oxide does not react with the anhydrous sulfates, which again is in line with the results obtained by treating water solutions of the sulfates with these various oxides. 2. The Reduction of Solid Insoluble Metal Halides by Metallic Zinc. The introduction of metallic zinc into a water solution of a salt of a metal that is more noble than zinc leads quickly to the deposition of the more noble metal. The deposit is usually black and pulverulent. The equation for this reduction, frequently known as "cementation," can be written: Me++
+ Zn" + Zn++ + Me'
(1)
(Me = Ag, Cu, Hg, etc.)
This equation, however, is not a perfectly correct representation of the precipitation of the metal from an aqueous solution, because the latter never contains metal ions, as such, but always the hydrated ions. Accordingly, the equation should be written:
+
Me(OH%),++ Zno + Zn(OHn).++
+ Meo
(2)
Anhydrous ions are present in dissociable metal salts only when the latter are in the solid (completely dehydrated) or molten condition. Equation (1) is valid under such circumstances. It W l y represents the action of solid metal salts (with ionic lattices) or of their melts, but i t does not represent the action of zinc with aqueous solutions of metal salts or hydrated ions. I t can be proved that the action expressed by equation (1) actually occurs. The cementation of certain solid metal salts by metallic zinc can be effected. These reactions constitute a reduction by the direct reaction of solids in the absence of a solvent. Anhydrous metal halides, not soluble in water, such as AgI, HgIz, CuzIz, TlI, and HgzC12are well suited to demonstrate this action. The dry powdered halide need only be ground with zinc dust a t room temperature; a deposit of the nobler metal and the formation of zinc halide ensues quickly. This displacement can be detected both by the change in color of the mixture to gray or black, and by the fact that the original dry powdery mass gradually becomes sticky. The zinc halide formed is responsible for the latter effect; it is hygroscopic and takes up moisture from the air. The
2MeX
+ Zno = 2Me0 + ZnXn
(Me = Ag. Cul, Hgr, H", TI.
X = CI, I)
Other baser metals, such as magnesium, can be used in place of zinc. It is remarkable that the cementation of the solid metal halides by metallic zinc proceeds even a t room temperature and a t high velocity. This is contrary to the usual rule that solids (not sublimable) react only gradually in the absence of a solvent, even when heated. Two explanations of this come to mind a t once. The first is that the zinc halide formed according to (1) attracts moisture from the air and the water of hydration then dissolves a slight portion of the metal halide and starts the ionic reaction (2). This would be a kind of "autocatalysis" or a catalytic action of water. The second, more plausible explanation is that the cementation of solid metal halides by zinc, according to (I), involves a reaction of the metal ions of the lattice. Consequently, this would be a true ionic reaction, which, like that of hydrated ions, proceeds rapidly. In contrast, most of the actions of solid materials discussed in this paper should be regarded as reactions of molecules (or groups of atoms) in molecular combination, which require a longer period of contact with the other reactant to get started. Evidence for this view is furnished by mercuric cyanide. This salt, though soluble in water, does not ionize. It accordingly undergoes no cementation when ground with metallic zinc. This is also true of AeCN and Cu2(CN)2. 3. The Formation of Color Lakes by the Dry Method. Certain hydroxides and oxides of 2-, 3-, and 4 - d e n t metals have decided adsorptive powers for some acid dyes. In many cases, the formation of the adsorption complex (which is water resistant) is accompanied by a deepening of the shade or by a color change. These adsorption complexes are known as "color lakes." Such systems are produced by the oxides or oxyhydrates of magnesium, aluminum, and beryllium with certain azo dyes, and with dyes derived from alizarin. Diphenylcarbazide and diphenylcarbazone (derivatives of urea) produce color lakes with MgO. This formation of lakes is the basis of sensitive tests for these metals (5). It can be shown that certain dyes, which, in water or alcohol solution, react with metal oxyhydrates, or which form color lakes when the oxyhydrate is precipitated in the presence of the dye, can also react in the solid state with the anhydrous oxides. In this connection, i t is worthy of note that those dyes which are sublimable or which melt a t comparatively low temperatures are particularly capable of forming lakes rapidly by the dry method when heated with oxides of divalent metals. The product is formed a t temperatures below the sublimation or melting point of the dye. It appears that lake formation with compounds that have no dyestuff character conforms to this same rule (see Table 1).
-
The formation of lakes hy the dry method is especially rapid with ignited MgO and BeO, whereas only slight quantities are produced by ignited AlzOs. The influence of the melting point or sublimation temperature is plainly visible in the behavior of MgO and Be0 toward the following lake formers.
direct action of solid dimethylglyoxime and an insoluble nickel salt. Anhydrous nickel cyanide is suitable for this purpose. Hydrated, insoluble nickel cyanide, Ni(CN)a 4He0, in water suspension, slowly forms the characteristic red salt when warmed with an alcuhol solution of dimethylglyoxime. When the blue-green hydrated TABLE 1 nickel cyanide, produced by precipitation, is heated Mc11ing P0i"l. Lake Formalio" to 180" to 200°C. i t loses all the water, and the anhyorgenic L ~ ~ Po r m ~w OC. A,,e, ~ l i z a r i n( ~ , z - d i ~ ~ d ~ ~ ~ zoo ~ ~ ~ t h ~ ~ ~z500.c, ~ i ~ drous ~ ~ ~ salt ) is left as a brown-yellow powder. Neither Purpurin (1.2.4-trihydroxyanthraquianhydrous nickel cyanide nor the salt dried a t 120°C., none) 256 Positive at 200°C. which still contains a little water, reacts with dimethylQuinalisarin (1,2,5.&tetrahydroryanthraquinone) 275 Positive at 200DC. glyoxime a t room temperature. If,however, the former Positive at 150DC. 0-Nitrobenzeneazo-=-naphthol 234 ~ i ~ h ~ ~ ~ ~ ~ ~ 163 ~ b ~pe.itiv..t ~ i 1d 5 0~~ ~ is . heated to about 200°C. and warmed dimethylglyDiphenylcsrbamne 157 at 1400C. oxime is stirred in, the red Ni-dimethylglyoxime forms Titan yellow Decomposer above 350 Ncgative st 25O0C almost a t once. Likewise, a mixture of dry, hydrated nickel cyanide, Ni(CN)a4H20, and dimethylglyoxime The rapid formation of lakes by the dry method reacts when warmed. In this case, i t can he seen with these hydroxyanthraquinones, which can be plainly that the reaction occurs only after the desublimed, or which have relatively low melting points, hydration of the blue-green nickel cyanide (change to and with diphenylcarbazide and diphenylcarbazone, yellow-brown). The following reaction has taken is doubtless due to the following circumstance. The place between the solid reactants in the absence of a temperatures employed are high enough to loosen or solvent: distort the molecule of the organic lake former suffiHON=C-CHI ciently to permit rapid adsorption on the solid oxide. I + Ignited Ti02, 2102, and Tho2 mixed with alizarin, Ni(CN)z HON=C-CHs purpurin, or quinalizarin show considerable lake forHO OH mation a t 250°C. or 200°C. within several minutes. I I Conseqnently, the acid-resistant oxides of the quadHIC-C==N\% .N=C-CH, rivalent metals behave exactly like the oxides of I 'Ni.' 1 + 2HCN H~C-C=N/ \N=c-CH* beryllium and magnesium, which are easily soluble II in acids. On the other hand, ignited A1203 (as already 0 I' 0 stated) is strikingly inactive. This is in contrast to The foregoing production of red nickel dimethylthe formation of lakes by the wet method, which, without exception, takes place a t once with the oxy- glyoxime is one of the most impressive and illuminating examples of the reactivity of solid reactants. I t is hydrates of the di-, tri-, and quadrivalent metals. The color lakes produced by the dry method some- also the first instance of the formation, by the dry times exhibit a shade differing from that of the ma- method,of aninnercomplexcompound. terial formed by the wet method. The covering The rapid formation of Ni-dimethylglyoxime from power of the "dry lakes" was found to be satisfactory. solid nickel cyanide and solid dimethylglyoxime 4. The Formation of Inner Complex Compounds obviously is due to the fact that dimethylglyoxime by the Dry Method. The production of color lakes by melts a t 235'C. and therefore is "activated" a t the the dry method showed that low-melting or sublimable temperature (180°C.) of the experiment. This leads organic compounds can form these systems a t even to the expectation that the dry method should be comparatively low temperatures with the oxides of successful for producing inner complex compounds di- or quadrivalent metals. Color lakes are not from other low-melting or sublimable complex-forming stoichiometrically defined compounds, hut the lake organic compounds. In fact, it has been possible to formation doubtless depends on the ability of the deform the inner complex compounds shown in Table oxides and the organic components to form inner com- 2 by heating a mixture of the solid reactants. Among these instances, the formation of mercuric plex salts. Consequently, it is proper to regard color lakes as mixtures of oxide (or oxyhydrate) with vary- diphenylcarbazide is particularly remarkable, because ing quantities of inner complex salt. This points to this double decomposition cannot be accomplished the possibility of producing complex, and particularly by the wet method, that is, by bringing together a inner complex compounds with definite formulas by solution of mercuric cyanide and an alcohol solution the dry method. Organic complex formers which are of diphenyl carbazide. This is an example of the sublimable or which melt a t a low temperature would reactions to be discussed later, which can be achieved seem to be the logical starting materials. In fact, by the dry method only. 5. Double Decomposition of Inner Complex Salts. it has now been found that the classical example of an inner complex salt, namely, red nickel dimethyl- The experiments described in 3 and 4 showed that cerglyoxime, can be made by the dry method through the tain color lakes and inner complex salts, which are
..
+
'
Cu-bex~zoinoxime Cu-rubcanate Pd-dimethylglyoxime Ni-rubeanate Ni-furildiorime Zn-difhironate Pb-dithizoaate Ag-9-dimethyhminobbbzylidbbcrhhdadiii Hg-dipbenylcnrbadde Boric cstcr of quinalirPrin K-dipierylamine
,Green
(7)
Black-violet (20) Yellow (24) Violet (20) Red (22) Red (10) Red (10) Red-violet (6) Violet (3.9) Red-violet (6) Orange-red (19)
A99raxitnalc Tm9rrolurc. 'C.
++
benzoinoxime (m. p. 163'C.) CuSO, (anhydrous) rubeanie acid (aubl.) Pd(CN). dimethylglyorime (m.p. 23S°C.) Ni(CN)r (anhydrous) rvbeanic acid (subl.) Ni(CN), (anhydtous) furildiorime (m.p. 248'C.) ZnO dithiron (m.p. 150°C.) PbCh difhizon (m. p. 150DC.) ApCN 9-dimethyl-inobenryliden~hbdddiii (m. p. 240°C.) HdCN)n diphenylesrbaride (m. p. 163'C.) B.0, quinnlizarin (m.p. 275-C.) KC1 dipicrylamine (m.p. 238°C.)
CuSO. (anhydmus)
+
++ + + +
formed rapidly in solution, can also be produced by the dry method by heating a mixture of the inorganic metal compound and the appropriate organic complexformer. These successes led to the expectation that inner complex salts would exhibit similar reactivity under these conditions. For instance, i t was expected that they could be transposed into other inner complex salts, or that they would acquire solubility in acids. Trials demonstrated that certain inner com~lexsalts of nickel, copper, cobalt, and zinc are transformed into more stable inner complex salts if they are heated, in the absence of a solvent, with stronger complexformers. Furthermore, i t has been found that inner complex salts can be converted into the corresponding oxalates by the dry method. It is merely necessary to heat the salt with anhydrous oxalic acid. Transpositions shown in Table 3 were accomplished by heating a mixture of the anhydrous reactants.
++
+
150 150 200 150 150 120 120 200 150 250 210
Anhydrons oxides, carbonates, and sulfides of other metals likewise are not affected when heated with anhydrous oxalic acid. 6. T h e Action of Manganese Dioxide o n Insoluble Metal Iodides. The addition of manganese dioxide to acidified solutions of metal iodides releases all of the iodine : MnOp
+ 21- + 4HC
-
Mn++
+ 2 8 0 + 11
Neutral solutions react in the same way, but part of the iodine is oxidized to hypoiodite and iodate. Water suspensions of insoluble metal iodides (AgI, HgIz, PbIa, CuzIz, TlI) do not react uniformly toward manganese dioxide. Those iodides that are fairly soluble in water (PbIa, TlI) produce considerable quantities of iodine when the suspension is boiled with manganese dioxide. AgI, HgIz, and Cuz4 remain unchanged under these conditions. The action of
TABLE 3
Product
Rcodontr
Red Red Red
Color
++ + + + +
Violet
Yellaa Salmon Bright green Yellow Red Red Salmon B r h h t meen yelionGreen Blue-green
Green Green Violet Red White
Yellon-green Salmon White Red
violet Bluc Blue Blue Bluc Violet Violet
Dimeth~lglyoxime Ni-oxime (I) Dimcthylglyoxime Ni-benzoinorime (7) Dimethylglyoxime Ni-ralicylaldoxime (4) Rubeanic acid Ni-mime (I) Oralie acid Ni-dimethylplyoximc O r d i c acid Ni-furildiorime (22) Oxalie acid Ni-benroinorime (7) Oxalic acid Ni-salievlaldorime (4)
+ +
~~~~~
The changes are easily recognizable through the altered color. Rapid and complete reaction occurs in all cases a t 150' to 200°C. It is rather remarkable that the foregoing double decompositions proceed quickly and to completion a t temperatures as low as 150' to 200°C. A prime factor obviously is that all these reactants melt or sublime a t relatively low temperatures (below 300°C.). Consequently the intermolecular linkages of the solid phase are weakened and the reactants are activated a t the temperature employed. This hypothesis is supported especially by the ease with which copper and nickel oxalate are formed from the inner complex salts of these metals. Inorganic, acidsoluble compounds of nickel and copper, which without exception can be activated only a t higher temperatures, do not metathesize with oxalic acid. They remain unaltered, while the oxalic acid sublimes away.
Green
MnOz is much more energetic when a mixture of the dry materials is heated to about 370°C. All the iodides, except one, react rapidly and give off iodine. (The exception is HgIz which sublimes and thus escapes from the field of action.) Consequently, the reactions Me12
+ Mn02
-
MnO
+ Me0 + In
(Me = Ag. Pb, TI,Cur)
have been accomplished in the absence of a solvent. In the cases of PbIa and TlI, the behavior corresponds exactly to the action of the wet method. With AgI and CneIz, the action, however, can be accomplished solely by the dry method. It is probable that the energetic attack of solid Mn02 on solid iodides a t elevated temperatures is
related to the fact that MnOa, when heated above 540°C. is known to decompose:
+0s
3Mn01 -r MnlO,
These equations are only summations. The actual course of the'reaction is as follows: On ignition Mn01 produces black-brown Mn80d:
-
The temperature (370°C.) used here is not sufficient 3 M n 0 ~ MnsO, O1 to bring about this decomposition or only to a very slight extent. However, this suffices to "excite" the The latter oxide behaves as though i t were an adMnOz so that it reacts more rapidly on contact with dition compound 2MnO.MnOz. Accordingly, the WOs solid iodides. (or Mooa) extracts the MnO, which of itself is not heat-stable: REACTIONS THAT OCCUR ONLY IN MIXTURES OF THE SOLID REACTANTS
This class of reactions includes particularly such changes as cannot be achieved by the wet method because of the slight solubility in water of the reactants and reaction products. Most instances are addition reactions, that is, the direct union of the reactants to form salts. The great influence of heat is noteworthy when addition reactions are accomplished in the dry way. It is frequently necessary to use strong ignition to achieve the nnion, particularly of reactants that withstand such temperatures. On the other hand, low-melting or sublimable compounds combine a t strikingly low temperatures. 7. The Union of Solid Acidic and Basic Oxides. An excellent example of the fact that materials react in the same manner whether they are dissolved or in the solid form is furnished by the union of acidic and basic oxides to form salts (neutralization). However, some acidic and basic metal oxides are so insoluble that salt formation is not possible by the wet method. Such cases demonstrate forcefully the significance of the reactivity of solid materials because such oxides do not form salts except when a mixture of the two oxides is heated. In other words, the dry method is the only feasible procedure. Perfectly anhydrous oxides can be used; extensive union can be achieved by heating a t 600' to 800°C.-temperatures that are below the melting points of the oxides and of the salts produced. This type of "salt formation by sintering" is shown in a striking fashion if the color of the salt differs from that of the mixed oxides. The following reactions, which are accomplished by brief ignition of a mixture of the solid oxides, are pertinent examples: WO, (y) WOs (Y) WO, (y) V20, (b)
----
+ ZnO (w)
ZnWO4 (w)
+ CdO (b)
M ~ W O I(w) CdWO, (w)
+ MgO (w) + ZnO (w)
+ PbO (y) Moos (w) + CdO (b)
Mooa (w)
(w = white;
r
=
Zn(VOd2 (w) PbMo01 (w) CdMoOd (w)
yellow; b = brown)
The reaction of MnOz with WOJ or Mooa is particularly striking. When the chocolate-brown mixture of these oxides is heated and stirred it becomes almost white because of the formation of MnWOd or MnMoOa:
MnO
+ WOI
+
-
MnWO,
The residual M n 0 ~again decomposes with evolution of oxygen. In the presence of an excess of the acidic oxide the consumption of MnO and decomposition of MnOe continue until all of the manganese has been converted into white MnwO4 or MnMoOd. 8. The Union of Oxides of Divalent Metals with
Pyrophos$hates or Pyroarsenutes of Dimlent Metals. The experiments described in 7 demonstrate that salts can be formed in the absence of a solvent by the direct union of solid insoluble oxides. Certain solid insoluble pyrophosphates and pyroarsenates likewise unite with basic oxides and thus simulate free acidic oxides. The action can be seen easily if a mixture of pyrophosphate, or pyroarsenate, with a colored metal oxide is heated and stirred. The color of the oxide disappears rapidly when an excess of the pyrosalt has been used. The union of magnesium pyrophosphate and cadmium oxide can be represented:
(white)
(brown)
(white)
The corresponding white zinc or manganons salts can be used in place of Mg2P20, or MgzAsz07. PbO, PbOz, MnOz, COO, and CuO react similarly to CdO. In the case of COO and CuO the ignited mixture is not colorless but assumes the color of the cobalt or the copper salt, respectively. The union of divalent metal oxides with the pyrosalt is observed most clearly when a chocolate-brown mixture of MnOz and pyrophosphate (or pyroarsenate) is heated. The MnOe is converted into MnsOa and the latter furnishes MnO to form the white manganous salt. The residual MnOe is again transformed into MnaOl, as described in the preceding set of experiments. Hedvall and Henberger (15) have shown that alkaline-earth oxides unite with pyrophosphates. These reactions, in contrast to the experiments given here, cannot be established directly through a color change.
It is noteworthy that a yellow mixture of equivalent quantities of ZnzPnO7and CdO, for instance, entirely loses its color after 30 minutes' ignition and stirring. Consequently, complete reaction has occurred to form the tertiary phosphate. Tertiary phosphates and arsenates of divalent metals can thus be obtained from pyrophosphate and pyroarsenate by "sintering." These products cannot be prepared by the wet method. The general reactions are
+ Me'O +
MezPaOl
M e l A s ~ h Me'O
--
Me9Me1(PO& MeMe'(AsO&
The oxides of trivalent metals are without action On pyrophosphates and pyroarsenates under the conditions used in the present experiments. 9. Formation of Silicates by the Dry Method. The union of insoluble metal oxides and silica, because of the insolubility of the reactants, does not take place a t all by the wet way or only a t an immeasurably low rate. However, i t is well known that this union is accomplished easily in melts. A familiar instance is the preparation of glass. However, silicates can be formed by merely heating a mixture of the metal oxide and silica, since the union occurs, in part a t least, a t temperatures below the fusion point. This can be demonstrated simply in the case of brown cadmium oxide (m. p. 1000°C.) and yellow lead oxide (m. P. 900°C.). When one of these oxides together with an excess of silica is heated in a porcelain crucible over a free flame until the crucible becomes redhot (about 700°C.), silicate is formed in about 1 hour. The brown-yellow or yellow mixture becomes colorless since CdSi03 and PbSiOa (or the corresponding polysilicates) are colorless (13). Higher melting oxides such as COO (m. p. 1800°C.), NiO (m. p. 2000°C.), and Mn301 (m. p. 1560°C.) exhibit no analogous action. l-he reason for the diverse behavior of the metal oxides obviously is that a temperatureof 2000 to 3 0 0 0 ~ .below the melting point is enough to or loosen the oxide molecules of the lower melting CdO and PbO so that they unite with the Si02 molecules. In contrast, the temperature(700°c.) used here is not to attain the same effect with the more difficultly fusible COO, NiO, and sicate formation cannotbe eapected in these cases until temperatures are reached that lie closer to the respective melting points (16). 10. The Trans#osition of Lead Sulfate and Barium carbonate. A double decomposition between lead sulfate and barium suspended in water can be expected because barium sulfate has the solubility product of the four concerned. strictly speaking, it is the dissolved of these salts that react: PbSO,
+ BaCOs
-
PbCOl
+ BaSO,
This transposition cannot be established directly, since neither of the initial materials nor either of the products is soluble in water, and all are white.
If, however, a mixture of finely pulverized lead sulfate and barium carbonate is heated to a dull red, with stirring, in a porcelain crucible, a reaction can be observed easily because the mixture turns bright yellow. The color is due to the fact that the lead carbonate formed decomposes into COa and yellow PbO a t even 315°C. On the other hand, BaCOJ is partially decomposed a t this temperature into Con and BaO. Consequently, PbO results not only from the primary reaction, but also from: PbSO,
+ BaO
-
Bas01
+ PbO
11. The Acceleration of the Onidative Decomposition of cuprous~ ~ by zinc d oxide. a ~ men white cuprous iodide is heated in the air, iodine is liberated and black cupric oxide is formed: Cud,
+ 0,
-
Xu0
+ Is
(3)
This oxidative decomposition is exceedingly slow a t moderate temperatures. For instance, when cuprous iodide is kept a t 250°C. for 5 hours, the white color is not altered and practically no decrease in weight can be found. On the other hand, if a white mixture of cuprous iodide and zinc oxide is heated to 250°C., a distinct gray, that gradually deepens to a black on stirring, will be seen within a few minutes. (One gram of Cud2 lost only 0.8 mg. when kept a t 250' for 5 hours. In contrast, a mixture of 1 g. CUZIZ and 1 g. ZnO under these same conditions lost 75 mg, The corresponding losses after 1 hour were 0 mg. and 19 mg.) If the gray or black product is digested with water, the filtrate contains neither I- nor Zn++ ions. Consequently, the reaction does not take the expected course: Cul12
+ ZnO + 0
-
2Cu0
+ Znb
(4)
Obviously zinc oxide accelerates reaction (3), namely, the action of atmospheric oxygen on CuA. It is likely that the hastening of the oxidative decomposition of cuprous iodide is due to the fact that both CuzIz and ZnO are excited by the rise in ternPerature. Evidence for this is furnished by the familiar observation that these white compounds turn yellow when heated; the color fades on cooling. This is an interesting case of catalysis in which both the catalyst (ZnO) and one of the participants in the catalyzed reaction (CunIz) are solids. All known cases of catalysis in homogeneous systems are intermediate reaction catalyses, that is, they involve the intermediate formation and action of compounds of the catalyst. If the hastening of the oxidative decomposition of cuprous iodide by zinc oxide is formulated as an intermediate reaction catalysis, the following sequence of reactions appears possible. The primary reaction (a) assumes the labile union of oxygen with ZnO (moloxide formation). In the secondary reaction (b) the moloxide reacts with Cuds, forming CuO. The ZnO is regenerated and the iodine is liberated. Summation of (a) and (b) gives (3), which represents the
oxidative decomposition of CuzIa, and in which the catalyst does not appear as a reactant:
CUJ*
--
+Cu&~ n+0 01 . 0 ~2 ~ u 0+?no + I? (b) 2Cu0 + 4 (a + b) = (3)
As a rnle, minimum quantities of a catalyst suffice to raise the reaction rate in homogeneous systems, particularly in solutious. In the present case, considerable quantities of zinc oxide are necessary to accelerate the oxidative decomposition of Cn21z. The reason is that the reaction sequence, just given, requires not only the formation of a moloxide of zinc oxide (a) but also contact of the moloxide with solid Cudz (b). However, in solution or in gas reactions, both, or a t least one, of the reactants have a mobility that is lacking here. The consequence is that larger quantities of catalyst are required in the reaction of a mixtdre of solids. 12. The Action of Selenium on Mercuric and Silver Cyanides. Among the reactions that can he accomplished only by heating a mixture of the solid reactants is the action of free selenium on the cyanides of mercury and silver. The reactions are Hg(CNh 2AgCN
--
+ Se
+ Se
HgSe
AglSe
+ (CNh
+ (CN),
-
HgSe
+
2T1I (yellow)
Evidence for this sequence of actions is provided by the following observation. If the mixture of mercuric cyanide and selenium is heated to 250°C. more raddlv. the material puffs up and forms masses quite like the Pharaoh's serpents that result when mercury thiocyanate is heated. An analogous transformation of Hg(CN)z into HgS can be accomalished hv heatinz a mixture of mercuric cyanide and sulfur to .about At this temperature, in contrast to the action of selenium, the reaction is not between two solids, but molten sulfur is involved. 13. The Double Decomfiosition of Thallium Iodide and Silver Chromate. If thallium iodide and silver chromate are suspended in water and kept a t the temperature of a boiling water bath and shaken occasionally, no noticeable change in the color of the
.
2 .
-
+ T12Cr04
2AgI (yellow)
(yellow)
which might be expected to occur because of the very low solubility of AgI, proceeds hardly a t all. On the other hand, a mixture of the dry salts heated for a short time a t 250°C. and stirred occasionally shows a very decided lightening of color. This indicates the conversion of AgzCrOd into AgI. Heating a mixture of the solid reactants therefore brings about a much quicker transposition than occurs in water suspension, where a reaction can take place only through the fractions of the reactants that are in solution. This d3erence may be due to the fact that TI1 (m. p. 431°C.) is activated a t 250°C. and so acts rapidly when it comes into contact with silver chromate. Thallous iodide is among those compounds whose color deepens considerably when they are heated (2). It is bright yellow a t room temperature and hrick-red a t 250°C. The original color returns on cooling. 14. The Union of Mercuric Iodide wilh Cuprous Iodide or Silver Iodide. When white insoluble cuprous iodide is added to solutions of mercuric salts, a red crystalline precipitate is formed:
+
+ (CNh + Se
~Eo'c.
AgzCrO, (red)
2 C u d ~ HgC+
The formation of the selenides is easily discernible because the gray mixtures of selenium, with the respective cyanides turn deep black even after only a brief heating a t 100' to 180°C. The rapid reaction is probably due, in part, to the fact that the reaction temperature is not much below the melting point of mercuric cyanide (275'C.), silver cyanide (325'C.), and selenium (220°C.). Consequently, excited or loosened molecules of the reactants are available. Furthermore, it is likely that the first step consists of addition of selenium to form selenocyanide. This then decomposes: Hg(CNSe)n
suspension can he seen even after several hours. Consequently, the reaction
-
HgCu2L
+ Cu++
The product has the composition just given or i t may be the isomeric compound Cu2Hg14. The complex mercuric-cuprous iodide becomes black when heated to 75OC.; the original red color is resumed on cooling. Silver salts and potassium mercuric iodide also give a complex iodide: zAg+
+ HgI4--
-
Ag,HgI,
This compound turns deep red a t 45°C. and recovers its yellow color on cooling. These and other temperaturevariable double iodides are rather well known. The reversible color change is probably due to a shift of the atoms on heating and cooling. Both HgCnzIl and AgaHgIa can be considered as addition compounds of insoluble metal iodides, which by themselves are stable. Consequently, it is reasonable to expect that the solids would react
and produce the same temperature-variable double iodides that are formed by the wet method. The following experiments bear out this expectation. The union of the solid iodides is accomplished within a few minutes a t even the temperature of boiling water. The rapid formation of the double iodides by the dry method probably rests on the fact that HgIz melts a t 253O and has a transformation point (red$yellow) a t 130°C. Consequently, a t the temperature used here, its crystal lattice is loosened enough to permit a rapid union with other iodides.
Some Experiments with Primary Cells in Silicic Acid Gels OYCO F. STEINBACH City College of Nelo York, New York
HE STUDY of primary cells can be nicely supTplemented by preparing them in a medium of a silicic acid gel. The results obtained are interesting and instructional. The familiar lead tree, which is generally obtained by pressing a piece of zinc or tin into the top of a silicic acid gel containing lead salt, can be obtained in 24 hours from the primary cell ZnlZnS0,11Pb(N03)nlPb. It seems to the author that the principle involved in the formation of the lead tree is more clearly illustrated by the primary cell than by the metho6 generally used. In an analogous fashion, it is possible to obtain a tin tree from the cell Zn ZnS0411SnClslSn. The following method was used to prepare these simple primary cells. Sodium silicate solution (sp. gr. 1.05) was mixed with an equal volume of acetic acid prepared from 60 ml. of the glacial acid diluted to 1 liter. The resulting silicic acid solution was divided into 20-ml. portions and 1 to 2 ml. of concentrated solutions of zinc sulfate, stannous chloride, or lead nitrate were added to the individual portions. The solutions were then poured into a U-tube in which a cotton plug had been placed in order to keep the catholyte and anolyte from freely miximg. After the gel bad set, strips of zinc and lead or tin were placed in their respective solutions. When the completed cells were tested with a galvanometer, a distinct deflection was obtained. The electrodes were then connected to each other (short circuited) and allowed to stand this way until the lead or tin trees had formed. The trees may be preserved by removing the electrodes and placing corks in the U-tubes. The Daniel cell, ZnlZnS0411CUSOrlCumay be prepared as another example of a primary cell. However, a gel must be prepared that is more acid than the above gels if a reasonable appearing copper tree is to be obtained with any rapidity, though copper is still deposited in the less acid gels. Another cell, which is also of the replacement type, is FelFeC1311CUS041Cu. The copper tree formed in this cell is somewhat superior
to the one formed in the Daniel cell. The last two cells are prepared by mixing equal volumes of 3 N HzS04 and sodium silicate solution of sp. gr. 1.16. Then 1 to 2 ml. of concentrated solutions of ferric chloride, copper sulfate, or zinc sulfate are added to 20-ml. portions of the silicic acid solutions. The solutions are poured into U-tubes as previously described and when the gels have set, the corresponding metal electrodes are placed in them. The cells may be tested by a galvanometer to show that they produce electrical energy and also to determine their polarity. The electrodes should then be short circuited and the cell should be observed over the period of a week as the copper tree forms. Upon removing the zinc or iron electrode, visual evidence of corrosion and solution can be distinctly observed. Closer inspection of the gel near the copper electrode will show that the gel is perceptibly less blue than portions further removed from the electrode. This is evidence of the diffusion and migration of cupric ions in the gel due to the cell reaction. The above cells may be modified by replacing the copper cathode by an inert electrode such as graphite. Upon short circuiting the electrodes, the graphite becomes copper plated a i ~ dlater a copper tree will form. Various combinations of metals and their salts may be used. For example, the cell MglMgS0411Pb(N03)2/Pb, when prepared in the gel of silicic acid made from acetic acid, produces a small lead tree in one hour while in three hours a fair-sized tree results, and in 24 hours a very dense tree is formed. Likewise, the cell MglMgSO,IISnC1$n was also prepared from the gel made from sodium silicate and acetic acid. In two hours a small tin tree was observed while in two days a good-sized tree resulted. A variety of primary cells have been prepared in gels of silicic acid. Trees of various metals have been obtained. The formation of the metal tree is clearly shown to be an electrochemical process.
THE DIRECT REACTIONS OF SOLIDS (Continued from Page 24) LITERATURE CITED
Bmc. "Die analytische Verwendung von Oxychinolin (Oxin)," Enke, Stuttgart, 1938. BLANK,J. CHEM.EDUC..20. 171 (1943). CAZENEUVE. Compt. rend.. 131, 346 (1900). EPHRAIM, Bn., 63, 1928 (1930); 64, 1210 (1931). FEIGL,"Qualitative Analysis by Spot Tests," translated b y Matthevs, 2nd ed., Nordeman Publishing Company, New York, 1939, pp. 115. 118, 119, 142. FEIGL, "Specific and Specml Reactions," translated by Oesper, Nordeman Publishing Company, New York, 1940, P 4 1 ; see also ( 5 ) , p. 224. FEIGL.Bn..56, 2083 (1923). FEIGL,Z. a d . Chem., 7 4 , 380 (1928). F E ~ AND L LEDERER,Monatsh., 45, 63 (1924). FISCHER, Z. angeu. Chem., 42, 1025 (1929). FUNKAND DITT.Z. anal. Cham., 93, 241 (1933).
32
(12) HEDVALL, "Reaktionsfiihigkeit fester Stoffe," Barth, Leipzig, 1938. This includes a comprehensive review and a good bibliography. AND ELDH.Z. anorg. C h m . , 226,192 (1936). (13) HEDVALL AND H E ~ E R G Eibid., R , 128, 1 (1923). (14) HEDVALL AND HEUBERGER. ibid., 135, 49 (1924). (15) HEDVALL AND SCHILLER, ibid., 221, 97 (1935). (16) HEDVALL (17) HUrr~o,see bibliography in (12) and indexes of Chemical A bdracts. (18) TANDER. see biblioeraohv in (12) and indexes of Chemical (19) (20) (21) (22) (23) (24)
