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The heat of adsorption of oxygen on charcoal Lotzkar, Harry 1935

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mm WAT OF ABSOTOOT OF Off&lff 01 0HAS00&L "by Hapyy Letafce?, B.A. A Thesis submitted as part fulfillment of the requirements for the Degree of Master of Arts in the Department of Chemistry I wish to take this opportunity to express nay appreciation to Br. M. J. Marshall, under whom this investigation was conducted, for his pleasant guidance during the course of this investigation. IMEOHJGTIOfF 1 APPARATUS 1 PROCEDURE 8 RESULTS S COICLTJSIOH. 5 SUPPIiESMEaSHPAL FORWARD.. • ; 9 RESULTS 9 SUMMARY. .11 THE HEAT OF ADSORPTION OF OXYGENON CHABOOA1. INTRODUCTION: The present investigation, although a continuation of similar investigations "by H. E. Bramston-Cook, D. H. Le Page, and Harold H. Herd under Dr. M. J. Marshall, was undertaken to determine the i n i t i a l differential heats of adsorption with the purpose of throwing some light on the mechanism of adsorption. The high i n i t i a l heats obtained by Harold H. Herd are compa-rable with the change in free energy when oxygen and charcoal unite to form carbon dioxide. References to previous work in this phase may he obtained from the paper of Marshall and Bramston-Gook {J.A.CS., 51, 2019, 1929). APPARATUS; The apparatus employed in the present investigation is essentially the same as that of previous Investigations mentioned above. However, the outer jacket of the ice calo-rimeter was built up so that cracked ice could be packed higher above the calorimeter, thus making conditions more stable for a longer period of time. The phosphorus pentoxide tube attached to the oxygen generator was replaced by a fresh quantity. The water in the calorimeter was driven out and freshly d i s t i l l e d water, free from air, was boiled into i t . The capillary tube was cleaned thoroughly to minimize sticking of the mercury. The ice mantle was built in the usual manner with liquid air and allowed a few days to come to equilibrium after -which a nor-mal positive leak of one mm. per hour was established. * The same sample of charcoal was used as in previous investigations but in this case, the quantity used was 25.6546 grms. The charcoal was outgassed for five hours at 1000° 0. in a quarta tube, sealed off arid weighed. The charcoal was removed and the tube was weighed empty but correction was made for the buoyancy of the a i r . The charcoal was placed in a quartz tube attached to the apparatus and was outgassed for sixty hours at 1000° C. after which i t was assumed that the charcoal was activ-ated to high enough degree. PROCEDURE: Y/ith the heat leak established and the charcoal out-gassed, the procedure involves the addition of oxygen measured by means of the pipette in the apparatus and the leak is f o l -lowed u n t i l the normal leak is picked up. At convenient inter-vals, the pressure was read by means of the MeLeod gauge. It was found that the normal heat leak is established within an hour of icing and that i t remains reasonably constant for a period of twelve hours. In view of the fact that the reaction i s complete and that the heat evolved is taken up by the mantle in several hours for i n i t i a l additions of oxygen, the constancy of the leak is f a i r l y dependable. RESULTS' Graph I constructed from table I, represents the molal heats of adsorption at concentrations varying from zero to 26.5265 x 10~e mols of 0 2 per gram of charcoal. Usually the molal heat of adsorption is determined from the slope of the graph in which total heat is plotted against total concentration but the method in which the molal heats of adsorption are de-termined from the ratio of Increments of heat (dq.) and incre-ments of concentration (dc) is found to be more accurate. Curves obtained by Bramston-Cook and Herd are shown for comparison. It was found that heat and pressure changes stop at nearly the same time and since this is in accord with observa-tions by Marshall and Herd, reference is made to Harold H. Herd, Thesis, 1934, B-A., U.B.C) . -/i'OOO . 10,000 0< 1 •c 0 1 0 t 1 t 111 \_ V) Ul H— • 1 r t IE J T T -J 1 ll "i r ! -hSOPO b0,O0C H E R D BRAM6TON - COOK L O T Z K A R . 11 i+ = CON-CSN-TRVTIOIST ~ 10 llf HOLS OF V* TABLE I. Baa Total concentration. Concentration Heat per run Heat of Adsorptii moIs x IQ-6 0g per per ran = C. in cal./gr. per gram charcoal : moIs x 10~6 cal. per gr. mol./gr. per gram dq charcoal dc . 1. 0.5362 0.5362 0.04038 75,310 2. 1.1949 0.6587 0.04965 75,370 3. 2.2376 1.0427 0.07803 74,830 4. 3.3327 1.0951 0.08130 74,240 5. 5.9925 1.5585 0.1907 70,370 6. 7.5510 1.5585 0.1031 66,170 7. 9.6735 2.1225 0.1402 66,060 8. 11.7960 2.1225 0.1364 64,260 9. 14.0478 2.2518 0*1430 63,480 10. 17.2128 3.1650 0.2095 66,200 11. 20.4612 3.2484 0.2019 62,140 12. 23.5616 3.1004 0.1822 58,700 IS. 26•5265 2.9649 0.1631 55,200 -5~ CONCLUSION: It was found that the graph for the differential heats of adsorption was practically horizontal at 75,000 calories for i n i t i a l additions of oxygen and then f e l l abruptly to lower values. However, the general behavior of adsorption in this case was very much the same as that observed by Marshall and Bramston-Cook and Marshall and Herd except for the f i r s t two or three additions of oxygen. In a previous series of runs made during this investigation, although conditions were not steady, evidence favoring a constant heat value of 75,000 calories was obtained for i n i t i a l concentrations. The nature of the graph points to two mechanisms of adsorption, namely, chemical and physical. If this is the case, then there are two rates of adsorption, the chemical rate being very fast as compared to the physical. Further, the energies involved in these two processes are different, the energy in-volved in the chemical process probably approaches the free energy of the chemical reaction, whereas the energy of the physical process is merely the heat of wetting. -The fact that the i n i t i a l quantities of oxygen added was adsorbed completely within twenty minutes strongly favors the chemical process. Not only the rate but also the heat evolved points to a chemical process. What the compound formed in the reaction i s open to speculation. But a compound of formula C„0,ris formed. However, a close relationship is seen "between the i n i t i a l differential heat of 75,000 calories and -52,510 calories and -94,260 calories for the free energies of formation of CO and COg resp. at 25° C. Marshall and Bramston-Cook found that for higli concentrations of oxygen that the heat of adsorption was in the region of 4000 calories which is ap-proximately the heat of wetting of carbon by oxygen. Therefore, experimentally, the two processes of adsorption are justified. From these two processes, the nature of the graph can be explained. Assume that the active carbon atoms are distributed in the surface of the charcoal and that i f any of these active atoms represent greater chemical affinity than others that they are a l l equally available to the oxygen mole-cules . The rate at which carbon and oxygen react is proportional to the surface density of active carbon atoms. For i n i t i a l ad-ditions of oxygen the probability of collision of oxygen mole-cules w i l l be removed from the gas phase rapidly and since the process of physical adsorption is comparatively slow, none or very l i t t l e of the oxygen is adsorbed in this manner. Therefore, the heat of adsorption for low concentrations is actually a measure of the chemical affinity of the reaction. As the active carbon atoms are removed from the "active group" the surface density of active atoms is diminished and the probability of col l i s i o n between active carbon atoms and oxygen molecules is less, therefore the time for reaction is longer and the physical process now is able to remove a substantial fraction of the ozygen from the gas phase. Therefore, the heat of adsorption is less than i t would have been i f a l l the oxygen had been adsorbed chemically. As the concentration of oxygen is increased less is adsorbed chemically and more is adsorbed physically; therefore the heat of adsorption f a l l s off progressively and fina l l y the heat of adsorption becomes the heat of wetting. Even i f some active carbon atoms represent greater chemical af-fi n i t y than others, each has an equal chance of reacting; there-fore the probability of some active carbon atom having greater energy than others does not enter the question of diminishing values for the heat of adsorption. The experimental fact that the rate at which oxygen is removed from the gas phase f a l l s off for diminishing heat of adsorption also favors the two mechanisms in adsorption. A constant heat leak is of the greatest importance in an investigation of this nature and any variation w i l l vitiate the results tremendously. One cause of variation is due to sticking of the mercury in the capillary tube and this sticking effect limits the size of the capillary which may be used satis-factorily. Water vapor was introduced into the capillary tube but when this failed to prevent sticking, a drop of water was placed in contact with the meniscus of the mercury with the result that the sticking effect was practically eliminated. This modification permits a capillary tube of finer bore to be used, thereby increasing the sensitivity of the leak. THE HEM? OF AIEOEPTIOff OF OXYSEH Off CHARCOAL F O R W A R D s Four series of results for the differential heats of ad-sorption of oxygen on activated charcoal were obtained and the results of the second series have been 'treated in the preceding text. However, th© values for the heats of the second series were obtained directly from the ratios of the increments of heat and of concentration. In this section the results of the four series are given and the values for the differential heats were determined from the slopes of the graph in which total concentrations as abscissae are plotted against total heats as ordlnates. In all the series except the fourth, the charcoal was out-gassed for f i f t y hours at 1000° C. and at the completion of this time the pumps were cut o f f at that temperature; but for the fourth series the charcoal was oatgassed for eighteen hours at 1050° C. and at the end of that time, the furnace was shut off and the charcoal was l e f t open to the pumps u n t i l the temperature f e l l to S00° C. As was suggested previously, water was banked against tho nraniscus of the mercury in the capillary tube to eliminate sticking. RESULTS; FIRST SERIES Run Concentration per run ( e c ) Heats per run (»q) in mola x 10"6 0g per in calories grata charcoal per gram charcoal 1, 1.0866 0.0785 Z. 0.0719 Zi 1.1789 0.0918 4* 1.2587 0.1033 5. 1.2916 0.1028 6. 1.2920 0.0893 7. 1.4050 0.0955 8. 1.6732 0.1123 9. 1.9247 0.1372 10. 1.9355 0,1510 •LJL « 2.9558 0.2136 IS. 2.3336 0.1665 15. 2.3160 0.1698 14. ' 2.3750 0.1472 S i X o o c 1;-' - ,7-' • • ' I .'.•'•:nl.:...-- . 1 ! 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J . - : : 'A'" x -1...-.ALA ;-.. , .. • " ' •'. -1 , ' -.-1 '".'j l .' - i ' -• i - . '•' ---•r~-' •1 »' :|: • •: .- i 1 1 " : " : T : : ' ' J3 A. : ; , ; 4 r : ; : :._| o i 17x1 JAkl* ".. I ' • . : S : " T " •i, . • • MP-. \ ) U j • : ! ' ' - r ! • . : ! • ; 'RAT ION R. QR i . r 7 ' A"! | J Total concentration la mols x 10~5 Og per gram charcoal Total heats i n calories per gram 1.0866 0.0785 2.2578 0.1504 0.2422 4.6754 . • • 0.3455 5,9570 0.4483 7.2590 0.5376 8.6640 0.6331 10.3372 0.7454 12*2619 0.8826 14.1972 1.0336 17.1530 1.2472 19.4856 1*43.37 21.8026 1.5835 24.1776 1.7307 Concentration Differential Heat of Adsorption i n molB x 10"6 G«> fla " -*S *£-. »n mil per gram charcoal calories per gram per tnol . ' per gram 0 .89,250 1 86,750 . ,2 85,750 3 84,000. 4 83,000 5 81,250 6 • 78,250 7 74,000 ' 8 • ' '71,000. 9 . . . ' .70,500'.. • 10 68,750 11 67,000 Graph II i s the graph for the Differential Heats of Adsorption for the f i r s t series. SECOHP SERIES Since the data for this series i s recorded i n Table I on page 4, only the differential heats of adsorption are treated in this section.,... • Concentration Differential Heat of Adsorption in raols x 10~S Os -per gram charcoal do calories per gram per raol per gram ...0" 76,700 1 76,500 z 75,750 5 75,2© 4 ° 74,500 5 72,500 6 71,000 69,000 S 68,500 9 68,500 10 68,500 11 68,500 12 68,500 I S 68,500 14 67,750 15 66,750 IS 66,250 1? 66,250 IS 66,000 19 65,000 20 64,250 21 62,500 22 61,000 23 60,250 24 59,000 25 57,750 26 56,750 27 55,500 These values of concentration and differential heats are plotted in graph III. wm> SERIES several runs were made i n order to follow the i n i t i a l d i f -ferential heats of adsorption. In this series covering a range of concentration up to 5 at 10"^ raols 0g per gram the differential heat of adsorption i s 70,000 calories. -12-BOCjgffi SERIES. jjarx concentration per run (=0) Heats per run (=g) in fflols x 10-6 o 2 per i n calories per gram charcoal ^ m charcoal 1 -1.5704- 0*1200 2 1.6844 ' 0.1820 S 1.5120 0.1151 4 2.1G73 0.1424 0 2.1296 0.1424 Total concentration. Total heats in mols x 10~6 0 2 per i n calories per grata gram charcoal 1.5704 0.1200 ' 3.2048 . 0.2521 4.716&- 0.3672 6:. 8241 0.5096 8.9537 0.6520 Concentration Differential Heat of Adsorption i n mols % 10"G Op per gram charcoal , . u , A * calorxes per mol Og 0 88,400 1 84,400 2 81,600 S 79,000 4 76,400 -• S 73,600 •& 70,000 t 87,000 .. 8 64,600 • © 61,400 The differential heats of this series are plotted, i n graph 17. -13-1. l a the present investigation no evidence of d i f -ferential heats of adsorption equal to the free energy of formation, of carbon dioxide was obtained. 2. The charcoal was activated as highly after eighteen hours of outgassing as i t was after sixty hours. 3. The effect of shutting off the pumps at 1000° C at the conclusion of the outgassing Is n i l , so far as the a c t i -vation of the charcoal is concerned. 4. By linking water agaiast the meniscus of the mercury in the capillary tube sticking was practically eliminated. 

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