Dadape, V. V.

Kulkarni, M. P.

English

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Indian Journal of ChemistryVol. 14A, December 1976, pp. 964-966Vapour Pressure of Barium Metal & Study of the EquilibriumReaction Ba(g)+BaCb(c) =2BaCl(g) at High Temperature"Nt P. KULKARNI & V. v. DADAPEtNational Chemical Laboratory, Poona 8Received 12 January 1976; accepted 3 June 1976The vapour pressure of barium metal has been determined at temperatures in the range1160-1333°Kemploying a transpiration technique. The second law calculations yield for theheat of sublimation (AH.os) a value of 40·43±4·94 kcal rrrole", which compares favourably withthird law value 43'54±0'57 kcal rnole", The equilibrium reaction Ba(g) + BaCl.(c) = 2BaCI(g)has been investigated between 12100 and 1305°Kemploying a flow method. Over the tempera-ture range t.H'1258 is found to be 77·76±8·97 kcal rnole", The heat of formation of BaCI(g)(AH,,_.) derived from this yields a value of -40·29±8·97 kcal mole ? by the second law whichis in good agreement with the corresponding third law value of - 43·67± 1·09 kcal rnole". Theentropy of the reaction (ASr125s) is calculated to be 49·26±7·12 e.u.THOUGH the existence of barium monochloridehas been reported by many investigators,the heat of formation of gaseous BaCl has notbeen determined experimentally. Available litera-ture values are only the estimates based on thedissociation energy of the BaCl(g) molecule. Herz-bergI reported the dissociation energy (Do) of thebarium sub-halide as 2'7 eV whereas Gaydon?obtained a value 5·0± 0·4 eV. Gurvich'' reported thedissociation energy Do of BaCl as 118 ± 5·0 kcalmole-'. Using heat of sublimation of barium- as41·74 kcal mole+ and the heat of formation of idealmonoatornic chlorine as 28'94 kcal mole= (refs. 5 and6) the values obtained for the heat of formation ofBaCl(g) are + 8·58, -44·32±9·2, -47·32 ± 5·0 kcalmole+ respectively. Feber? calculated the heat offormation at 298'15°K of the gaseous bariummonochloride as -49,0 kcal mole'<.Quill in his books mentions that Eastmann andcoworkers claim that there are no stable solid sub-halides, and Cubicciorti reported the phase diagramfor Ba-BaC12, Ca-CaCl2 and Sr-SrC12 systems. Inall these cases the phase diagrams show no evidenceof Solid sub-halide though there was appreciablesolubility of the metal in the halide. The existenceof bands due to BaCl(g) was reported by King? in1905 and Parker-? in 1934. In view of the non-availability of any experimental value for the heatof formation of BaCl(g) it was thought worth whileto study the equilibrium reactionBa(g) +BaC12(c) <=2BaCl(g) ... (1)at high temperature. The vapour pressure of Ba(g)in equilibrium with Ba(l) was also evaluated.Vapour pressure of Ba metal by the boiling pointmethod was determined for the first time in 1924{ref. 12) and later in 1935 (ref. 13) employingKnudsen effusion method. In these studies, thepurity of the metal sample was not quite high and*NCL Communication No. 1986.+Present address: Development Manager, MIs DharamsiMorarji Chemicals Ltd, P.O. Ambernath, Thane.964also it was difficult to obtain uniform temperaturesof the effusion chambers. The results, therefore,were influenced by these drawbackst-, It was there-fore necessary to study the vaporization of Bametal at high temperatures.The present study includes (i) vaporization ofBa metal at temperatures between 11600 and 1330GKand (ii) equilibrium reaction of Ba(g) with BaCI2(c)at temperatures between 1210° and 1305°K.Materials and MethodsBarium chloride (BaCI2.2H20), BDH Analar grade,was dehydrated in a stream of HCl at 400°C for 2 nrusi ng purified nitrogen as carrier gas. The di-chloride was chemically analysed for Chloride andbarium by standard methods of analysis. The puritywas found to be more than 99·9%. TIle anhydrousdichloride was freshly prepared for each experiment.Ba metal (Fluka, AG, Switzorlarid] was chemicallyanalysed in a dry box (having argon atmosphere).The purity was found to be more than 99'5%.Argon was used as a carrier gas. Impurities likeoxygen, nitrogen, carbon dioxide and moisture wereremoved by methods described earlier by Rao15.In addi tion titanium metal at 1000°C was used toremove traces of oxygen and nitrogen.Apparatus and procedure - The experimental set-up consists of a tube furnace which can be easilyheated in a furnace in the range 1100° to l3000Khaving two constant temperature zones of about20 ern length. Transparent silica boats were usedas containers for the solid materials and mullitetube with stanard glass joints was used as tnereaction tube. Temperatures were measured bymeans of calibrated chrornel-alumel thermocouplesand a flow meter was employed to measure the rateof flow of carrier gas.Results and DiscussionVaporization of Ba metal- The barium metalwas cut into small pieces under petroleum. Tnepieces were quickly and carefully dried with filterKULKARNI & DADAPE: VAPOUR PRESSURE OF BARIUM METALpaper and then transferred to the boat. Theseoperations were carried out ir a dry box throughwhicn argon gas was passed.The reaction tube was completely flushed withargon gas. After attaining the required cor stanttemperature and maintainir g desired flow of argongas, the boat containirg barium metal was intro-duced from the downstream end. The experiments\\ere carried out for 1 nr. After completing thereaction the boat was pushed to a chamber contain-ing dry argon and tnen weighed. The vaporizationwas carried out at different temperatures andthe results are recorded in Table 1. The vapourpressure data have been employed in the studyof the equilibrium reaction (1).Study Of the equilibrium. reaction - The experi-mental set-up was the same as described above. Thetemperature of the zone in wnicn the vapour of Bametal was generated was maintained as 12000Kand otner zone was kept at the desired temperaturefor tne reaction. Refractory plugs of sui tablesizes were introduced at the proper points to actas radiation shields and diffusion barriers withoutwhich the equilibrium could not be easily attained.To study the equilibrium reaction the argon gasrate was regulated to the desired value (3·1 to 3·3litresjhr) and the experiment was started bypushing the boats containing metal and metalchloride in the respective zones. Experiments werecarried out in the temperature range 1210-1305°Kand the results are recorded in Table 2. After C01'.-tir.uing the run for a definite time (1 hr) the boatcontaining barium metal was pushed out of the hotzone; thereby arresting the reaction. The heatingwas then stopped, the system allowed to cool inargon and both the boats were weighed.Further the vapour pressure of BaCl2 at tempera-ture studied is quite negligible. But when thevapour of barium is passed over BaCI2, a loss inBaCl2 was observed (due to the fo;rmation of BaCl).This loss combined with the observation of BaCl~and Ba (from BaCl) on a cold finger conclusivelyprove the formation of BaCl.Identification of the gaseous products was carriedout by condensing them on a silica cold finger.After the reaction the deposit on the cold finger onchemical analysis showed the presence of BaCI2•It is possible that the deposit of BaC12(c) may bedue to the disproportion of BaCl(g) in accordancewith Eq. (2)2BaCl(g) = Ba(g)+BaC12(c) ... (2)The weight loss data were used to calculate the partialpressures of BaCl(g) and Ba(c) using the following:reactions:RTrPBaCl = na TTrRTrPBa = nb V,where na and nb are the number of moles of BaCland Ba respectively. The equilibrium constantKp was calculated from Eq. (5)K - (PBaC1Fp - PBaThe free energy and heat of reaction at varioustemperatures were calculated employing Eqs. (6)and (7)t::.Go = -RT In Kt::.H~98_ R 1 K Gy - HfJ.98(P d)-r -- n - T ro+Gr - H~98T (react)... (3)... (4)... (5)... (6)... (7)TABLE 1 - VAPORIZATIONOF Ba(l) = Ba(g) AND THIRD LAW HEATSTemp. Argon gas Ba metal P xl03 -RT In Kp t:.jel Tt:.jej t:.Ho298OK passed transported atm. kcal mole'< cal mole"! kcal mole'? kcal mole"!Iitre/hr (mg)1160 3·09 17·0 0·9794 15·97 23·50 27·26 43·231195 3·09 22·0 1·2675 15·84 23·38 27·93 43·771225 2·90 28·0 1·7188 15·49 23·31 28·55 44·041240 2·76 46·0 2·0671 14·34 23·27 28·85 43·191273 2·40 67·0 4·9698 13·42 23·18 29·51 42·931333 2·10 74·0 6·2733 13·43 23·02 30·68 44·11Duration of experiment = 1 hr. Av. = 43·54 ± 0·57.TABLE 2 - TRANSPIRATIONDATA AND RELATED THIRD LAW HEATS FOR THE REACTIONBa(g) + BaCI.(c) = 2BaCI(g)Temp. Argon gas Ba(g) BaCI.(c) P X 10-3 -log Kp -RT In K t:.Hor'98 t:.Ho'98OK passed passed transported BaCI(g) kcal/mole kcal/mole=! BaCI (g)litrejhr (mg) (mg) atm. kcal molc-!1210 3·00 22·0 11·5 0·9009 3·2072 17·75 74-88 43·591235 3·09 24·0 13-5 1·0259 3·1185 17·62 75·76 43·151255 3·21 53·0 29.0 2·1217 2·8150 16·16 75·04 43·511270 3·00 22·0 25·5 1·9568 2·5326 14·71 74·09 43·981285 2·90 17·0 25·5 2·0646 2·3888 14·04 73·97 44·051305 3·30 64·0 58·0 4·1269 2·3068 13-77 74·47 43·79Duration of each run is 1 hr ; t:.H~98 BaCI(g) = 43·67 ± 1·09 kcal mole-! (av.); t:.H~98 of Ba(g) = 43·54 ± 0·57 takenfrom third law of Table 2; and generation temperature of Ba(g) = 1200°K.965INDIAN J. CHEM., VOL. 14A, DECEMBER 197635 !03000.:x:o0 1°...J 2520I·B7·5 B·O 8'51/ T X 104Fig. 1 - Plot of -log Kp versus 1/1' for the reaction Ba(g)+ BaCl.{c} ? 2BaCl(g)The experimental results have been summarizedin Table 2. The values of log Kp are plotted againstthe reciprocal of the absolute temperature (Fig. 1).From the slope of the plot the heat of reaction (6.H'1258)was found to be 77·76 + 8·97 kcal mole:<. Accordingto King and Kelleyllthe cnange in Cp value is 9·01kcal mole'<. When this value is integrated betweenthe limits 298° and 1258~K tne value of 8·65 kcalmole'< is obtained. This when added to the valueof 6.H'I258 yields 86·41 ± 8·97 kcal mole+ forAH'298'The third law calculation is carried out usingfree energy functions which are calculated at desiredtempe rature using en tropy an d HT- H298 values tabu-lated by King and Kelley!", The average experi-mental value for the heat of reaction at 298°K is74'70 + 1·64 kcal mole-'.Vaiaur pressure data of Ba - Employing theVant Hoff's reaction,d In P AH'Vd(lIT) = ~ ...(8)the heat of vaporization of Ba(c) has beendetermined from the values of log P and lIT. Theconstants from tne least square line led to a valueof 36·80 ± 4·94 kcal molet for AH~ap at 1246°K.It represents the mean value over the temperaturerange studied. According to Stull and Sinke+ tne966change in Cp is 1·89 kcal mole+, This differencein the Cp when integrated between tne limits 298°and 1246°K gives a value of 1'79 kcal mole-t, Thiswhen added to the above value of heat of sublima-tion at 1246"K yields AH~98 = 38·60 ± 4·94 kcalmole+. Adding the heat of fusion- AH1298 = 1·83kcal mole:", AH298 has a value of 40'43 ± 4·94kcal mole-I.For the calculation by third law method, AIelhas been obtained employing values of Stull andSinke4, for Ba(g) and Ba(IC); AH, is calculated fromexperimentally determined free energy (6.G~). Fromthe value of !1H" AH/~98 of Ba(g) is found to be43· 54 + O'57 kcal mole+.Equilibrium rea-ction- In the study of equilibriumreaction (1)AH'298 = 26.Hf~98 BaCI(g)- 6.H/~98 Ba(g)-L\Hf~98(BaCI2) ... (9)The second law value for the heat of formationof BaCI(g) AHf298 (-40'29 ± 8·97 kcal mole'<] is ingood agreement with that obtained by the third law(t1Hf298 = - 43·67±1·09 kcal mole+). The experi-mentally obtained value for the heat of formationfor BaCI(g) compares well with that calculatedfrom the dissociations (t1Hf298 = -44'32 ± 9·2 kcalmole-t).The entropy of the reaction was found to be 49·26± 7·13 e.u, at 1258°K.References1. HERZBERG, G., Molecular spectra and molecular structure(Prentice-Hall, New York), 1950.2. GAYDON, A. G., Dissociation energies and spectra of di-atomic molecules (Chapman & Hall, London), 1968.3. GURVICH, L. V. & H.YABOVA, V. G., cu-« Abstr., 61(1964), 11349.4. STULL, D. K & SINKE, G. C., Thermodynamic propertiesof elements (American Chemical Society, Washington),1956, 52.5. BREWER, L., (private communication).6. BARROW, R. F., Trans. Faraday Soc., 56 (1960), 952.7. FEBER, R. C,; Heat of dissociation of gaseous halides(Los Alamos Scientific Laboratory, California), 1964,173.8. QUILL, L. L., The chemistry and metallurgy of miscel-laneous materials (McGraw-Hill, New York), 1950, 119.9. KING, A. S., Astrophvs. J., 21 (1905). 236.10. PARKER, A. E., Phys. Rev., 46 (1934), 30t.11. KELLEY, K. K. & KI~G, E. G., Contribution to the dataon theoretical metallurgy (Bureau of Mines, USA), Bul-letin 584, 592.12. RUFF, O. & HARTMA~, H., Z. a'wrg. allgem, Chem., 133(1924), 29.13. RUDBERG, E. & LEMPERT, L., J. chem, Phys., 3 (1938),627.14. NESMEYANOV, A. N., Vapour pressure of the chemicalelements (Elsevier, New York), 1963, 186.15. BHOGESWARA RAo, D. & DADAPE, V. V., J. phys. cu«,70 (1966), 1349.

Vapour Pressure of Barium Metal & Study of the Equilibrium Reaction Ba(g)+BaCl2(c) =2BaCl(g) at High Temperature

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Vapour Pressure of Barium Metal & Study of the Equilibrium Reaction Ba(g)+BaCl2(c) =2BaCl(g) at High Temperature

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