HEATS O F ADSORPTIOS 01: SULFUR DIOXIDE X S D OF WATER VAPOR BY PILlCX GEL AT o o c * B Y \Y. .4. PATRICK A S D C. E. C R E I D E H
Heats of adsorption from the gas phase have been measured by a number of previous w o r k w including Favrcl Titfoff2,and Lamb nnd Coolidges. In each cnse note is mnde of the fact that the hcat of adsorption is greater per gram of vapor actsorbed than the corresponding heat of liquefaction at the temperature. Since it is assuincd that thc vapor is liquefied in the pores or on the surface of the adsorbent, various csplanntions have been advanced to account for the difference bctween thc heat of adsorption and t h e heat of liquefaction. This difference should at s:tturation be equal to the heat of wetting, and is called by I,,zinb the net heat of adsorption. He assumes that this “net heat” is due t o compression of the adsorbed liquid by the forces of inolecular attrncticm, :tnd from this hc c:ilciil:it)es that in the cases with which he workccl t h e i x h r b e d liquid w:is under :L prcssurtt of about, twenty thousand at I n ospli e re s . Ilarkins and EwingJ h v e suggested that t hc heat of wetting, which they call the heat of spreading, is due to the change in surface energy involved, and derive equ,ztions to express their ideas, but do not dttempt any quantitative calculations. Recent work in this laboratory5 has shown that, when silica gel is used as the adsorbcnt, it is possible to calculatc quantitatively the heat of wctting from changes in surface e n e r n . The most active saniples of silica gel, activated by heating at 2 j o o - j o o o in vacuo for a half hour or niore, contain from three t o sis percent of water. It, is therefore assumed that the gel before wetting eshibits a water surface. Wetting by water would therefore simply fill u p the pores of thc gel and reduce the water surface from its original vet?. large value to practically zero. The heat of wetting of silica gel by water was measured at 2j0. and the specific surface of the gel calculated on the assumption that the heat measured was due entirely to liberation of the total surface energy of the water. The specific surface obtained in this way agreed 1w-y well with that obtained from ultramicroscopic investigations. The purpose of this work was to measure heats of adsorption a t o o , with silica gel as the adsorbent. I n the adsorption of water vapor at oo, the net heat of adsorption should bc negative if it is due to compression of the adsorbed water. If, on the other hand, the net heat is due entirely to liberation of surface energy, it should at saturation be equal t o the heat of wetting as * Contribution frorn t h e Clicmicnl Lnboratory of Johns Hopkixu University. Ann. Chim. Phys. (5) 1, 209 (1874). Z. physik. Chem. 74,641 (1910). 3 J . Am. Chem. Soc. 42, 1146 (1920). 4 Proc. S:tt. Xcnd. Sci. 6,49 (1920). 5Patrirk anti Grimm: J. Am. CheS. SOC.43, 2144 (1921).
1
2
C
y“ FIG. I
I’IC.
2
The mcasumnents of heat at this temperature were made with a modified Bunsen ice caloiimcter., as shown in Fig. 2 . M is :t mercury reservoir, and N n. c‘npillnry tube ending in n tip of the shape recomncndcd by Ostwald and I,ut8hcr.l Thc heat given off (or absorbcd) is followed by the change in weight of sin:tll ~ncrciirycups placed under. the tip of N. The inner tube of the caloriinetci. has in it sufficient mercury to stand t o about t h e height shown in t h e figure. in order that thermal equilibrium between the adsorption bulb and the calorimeter might be more quickly attained.
HEATS O F A D S O R P T I O S
'033
The calorimctcr is placed in a two quart "Ferrostat" vacullrn jar, filled with a rnist(urc of icr, and watcr. It rws found that ordinary ice if \vel1 Ivashed would kecp the tcn;pcraturc at such a point that heat, intercliangc 1)ctwcen the calorimeter and its surroundings was ricgligiblc. This blank was, however, re-detrermincd at each filling:
Materials The nicrc~ii*y nnd the ruther connections were clc:inccl in the ilsu:il manner. The sulfur cliosictc was taken from a tank of the cornrricrcial material, from which thc first portions ~vercallo~vcdto cscape. I t wts dried hy p,zssagc through :t U-tul;e filled wit 11 phosphorus pcntoxiclc. The silicn gel was prcpared by the procoss used hy Patrick and hXcGavack' and when dried for two hours at 300°, various preparations contained from 3 . j to j . j percent, of watcr.
Procedure In the adsorption of sulfur dioxide the following procedure was adopted. From 0.5 to 2 granis of the gcl, prcviously heated for at least a half hour a t z j o o - ~ ~ o Oino vaciio, wits pl:tcccl in n sninll h i l t > , which was thcn scaled to the stopcock :Inti o\itei*Mf of t h e grouncl-gI:iss joint I-I, ~nakingthe complete adsoi*ptionl)iiIb I. T h c joint was 1ubric:Lted with stopcock grease and fitted in place, and t h o system, including the bulb containing the gel, evacuated to a prcssure of less t h m 0.00 I m 1 i . The stopcock of the bulb was thcn closed, and after equalizing pressure in the system, the bulb was detached, the lubricant on the joint removed with a clean cloth, and the bulb weighed. Experiments were nmde to determine if the weight of the bulb, as determined in this way, was reproducible. The bulb was weighed, stopcock grease applied to the joint, then removed with a clean cloth, and the bulb re-weighed. .I series of six such weighings agrccd within 0 . 2 mg. The h l b was placed in the caloriinetcr and again attached t o the system, which vas thcn evacuated, as before, to a pressure less than 0.001 min. K h i l e the evacuation was in progress, sulfur diosicle i m s allowed to sweep out the dii-ing train, connected to the system a t E, the sulfur clioside escaping at F. This drying train contained also a n open manometer, in order better to regulate the admission of the gas. It JWLS assumed that the air was completely displaced from the train between the tank and the stopcock D after sweeping the gas through at a moderate rate for half a n hour. This was further checked by the time required for equilibrium to be established w-hcn the gas was adsorbed by the &,--it having been s1ion.n by Patrick and McGavack that a pressure of air over the gel too small to niatcrially affect the calculated pressure of sulfur dioxide would increase by several hours the time required for equilibrium to be attained. Upon coniplction of the evacuation, the stopcock of the adsorption bulb was opened and the mercury reservoirs raised so as to close traps A and C. The caloriinetcr and adsorption bulb had come to thermal equilibrium at the end of the evacuation. The stopcock 31, Fig. 2 , was opened momentarily,
allowing t h c c:ipillnry S t o bccornc corriplctcly fillctl with mercury, nncl a small \vcighctl clip of Incrcup- so placed that the tip of S was inlmcwecl in it. The stopcock D, Fig. I , wts then opened to the system for a few seconds, admitting a small amount of sulfur dioxide. During a run, a moderate current of sulfur cliosicici wis continuously passing through the stopcock D and out of the opening 17, csccpt when it W,LS k i n g admitted to t h c adsorption i~ulb. The rncrcury cups iisecl to (leterrnine the heat were changcd ever?. fifteen iniiiutw until the weight W:LS constant (witchin 0.5 rng.). Attainment of cquilibriiirn W:LS alw follo\vcti \)y o h r v i n g the I)rwsiIre of gas over the gcl, rncnsurctl :is the diffcrencc lwt \vccw the rncrcury Icvcls in tlic t)wo arms of the tr;ip C', :tilt1 rc:td with a c:ithctonwtcr telescope. I t was obscrved that at lower prcssiircu cqiiilihriiim was rc~ichcclin fift ecn minutes or less, and that if e\':wi:tt ion i v n s propcrl~*cnrrietl out', it requirccl in no caw more than half an hour t o c.;t:il)lish itself. \\'hen cc~iiilil~riii~i~ 1 ~ : ~rcachcct, s as shown by both pressure and heat measi i r e ~ n ~ ~tlht es ,total loss in weight of the mcrcury cups, and thc pressurc were r c w r d ~ ( l , l'hc stopcock to the bulb was then closcd and pressure equalized, \vhereupon the Inilb W;LS removed, dried and iveighed. The increase in weight, corrected for the a i i : c u i i t l of sulfur tliositlc in the free space of the bulb a t the pressure obscrvcti, gave thc weight of the sulfur dioxide adsorbed. The bulb was then replaced in the calorimeter and again attached to the system, which was evacuated 8 s before, the stopcock to the bulb being kept closcd during the evacuation. \\'hen evacuation was complete the mercury traps \\'ere again closed, the stopcock to the bulb opened, ancl a second portion of gas admitted in the same manner. For the new pressure the heat of adsorption was recorded as the sum of the two heats measured, and the : m o u n t aclsorbed is also taken as the total (corrected) increase in weight of the bulb, rather than the difference between two successive weighings. I n this manner four successive portions of gas were admitted, and the equilihium pressures, total heat, and weight of substance adsorbed recorded for. each point. The final pressure was never much greater t h a n 600 mm., since if this pressure esists in the bulb a t oc, the pressure therein becomes greater than the estcrnal pressure when the bulb is allowed to corne to room t emperatur-e. After its final weighing. the bulb was attached to the system and evacuated. In some cases it was first placed in the calorimeter and the heat of desorption measured. It was found that about g j percent of the gas could be removed by evacuation at oo for one hour. I n any case, the evacuation was completed while heating the gel at 2jo0-3oo0 for half an hour, after which the gel was ready for R second run. Several successive nins with the same sample of gel did riot show any falling oft' in activity, the curyes being practically coincident. I n the runs using water vapor, the method was, in general, t h e same. A bulb containing distilled water was sealed to the system at E. T h e water and the free space between the stopcock and the bulb containing the water were freed from air by repeated espansion into the remainder of the system, and were then kept air-free.
HEATS OF ADSCaRPTIOS
105 j
The time required for evacuation of the system was materially lowered by sweeping it out with water vapor by 3 suitable manipulation of the trap A and the stopcock D. Practically complete evacuation, except for water vapor, could be brought about in ten or fifteen minutes by this method. The stopcock to the adsorption bulb, which had been previously evacuated, was of course kept closed during this process. A higher degree of evacuation was required for water vapor th:tn for sulfur dioxide. in order that. equilibrium might be cstnblished in thc smw tirnc. The correction for rvater vapor in tfic free space of the bulb is negligible. I
I
I
I
I
I
I
1
Attempts t o measure the heat of desorption were unsatisfactov, since only a small amount of water is removed from unit weight of geI on evacuation for one hour at 0'.
Discussion The heat! of adsorption of water vapor follows the upper curve of Fig. 3, which also s h o w the heat of liqucfaction (vaporization) of water at 0'. The value of the latter is taken as 596.8 calories per gram, as measured by Dicterici'. The heat of adsorption is found to follow the equation H = KX"" of Lamb and Coolidge, the values of I< and I /xi being, respectively, I .og I and 0.9 1 4 . This is obtained from esperiments 6 and g (Table I), which give partial heats of adsorption as well as the value at saturation. The other two runs give only the values at, saturation of X and H. The mean weight of water adsorbed at saturation is 361.2 mg. per gram of gel, and the mean heat of adsorption at saturation is 236.2 calories per gram of gel. The heat of liquefaction a t oo of this weight for water is 2 1 j . 6 calories, thus the net heat at saturation is 20.6 calories per gram of gel. The fact that this net heat is positive, while it
doc$ riot pi*cc~luclcthc ;iossibilitj*of compression of the nctsorbccl liquid by the forcoi: of 111olccul:tr. at t r w t ion, makes it secrri improbnblc that this comprc)sionqi f :tn?*, 11:~s :t11~-riiatcrid cffcct on the heat of adsorption, since the coin piwsion of ~ w t e at r o' woi~ld,according to the principle of LeChAtelier, rcsult in an :~t,sorptionrather than an cvdution of heat,
Run 9
li
0,s
2; .O
32.9
83.4
114.6
2.1
157 .o
2.31.
4.6
2;s. I
361.7
0.3
Itur1 4.6
j
IO
23;
362 . o
T h e n e t heat may, however, be satisfactorily esplained hy a consideration of the changes in surface a n d in surface e n c r m involved. As has heen pointed o u t i n n pwvious paper,' silica gel is assumed t o have 3, water surface before netting 01' adsorption, and the process of wetting or adsorption in the case of water simply consists in the filling of the pores of thc gel, which reduces the original, very large surface to practically zero. Thc heat of wetting of silica gel by water 3s measured bj, Patrick and Grimm at 25' was 1 9 . 2 2 calories per gram of gel, while the net heat of adsorption of water vapor st 0" was shown by our niea~urenientsto he equal, at saturation, t o 20.6 calories per gram of gel. The esperirnerltal error in the latter measurement is magnified considerably by the f s c t that the net heat, as is apparent from the curve, is less than one tenth of t h e total heat of adsorption as 1ne:isured. It may be rernarked in passing that in most of the cases observed by Imnb mid Coolidge, the net heat was a h u t one hnlf of the total heat measured, and in our measurements of the heat of adsorption of sulfur dioxide on silica gel, shown below, t h e net heat also approaches this fraction of the total. That t h e heat of I
Patrick and G r i m m : J. Am. Chcni. Soc. 43, 2144 (1921).
11, are shown graphically in Fig. 4. 50
-1-
\V. A . Pr\TRTCR .4XD C. E. GREIDER
TABLE I1 Heat of Adsorption-Sulfur Dioxide a t Sample gel = 1.813 gm.
0’.
1I ill ijir:ms : d ~ o r l ) c c l per grnm gel
(S)
11.8 j8.j 290.0
577.5 57.9
16j 2j6 ,184
IO.
7
1 0 2 .
