THERMODYSAMIC PROPRTIES OF BICL GAS
.June, I960
HT than labeled acid. This bias is most pronounced in acids containing a large proportion of primary H atoms. ( 5 ) Llttai>kon a C-C bond to form a laheled hydrocarbon LS much more probable at the more ex-
79 1
posed terminal C-C bonds. (6) Excitation energy introduced during the displacement may cause decomposition of a labeled acid. This is particularly noticeable with acids which tend to decarboxylate readily.
+
THE EQVILIBRIUAI /3Bi(l) 1,'3RiC13ig) = RiCl(g) AXD THE THF,RRIOT)YS,YXIC PI-1OPEKTTES OF RiCl GAS' BY D.%SIELCUBICCIOTTI Stanford Research Znstztute, . I f d o P a r k , California Recmed Januat
v 12, 1960
+
The reaction 2/3Bi(l) l/aBiCIJ(g) = BiCl(g) was investigated by a transpiration technique in the range GOO t o 700". The stoichiometry of the reaction was checked both by varying the activity of the gaseous reactants and by comparing the measured entropy (18 e.u.) with a calculated value. The enthalpy change for the reaction was found to be 24 kcal., which led t o a v n l i i ~of 7 Ircal. per mole for the enthalpy of formation of BiC1, gas, a t 25".
Introduction I n the vapor oTw liquid Bi-BiCla mixtures there i? evidenrf that some Ri species exists of lower valence than three. For example, one ran observe the trunspclrt of Bi through a vapor phase containiiig BiCla under coiidit'ioiis in which the vapor pressure of Bi itself is much too small to be coiisidered. ;\,lso. the existence of the gaseous diatomic molecule RiCl under certain conditions is known froni spectroscopic observation. The present paper is a report of work performed to determine the molecular species t o be found in the mpor over Bi--RiCl, mixtures and the eyuilibi~iiimpressures of such species. Experimental
the mouth of a collector. In that n-aj-, matrri:il v a s deposited in the collector only by diffusion. Diffiixion measurements were made over the entire range of temperature? of these experiments iinder varying pressures of H i 4 1 species. The results allowed a correction to be made for each transpiration experiment. I n this correction it was :issumcd that the composition of the diffused material wzs thtl same as the eqiiilibrium mixture. Transpiration experiments were made a t Bi temperatures from 600 to 700'. Below 600' the proportion of lover-valent Bi species was too small for convenient determination. The experimental BiCb pressure a t each of the several temperatures was varied over a t,enfold or greater rniige so that' the stoichiometry of the equilibrium could bc drtrrmined. I t should be noted that vapor pressure2 of Bi in the temperature range studied was too small t o influence the results. .it 700" its vapor pressure is about l o + mm. and that of the lon-er-valent Bi species a few mm. At 600" the Bi pressurr is ahout 10-3 mm. and that of the lower-valent species about onr mm. The EiCI, used was dried by vigorous boiling and doubly distilled in an PIT2 stream. The Bi used was Johnson-JIatthey's "specpure" grade.
The reaction of BiCli vapor 1%-ithliquid Ri was studied in a txpe of tranipiration c,xperiment. BiCl:, carried in a h'2 stream, was tsquilibratcd with liquid Bi and the amount and ouer-all composition of the condensed vapor iyas determined. To obtain thl: greatrst fraction of loFer-valent Bi species in the vapor, tl- e equilibrium between liquid Bi and relatively Results lo^ pressures of RiC13T K ~ S studied. Equilibrium Constants.-The measured quaaA stream of high purity S,n-:is dried by passage over silica gel at, solid C;?? tcmperatures and deoxidized over Cii and Fe tities, pertinent to t.he equilibrium under studg, filings at 400 . I t was then divided into two streams, whose r e r e the moles of Napassed, the weight of Bi-C1 relative flow i'ates m i l d be controlled; one stream was passed over liquid BiCl, :it about 240" while the other bypassed the species condensed, t'he weight of AgCl formed from 13iC1, to rejoin the first S, downstream. In that way a the Bi-C1 sample, and a diffusion correction. stream of S, m-ith :I controllable pressure of BiCl, was proFrom these one can calculate the partial pressures driced. This s t r a m was bubbled siiccessively through of the Bi-C1 species in the equilibrium gas! prothrcle small T-ycor buhl)lers, each containing 3 to 4 g. of Ri at vided he has independent' information of their 600 to TOO". Thc stream was then passed out of the furnace through a collector tithe in ~ h i c hthe various Bi-C1 species molecular weights. It will be shown that there is condensed n : i i l ~tlir S?w:ts collected and determined in a no substantial contribution from species n-ith more ronst:int pressure elldiometer. The condensed Bi-C1 sample than one chlorine per molecule, thus excluding ivns weighed snd the C1 determined gravimetrically. To dc,terminc whethw equilibrium had been established species with Bi oxidation number two. The press than doubled in several experiments ence of species with low oxidation nuniberr is also the flow rate ~ m morr and the result lint equilibrium constant checked for agreement. demonstrated by average oxidation numbers of the This may be seen in Table I by comparing the following pairs Bi in the transported gas of less t,han t v o . It is not of experiments: at 601-602", the 3rd and 4th, the Fjth and possible to eliminate species such as BiX1 (i.c.. Bth, tlir 7th ;,nd 8th; at 650", the 5th and 6th. In this type of flax experiment condensnblr material is Hi2C1,Hi,Cl) a priori: however, as n-ill he seen, the cl out 01' the hot zone by diffusion as well as hy the NZ agreement of the enthalpy and entropy of reaction treani. In order t o ca:tlculatr the pressures of condensable ohserved in these experiments with thow calculated y w i c s c.orrec%ly, the amount of material that reached the c~old zone by diffusion must be subtracted from the total from spectroscopic data for RiCl indicate that the xnoiint depcdtcd. Therefore an auxiliary study of the contribution from such species must have heen Bi-C1 species in t h r N? stream was small. ing the Bi-Cl ]:den 1-2 stream around . (1) This work wan made possible by t h e financial support of t h e Resrarrti Divinion c d thr. T'nitrd State8 .Atomic Energy Cornmimion.
~ ~ LIiscrllaneous ( 2 ) L. Brewer, "The Chemistry and ~ 1 e t i i l l i i r oi ;\Iaterials," XN'ES. I V - I g R , hlrGra~r-Hill R o o k ( ' < I . , J r l c , . , Si,n- \-ark, Y..IRRO, 11. 31.
s.
DANELCVBICCIOTTI
\'d. 64
TABLE I EXPERIMENTAL REsurrs A N D EQUILIBRIUM CONSTANTS Moles
0.414 , l
