XXXIV. -React ions between Solid Substawcs.

By LESLIE: HENRY PARKER.

IN a previous communication by the author (T., 1914, 105, 1504), itt has been showii that under certain conditions it is possible to bring about interaction between various pairs of solid substances by means of shearing stress, even at ordinary temperatures, or a t least very greatly to increase the velocity of these reactions above that’ which normally pertains a t these temperatures. The experi- nieiits described in that paper led the author t o the conclusion that shearing stress, such as could be applied by hand between a pestle and mortar, is widely different in its effect+s from simple pressure, and that one of the main reasons why i t is able to bring about reactions between apparently solid substances is that local or surface fusion of the reacting substances is occasioned-

This phenomenon seemed of sufficient interestl t o warrant further study, as it is a well-known fact; that salts in the fused state are capable of reacting together.

I n considering the conditions under which solid salt pairs are capable of reacting, it has t o be remembered that most of the ordinary salts melt a t a temperature that is high compared with that a t which one commonly works with these substaiices ; more- over, that a slight rise in temperature very often occasions a very rapid increase in the reactioii velocity between two substaiices. From this it follows that the readiness with which two salts react in the fused state may be due to the influence of one or both of the following factors, namely, (1) the high temperuture necessary

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PARKER : KEACTIONS BETWEEN SOLID SUBSTANCES. 397

to bring one or both of the salts into a state of fusion, or (2) the existence of one or both of the salts in the Zipz~id state , and the intimate contlact occasioned thereby.

The object of the presertt investigatiou was t o attempt to gain som8 idea of the relative importance of these t w o factors, because iE high temperature is the main influence in causing two salts to react, some further agency must be loolted for than mere fusion when reactions are brought about under shearing stress. I f , on the other hand, the existence of the liquid state is the predominating influence, there appears to he no reason why salts should not react with appreciable velocity a t ordinary temperatures should one or more of them be brought< into a state of fusion, however transitory.

An iuvestigation was therefore commenced on the velocity of reaction between various saltl pairs a t temperatures u p t o and through the melting point of: the mixtures. The form of the curve obtained. by plotting velocity of reaction against temperature should give iiif ormation on the point desired.

Let us suppose that’ the temperat’ure is the main influence in accelerating the velocity of’ reaction. The latter will go on in- creasing as the temperature rises until the melting point of the mixture is reached. A% this point, however, them should be no marked increase in velocity, as we have assumed the existence of the liquid state to be of secondary importlance. The curve should theref ore exhibit 110 discontinuity a t this temperature, but pass smoothly through the melting point.

On the other hand, let us suppose that the existence of the liquid state is of greater importance to the progress of the reaction. There may be, and probably will be, an increase of reaction velocit,y between the two mixed salts while still both iii the solid state. J”17he~~ the point of fusion of the mixture is reaclied, however, there will be a sudden increase in the reactioii velocity, and the curve will exhibit a sudden break at this temperature.

E x P E n I M E N TI A I..

,Sodiu I ~ L C’artr oiiut c LI ti d Bar L‘iEtri Sidirhat c . The interaction between these substances has already been in-

vestigated under shearing stress, as described in the previous paper

10.6 Grains O F sodium carlmnatc, prepared froiii purified sodium hydrogen carbonate and dried by Imlonged hetttirig, were mixed with 23.34 g r a m (equivalent quantity) of bar.ium sulphate, pre- cipitated from purified barium chloride, arid dried by heating in a current’ of dry air. The two salts were placed in a dry, airt ight

(ZOC. cit .) .

R 2

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398 PARKER : REACTIONS BETWEEN SOLID SUBSTANCES.

bottle, stirred with a glass rod, and shaken for fifteen minutes to mix them thoroughly. An arbitrary sample of this mixture was then taken for analysis.

A quantity equivalent t o 1.06 grhnis of sodium carbonate (3.394 grams) was weighed out, treated with cold water, and lixiviated through a specially prepared Gooch crucible, the filtrate being treated with N - and W/ 10-hydrochloric acid, as previously described (Zoc. c i t . ) . An amount of acid was neutralised corre- sponding with exactly 1.06 grams of sodium carbonate, showing that the original mixture was homogeneous and not affected by water. If sufficient' care were taken t o ensure that' the salts were in a fine state of subdivision, a i d that they were stirred (not. ground) and shaken together sufficiently, this method of sampling gave a similar result with many other experiments. No variation in the composition of the mixture in various parts was therefore to be feared, especially as only sufficient mixture was made up a t any one time for nine experiments and one sample.

A nickel boat was then constructed to accommodate, when about half full, 3.394 grams of the mixture. This was heated t o various temperatures for various lengths of time in an electric furnace, constructed by winding about 900 cia. of nichrome wire (4.1 ohms per metre) on 20 cm. of opaque silica tubing of 4 - 4 cm. internal bore. This was insulated with about six turns of asbestos paper and two turns of asbestos cloth. The ends were closed with asbestos card, and the whole was connected with a 110-volt lighting circuit, when suitable temperatures, as registered by a thermo- element, could be obtained and adjiisted by variable resistances.

The procedure f o r each experimentl was to heat the furnace to the required temperature, open one end, quickly insertl the boat

TABLE I. Time in

minutes. 30 60 90

120 150 180 210

.? 5

10 30

Sodium carbonate transformed. Per cent. -----I------ J---_---- - I - - -

105" 210-215" 310" 360" 400" 600" 600" 6SO-685a 0-55 0.48 1-07 0.96 1.37 8.42 30.5 48.1 1-37 0.30 1.31 1-25 2.27 11.7 34.4 55.8 - 1-19 1.97 2-26 3.34 11.1 37.6 57.6 - 1.37 2.62 - 3-64 13.1 40.6 58.8

0.55 - 2.98 - 3.94 13-2 44.0 - - 1.79 - - - - - -

1.37 1.79 - 2.86 3.76 17.4 46.3 60.6

700-720° (fusion). 60.0 60.6 60.0 59.2

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PARKER : REACTIONS BETWEEN SOLID SUBSTANCES. 399

containing the mixture, so adjusting its position in the middle of the furnace on some silica supports that it was clear of the walls of the furnace and almost touching the thermo-element, and close the furnace for the required time. At. the expiration of a definite

period, the boat was withdrawn and the residual sodium carbonat6 was estimated. The results are shown in the table on p. 398.

These results are expressed graphically in Fig. 1. It will be iioticed that this mixture fuses at about 720° (as

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registered by the thermo-element used), and i n this state the velocity of reactbon, so far as can be seen, is practically instantlaneous, equilibrinm being setl up for t he reaction

when about 60 per cent of the sodium carbonate is transformed. It will also be remarlred tha t up t o 400°, the amount of sodium

carbonate tha t has entered into reaction with the barium siilphats is insignificant!, even after three and a-half hours. W h a t little double decomposition has occurred has taken place fairly regularly, and it is fairly easy to compute the average reaction velocity for the temperatures up t o 400O. The curves €or 500°, 600°, and 685O, however, show some peculiarities. f t will be noticed tha t during the first thirty minutes, in each case the reaction has started fairly rapidly, but' after this time, has slowed down to B regular velocity, t h e points lying practically on a straight line.

This might be explained by the' presence of a trace of moisture occluded in the particles of the salts, which is always very difficult' to remove. A t the high temperature of the experiments, this is rapidly expelled, b u t caiises a certain amount of reaction between the sa1t.s on its own account. Once driven off, however, i ts influence is lost, and the straight part of the curve represents the t rue velocity of reaction between sodium carbonate and barium sulphate at any particular temperature.

T t is possible tha t this moisture may have occasioned t h e small amount of change a t t he lower temperatures, and tha t had it, been possible completely to dry the salts, the velocity of reaction up t o 400° would have been negligible. In t;he curve for 685O, the p r - Lion between the points A and U has been talien as representing the reaction velocity, as the point (7 represent,^ eqiiilibrium, which was probably attained before -the l ime represented in tho figure (210 minn tes). Tf we now plot reaction velocity against temperature, we obtain the curve shown it1 Fig. 2 (continuous line), which is remarkably regular up t o the point €or 720°, when, the velocity of t he reaction atl t ha t point becoming instantaneous, a suddcii hrealr i n the continuity of the curve occurs.

I f the equilibrium of this systlem is considered, it, will be seen t,hst i n the solid state there are present four solid phases, which can be defined by three components. This gives one degree of freedom for t h e system, and the equilibrium is therefore determined by the temperature ; it is therefore probable tha t there is a definite equilibrium mixture for each temperature, arid tha t the lower curves in Fig. 1 would not all of necessity approach the equilibrium attained 011 fusion, namely, the transformation of 60 per cent. of the sodium carbonate.

Na,CO, + BaSO, BaCO, + Na,SO,

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It was also endeavoured to reconcile the ve810citly-temperature curve in Fig. 2 with the formula put forward by Arrhenius and van't Roff, namely,

0.14 07 u

which follows the experimental curve very closely except, a t the higher temperatures as the point of fusion is approached.

The next two cases were chosen t o be of such a nature that no limit!ing equilibrium could be obtained, itl being possible for the reaction t o proceed to completion in one sense.

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402 PARKER : RXAOTIONS BETWEEN SOLID SUBSTANCES.

Silver Nitrate and Sodium Carbonate.

The products of this interaction are silver carbonate and sodium nitrate, and as the former is unstable, decomposing with rapid evolution of carbon dioxide a t looo, the rate of evolution of this gas from a mixture of silver nitrate and sodium carbonate could be used as a measure of the velocity of reaction between these two salts. The reactJon may be regarded as capable of proceeding to completion in the sense

Na,@O, + 2AgN0, = 2NaN0, + Ag,CO,, as the only equilibrium which can be est'ablished is tha t between silver oxide and carbon dioxide, which may be neglect'ed.

Silver nitrate crystals were purified by recrystallisation from dilute nitric acid solution and dried by heating just above the fusion point f o r two hours. The sodium carbonate was prepared and purified as before. The apparatus employed in these experi- ments consisbed of a thick Jena-glass test-tube, somewhat longer than the electric furnace used to heat it, with a tube of soft glass ground into its open end. This tube was sealed to the gauge column of a Sprengel exhaust pump. The mixture of silver nitrate and sodium carbonate (always 1.699 grams of the former and 0.53 gram of the latter) was placed in a porcelain boat, which was then introduced into the Jena-glass tube. This was then connected t o the pump, the apparatus exhausted, and the Jena-glass tube and con- tents were heated by the electric furnace. The rate of fall of the manometer on the1 pump was noted, giving a measure of the velocity of reaction proceeding between the mixed salts. The salts were tested for occluded gases by being heated separately in a vacuum. No appreciable fall in the mercury was to be noted. I n any series of readings taken at any particular temperature, after the last reading the tube and contents were allowed to cool to the ordinary temperature, when the manometer was again read, and all previous readings for that experiment were corrected accordingly, thus eliminating the effect of the various temperatures on the partial pressure of the carbon dioxide. The results are shown in the table on p. 403.

These results are expressed graphically in Fig. 3. The characteristics previously noted are again in evidence.

After a short period of induction over t h s first five minutes, due no doubt t o the gradual heating up of the mixture, the curves for the temperatures between l l O o and 170° show an increasing rapidity for the reaction a t the start, over a period of about' a

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PARKER : REACTIONS BETWEEN SOLID SUBSTANQES. 403

TABLE 11.

in /------- \

Time Corrected partial pressure of carbon dioxide in mm. of mercury. -,L

minutes.

10 15 20 25 30 60 90

120 150 180 210 270 300

Time in

inir iutos. 5 6 7 8 9

10

11 104

:at 124 13 134 14 14; 15 154 16

17 17!, 1s 19

16.1,

110" 130"

- 0-5

I 4.5

5.5 9-5 12.7 17.2 15.8 20.0 17-0 - 18.0 29.0 19.0 29.0

- - - -

- I

155-1 60"

1-8

12.9

20.2 32.2 37.7 41-4 45.0 46-0

-

-

- - -

165" 170"

2.7 15.3 12.6 30.6 22.4 42.3 28.6 50.4 34-0 53.7 43.8 63-0 48.3 69.3 52-8 - 57.4 77.4, 60.0 81.0 1 -

Corrected partial yressuro of carbon dioxido in mm. of mercury.

180' (fusion). 3-2 7.2

13.6 25.4 59.7 97.0

112.0 125.0 135-0 146.0 157.0 168.0 180.0 191.0 202.0 211.0 2 19.0 225.0 230.0 235.0 239.0 243.0 247.0

240-250" (fusion). 1.5 3.7 6.4

10.1 14.7 26.6 43.1 62-4 59.0

114.0 134.0 153-0 170.0 186.0 205.0 321.0 237.0

further fifty minutes. After that time, the rate of reaction in each case becomes practically uniform.

The curves for 180°, where fusion took place, and for 245O, although apparently almost coincident' on the diagram a t the scale given, are by no means so in reality. Calculated from the figures in table 11, the curve for 245O represents almostl t'wice the reac- tion velocity as does the curve for 1 8 0 O .

The graphical representation of reaction velocity against tempera- R*

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404 PARKER REACTIONS BETWEEN SOLID SUBSTANCES.

ture is shown in Fig. 4, and again a sudden increase is to be noted at the tempe'ratnre of fusion.

I n the solid state, there are here present five phases, comprised of four solid phases (AgNO,, Ag,O, NaN03, Na,CO,) and one gaseous phase ( C 0 2 ) ; all these are defined by the four components Ag,O, Na,O, N,O,, and CO,. The pressure of the system is there-

FIG. 3.

T i m e in minutes.

fore a function of the temperature, an example of the active masses of the reacting substances remainirig consbant in the solid state.

In the partly fused state there existl four solid phases, one liquid phase, and one gaseous phase, with the same number of components as belfore. There are, therefore, n o degrees of freedoin, and the point K is analogous to the triple point in the system ice-watdr- water vapour.

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PARKER : REACTIONS BETWEEN SOLID STJBSTA'NCES. 405

C.uproz~s Chloride and Sodium Carbonate.

This mixture was chosen more from the point of view of dis- covering what' happened after fusion than actually a t the fusion point. The cuprous chloride was prepared from the commercial substance by dissolving i t in concentrated hydrochloric acid and pouring the solution into water. The precipitated saltl was quickIy

F I G . 4.

1 1300 170" 2iu0 250"

Temperature.

collected, washed with alcohol and ether, and dried for many hours a t looo.

It remained quite white, apparently indefinitely, in a dry, air- tight bottle. For each determination, a mixt:ire of 0.991 gram of this salt with 0.53 gram of sodium carbonate was taken (equivalent quantities), and the experiments were conducted with the same apparatus as in the last reaction. The results are shown in the following table.

EL* 2

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406 PARKER : REAUTZONS BETWEEN SOLID SUBSTANCES.

Timc in

ini iiut cs , 6

10 11 12 1 3 I 4 15 16 17 18 19 30 2 5 3 0 40 50 A0 90

120 160 1 so

6 6

7

8

6 ;

74

.8* 84

9' 92 94 0.;

1 0 1 0$ 104

114 114 11;

12: 12+ 1 2; 1 3 134 14

8 3

] 0:' 1 l4

12

TABLE 111. Corrected partial pressure of carbon dioxide

in mm. of mercury.

100'

I i i l

300' 7 - 3

13.4

53-0 83.5

119.0 138.0 155.0 172.0 1'32.0 207.0 223.0 237.0 250.0 262.0 274.0 283.0 291.0

309-0

324.0

338.0

350.0 362.0 370-0

-.I-

-

--

-

r - Ihesc results axe expressed Fusion took place at about

reaction velocity.

330' 17-5 24.0 35.0 53.0 84.0

123.0 141.0 163.0 181.0 200.0 2 17.0 238.0 258.0 281.0 306.0 334.0 361.0 389.0 420-0 446.0 4'71.0 487.0 515.0 531-0 548.0 --

260' 3.6

48.0 83.0

116.0 147.0 1 74.0 199.0 2 17.0 234.0 249.0 269.0 268.0 310.0 336.0 370.0

41'3.0 1

- _ -- -

360° 9.2

35.0 68.0

119.0 178.0 256.0 307.0 353.0 404.0 4 ;; 4 * 0

52 1.0

569.0

--

-

__ -- .-

-- -

I-

- --_

I-

-

-

graphically in Figs. 5 and G . 260°, causing a large increase in tlie

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PARKER : REA CTTONS BETWEEN 8C)TdT) SUBSTANCES. 407

Itt seems fairly well established, therefore, that the process of fusion of itself causes a marked increase in the reaction velocity of the mixtures, the ratio of velocity in the liquid state to that at the highest temperature in the solid state varying from infinity

F I G . 6.

6 0 90 120 150 180

X i m e in minutcs.

in Case 1, through about 150 in Case 3, t o about 20 times in Case 3.

It may be taken, therefore, that the liquid state per se has a direct bearing on the interaction of salt pairs. I t should be noted, however, that after fusion the velocity of reaction is not quite so

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independent! of the bemperature as in the solid stat,e. The velocity liecoming practically infinite on fiision in Case 1, this could be investigated no further, but in Cases 3 and 3 it, is seen that further rise in temperature causes a f-iirther great increase in reac- tion velocity, although notl nearly so great' as the increase pro-

100" 140" 180" 320" 260' 300" 340' 380'

Temperntu.rP.

dnced by fusion over -the velocity in the solid state. I n Case 2, a rise of 6 5 O nearly doubles the reaction velocity, and in Case 3, a rise of 90° multiplies the rnt'e about six times. The reacting mole- cules now being in intimate contact, the temperatlira begins to exert a well-dt.fixle~~ infinfwcc?.

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There seems therefore to be no inherent objection t o the ex- planation, put' forward in the previous communication, of the reactions brought athont by shearing stress between apparently solid substances, namely, that the stress causes local or surface fusion of the salts; it has been shown that the liquid state, quite apart from temperature effects, has an important bearing on the capacity for interaction.

It is known that the size of particle may have a slight influence on the melting point of a substance, as in the case of salol investi- gated by Pavlov ( J . Buss. Phys. Chem. Soc., 1910, 40, 1022), who showed that the temperature of fusion was, however, only lowered 2.8* for one hundred times the increase of surface (ten times decrease of radius). This therefore is probably a factor of no great magnitude.

Tammann ( A n r ~ . Phj/s. L'henz., 1899, [ii], 68, 553, 629) showed that simple pressure was capable of influencing the melting and transition points of a large number of substances. He also showed (Zei tsck. pli:ysiknk. Glaem., 1903, 46, 818) that the fusion curve for sodium sulphate rose t o a maxirnnin at a pressure of 750 kilos. per sq. cm., and then fell considerably.

Most inorganic salts, however, have their fusion pointjs raised by increase of pressure, and even in cases of the reverse, such as that of silver bromide, i t is necessary to apply pressures of the order of lo5 atmospheres t o cause fusion a t ordinary temperatures. Although the experimental facts do not yet seem t o offer a com- plete explanation of the action of shearing stress as applied to solid substances, it thus appears probable that such stress has a peculiar action of its own towards the production on the surface of such materials of a state analogous to, if no t identical with, fusion.

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