Equilibrium in the System Hydrogen, Water Vapor and the Oxides of Tin Citation Magill, Paul Lafrone (1924) Equilibrium in the System Hydrogen, Water Vapor and the Oxides of Tin. Bachelor's thesis, California Institute of Technology. doi:10.7907/GHDG-PP88. https://resolver.caltech.edu/CaltechETD:etd-06072004-150521 Abstract No Abstract. Item Type: Thesis (Bachelor's thesis) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Unknown, Unknown Thesis Committee: None, None Defense Date: 1923 Record Number: CaltechETD:etd-06072004-150521 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-06072004-150521 DOI: 10.7907/GHDG-PP88 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2489 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 08 Jun 2004 Last Modified: 03 Oct 2019 23:18 Thesis Files Preview PDF (Magill_pl_1924.pdf) - Final Version See Usage Policy. 558kB Repository Staff Only: item control page

EQUILIBRIUM IN THE SYSTEM HYDROGEN, WATER VAPOR: AND THE OXIDES OF TIN Thesis by Paul L. Magill In partial fulfillment of the requirements for the degree of Bachelor of Science in Chemistry CALIFORNIA INSTITUTE OF TECHNOLOGY > Pasadena, California 1923 Outline of Investigation and Review of Previous Work Wohler end Balz ( zs. El-Chemie, 1921, pg. 414) determined the equilibrium for the reactions Sn02 + Hp * Sn0 + Hp0, Sn0 + i= _ Sa + Hod and SnOo + 2Hp ===> Sn + 2d at 700° and 800°. Tt was thought desirable to obtain the equilibriwn conditions at sone lower tenperature and thus avoid some of the error in extropolation to stendard teuxperature. Furthermore, the heat effect calculated from | the results of Wohler and Balz aiffers by over 9,000. calories from that calculated from the thermochemical data of W. G. Mixter ( Am. J. Sei., 27, 220-34) who determined the heats of formation of stannous and of stannic oxide. The purpose cf the present investigation was to obtain the equilibrium constants of the above reacticna at as low a temperature as practicable. A series of experiments was made to determine whether or not the reaction could be carried on at temperatures as low as 350° and 450°. Experiments were made to prove the décomposition - ~. 2 of stannis oxide from 300° to 600° was negligible. The final equilibrium measurements for the above reactions were made at Yuko. | The atomic weights used throughout this work are as follows: Sn, 118.7; H, 1.0; 0, 16.0 Preparation of Materidls The stannic oxide used was made by treating a good grade of in- ported tin with C.P. concentrated nitric and then drying and heating the product at a-red heat for eight hours. The stannous oxide was «repared by boiling a good grade of do- _mestic stannous chloride with finely divided metellic tin in aetd solution until the liquid became clear. The flask containing the stannous chloride was then washed clean of all air by means of hydro- gen. From this point to the drying of the final product the solutions were kept in an atmosphere of hydrogen. Stannous hydroxide was first precipitated by neans of sodium hydrcxide. The stannous oxide, in the form of blue black crystals, was obtained by boiling the stannous hydroxide in a dilute alkaline solution. The oxide thus obtained was washed by decantation until the washings gave no test for chlorides. After allowing to drain, the oxide was washed first with alcohol and then with ether. It wag allowed to dry in a vacuum desiccator, over sulfuric acid for two weeks. The results of later experiments seemed to show that the stannous oxide prepared in this way contained water. The black crystals of stannous oxide wereithen placed in a hard glass tube attached to a vacuum pump and heated for several hours in an electric furnace at 600°. Upon removal from the furnace, the stannous oxide apreared as an alive green powder. | ‘The metallic tin was precipitated from a solution of stannous hydroxide, prepared as above, by boiling the hydroxide in 4 strongly &lkaline solution. The finely-divided tin, prepared in this way, was washed until free from chlowides then melted to a button in an atmos- phere of hydrogen, efter woich it was cut into small pieces. The hydrogen used was rrenared by the action of hydrochlorie acid on gine. It was washed by means of concentrated sulfuric acid and alkuline pyrogallol. f Apparatus and Method In the early part of the work a flow method was tried. However, at the temperature 500° it was found that equilibrium could not be ob- tained in a reasonable length of time. The static method was then tried at 357°. The method was as follows. Into a flask of about sixty cubic eentimeters capacity was introduced several grams of a mix- gure consisting of the initial and final solid substances which took part in the reaction. In the ease of the reaction Snd + H0 = Sn0o + Ho and Sn + HpQ ==* SnO + Ho, in which water was one of the initial reacting substances, a small weighed quantity of water wag in- troduced into the flask by means of a sealed bulb. With the bulb was introduced a small solid glass rod. The neck of the flask was drawn down to a small capillary and the whole attached to a vacuum pump, capable of evacuating to a pressure of less than one one-hundredth of amilli-meter. After the flask had been evacuated, it was sealed by fusing the capillary neck. The bulb containing the water was. then fractured by jarring the flask sufficiently to ceuse the impact of the ‘rod to crack the bulb. The flask was them placed in a vapor bath and allowed to heat for several days. When it was thought that equilibrium was reached, the flask was removed from the bath and rapidly ccoled in a strong blast of air. The cooling usually required from one and one- half to two mimutes. It was then weighed after which the tip was broken off under water and the water allowed to rise in the flask. There was a second weighing to determine the volume oceupied by the water vapor at equilibrium. The space left above the water was filled with hydrogen in the quantity at which it was present at equilibrium. ‘The partially filled flask was then filled completely with water and weighed again. By making the proper corrections for barometric rressure, heights’ of water, temperature of water and air, vapor tension ete., the quantity of hydrogen was computed. It was rossible from these results, as wkll be ghowm later, to calculate the equilibrium constants. | | In the case of the reactions Sn0 + Hp <=> Sn + HpO and Sn0p + Hp =~ 5n0 + Hp0, in which hydrogen was one of the initial reacting sub- stances,. the flask was filled with the substances and the neck was - drawn dowm as before. Connection to the flask was made by means of a two-way stop cock. One position would connect the flask to the vacuum pump and the other position would connect it to a source of hydrogen. | The pressure at which the flask waa filled was measured by means of a mercury manometer. In order to facilitate the reactions in which metallic tin was one of the reacting substances an apparatus was devised which would cause the molten tin to be agitated. It consisted of a flask of about thirty cubic centimeters capacity whose shape was somewhat conical. then the flask was rapidly rotated the molten tin would be thrown on the wall in @ thin layer, thereby increasing the surface of the tin and keeping the metal separated from its oxide. The determination: of the equilibrium © mixture was nade in the same manner as the above. | Exrerimental Results The first part of the work was tried at 357°. At this temerature the reactions SnO0 + Ho ===Sn + HpO and Sn0o + Hp ===> Sn0 + HO seemed to go rather slow. The reaction Sn0o + 2Ho =—— Sn + 2H20 after heating | for two days was found to have gone so far to the right that an accurate determination of the constant was not feasible. Befcre going to a higher temperature an experiment was made to determine whether or not stannic oxide had an appreciable dissociation pressure. It was fund by the use of a mercury nanometer that between 300° and 600° the rressure of the oxygen was negligible. | The temperature was increased to 44uko and the following results were: obtained. Table 1 “Initial sub-frotel mols! Totel Vol.| Py |Py, |@#20= x/Totel | Hours stances ~~ (ee.) (atm. )} (atm. ) Py, . [Pressure heated Sn0,Sn0o,H» [0.00256 | 64.40 0.454 [1.88 |0.cho | 2.34 yl SnO,Sn09,Hp [0.00252 64. 20 0.477 |1.235 |0. 260 | 2.32 72 $n0,Sn02,Hp [0.00262 | 66.50 0.481 |1.820 10.270 2.31 &6 SnO>,Sn0,HpO0 |0.00312 =| 64.40 1.01 1.75 0.576 2.76 41 Sn0>,Sn0,Hp0 [0.00165 | 66.36 0.425 11.035 0.4Os | 1.46 66 Sn0z,5n0,HpO |0.00106 =| 63.79 0.208 |0.638|0.326 | 0.8h6 &6 Sn0,Sn,H20 [0.00269 | 47.80 1.775 |2.530 [1.150 3.32 86 Sn0,Sn,Ho0 0.00250 66.67 0.730 [1.470 10.667 | 2.20 al. Sn0,Sn,H20 0.00200 65.69 0.639 |1.790 0.554 1.79 41 Sn,Sn0,Hp (0.00192 (48.34 0.807 [1.533 (0.415 2034 se Sn,Sn0,Hp 0.00272 68.40 0.808 /1.512 (0.528 2.32 4o | Although there is a close agreement of the results in several cases, the variation in the remaining determinations seems te be greater than that accounted for by experimental error. An experiment was made to determine if the reaction 25n0 Sn + Sn0e would teke place at 4Wuy°. After heating a sample of stannous : oxide in boiling sulfur for ten hours, the following differences were noticed. -) Before heating. The stannous oxide existed as blue black crystals. ‘The crystals diesolved in concentrated hydrochloric acid leaving no residue. No gas was evolved during the solution of the black crystals. _ After heating. - The material existed as an olive green powder. The powder partislly dis- solved leaving a white residue. Gas seemed to be evolved in small amount | wpon the addition of concentrated hydrochloric acid. | The reaction Sn0zg + 2H>——~*Sn + 2H20 was tried at W4U° and the following results were obtained. a Table 2 ie sub-| Total mols | Total vol. P30 Pu Py 0 = x|Totel | Houre pvance (atm.)| (atm.) Pi pres. | heated Sn02,8n,Hs | 0.00252 | 6420 1.16 | 1.26 | 1.00 | 232] 62 Sn0>,Sn,Hp | 0.00148 37-00 1.563 | 0.803 | 1.938 | 2.36 n Sn0g,Sn,H20 | 0.00266 46.40 3.100 | 0.280 | 10.99 | 3.38 62 ISnOo,Sn,H20 | 0.00304 66.20 2.63 | 0.0703) 42.uo 2.70 62 Calculations The heat effect for the reaction Sn0o + lp —* 220 + Sn was calculated from the results of Wohler and Balz by means of the integrated van't Hoff equation from the constants 107.0 at 700 and 184.5 at 200° C. The heat effect is -/AE = 11,28 between 700 and 80°. By using the specific heat | of stannic oxide as 0.0938 the specific heat of tin as 0.05876 and the heat of gusion of tin as 1,700 calories for 1 gm. atom, the heat effect was cal- culated to be - AH = 23,380 - 1.50 T - 0.00287" + 0.000,001, 489 above 233° C. Or =A H = 20,530 calories at 750°C. The heat effect below 233° is - AH = 22,167 - 2.67 T = 0.002877 + 0.000,00148 T”. By using the value 11,28 for - AH and calculating | the equilibrium constant from 800° to Wile C, the value tor( Piso sie)" was found to be 13.13. If the value for - /\H is used that was determined from heat data, the value tore oy is found to be 1.398 when calculated from 800° to HUY, and i when calculated from 700° to 4uk°. ‘The equilibrium con- 10. stant (#20 )* when calculated from 800° to 25° is 7.95 x 10 and en Galculated from 700° to 25° is 14.4 x 10 10 These values give the free energy of stannic oxide to be 121,364 calories and —}21,134 calories respectively when the free energy of #20 (gas) is taken as ~54,507 calories at 25°. An average of the best experimental results of the present work P was taken and the value for (ey was found to be 0.158 at Yuko C. when this values is calculated to 233° C inet snerpoint of tin) al PHO it was found to be 3.19 x10 . The value for /——— was found to Z ~-10 be 1.63 x 10 at 25°. ‘These values give the free energy of stannic “oxide to be -126,488 calories at 506° A and -122,364 calories at 298° A. An average value for Py2o for the reaction Sn0 + Hy =—$n + H20 P. was found to be 0.541 at ue 6. When this is calculated to 233° C it was found to be 0.0633 and when calculated to 25° C it was 3.96 x 10” This gives the free energy of stannous oxide to be -62,009 calories at 233° C and -60,507 calories at 25° C. The heat effect used was - A= 7,947 - 1.1 - o.colye® + 7.4 x10’ below 233° C, and - AH = 895 = 1.47 - 0.001482 + 7.4 x 10° 'n”, above 233° C. é. Another calculation was made to determine at what temperature ‘stannic oxide, | stannous oxide, and tin are in equilibrium. Values taken from the results of Wohler and Balz give 100° A, experimental data from the present work give 786° A. The change of the free energy with the temperature of the reaction 25n0 =—— Sn0o + Sn is in such a direction that at temperatures below those given tin will reduce stannic oxide to stannous oxide. Discussion of Results Although the results of this research are not conclusively quan~ titative, it has been shown that the above reactions do take place at yuye C at a moderately rapid rate. Application of the phase rule to the system Sn, SnO, SnOs, HeO and Ho shows that there is but one temperature . @t which the three solid phases can be in equilibrium, and at this tem- perature oth the total pressure and the ratio of H20 to Ho are fixed. At temperatures below this point the foregoing calculations show that the stable phases are Sn and Sn0, above this SnOo and Sn’ are the stable phases. This, however, does not exclude the possibility of a metastable equilibriug between any two of these three phases at any temperature where the rates of reaction between the solide (such ae Sn + Sn0g —s 28n0) are very slow. But to obtain this metastable equilibrium the ratios of H20 to HZ and “the total pressure would have to be such as not to produce a third phase or else the rate of attainment of equilibriwn with any third phase appear- ing would have to be slow. This would doubtless be the case when only small quantities of the third phase appeared. - It is possible though improbable that the equilibrium changed during the cooling of the tube. It is likely that a continuous flow method.could be used; to better advantage. The disagreement of the data in the second table was due, no doubt, to the reduction of the stannic oxide by the tin; thereby preventing equilibrium from being reached. The calculations show that this is possible at Whe C It can be seen from the data in the first table that with the exclusion of one value, the greatest variation is about two fold. In calculating the free energy of stannic oxide at 25° a two fold varia- tion in the equilibrium constant would make a difference of about 360 calories. The free energy of stannic oxide was found by electromotive- force measurements to be 122,374 cal. at 25° which 1s only ten calor- ies different from the above value. | Summary In this article results are given to show that the reactions Sn0g + Hp "Sn0 + Hy0, SnO + Hp=—>S m+ HO and Sn02 + 2Hy = Sn + 220 take place at a fairly rapid rate at uke c. Results have been obtained which indicate that the reaction Sn + SnOo—~2Sn0 takes place below 513° C. The decomposition? stannic oxide was found to be negligible between 300° ahd 400° Cc. The equilibrium constant for the reaction Sndz- + 22 = Sn + 2H0 was found to be 1.63 x 10° 10 at 25°. It was calculated from the results of Wohler and Balz to lie between 7-95 x 10 °° and 14.4 x 102° at 25° C. The equilibrium constant for the reaction Sn0 + Hp —-—+Sn + H20 was found to be 3.96 x 107) at 25° C. * Palmer Miller's Thesis, C.I.T., 1923. 10. The free energy of stannic oxide was found to be -122, 364 calories at 25°. It was calculated from the results of Wohler and Balz to lie between -121,364 calories and -121,134 calories at 25°. The free energy of stannous oxide was found to be -60,507 Calories at 25° C. Note:. From the results presented in an abstract (Chem. Abstr., 17, 3124) of the data obtained by Teutommuu Maeda (Bull. Inet. Phys. Chem., Ree., Japen) on the equilibrium SnO2 + 2CO = Sn + 2COo, the reaction 2Sn0 = SnOo + Sn was found to take place in the temperature interval from 700%e 950° C. From the calculated pressure of oxygen of 4.47 x 10725 atm at 927° C, as reported in the abstract, the free energy of SnOo at 25° is calculated by me to be -122,350 cal. | Moreover, from the temperature coefficient of the equilibrium, Maeda calculates (\H for the reaction Sn0z = Sn + 02 to be 130,260 cale, while thermochemical data give 133,570 cal.which is perhaps. as good agreement as could be expected. | - It seems certain then that the frea energy of Sn0o at 25° is very close to the value -122,360 cal. and aleo that at temperatures above about 500° C Sn and Sn0o are the stable phases ‘and below this temperature Sn and SnO are the stable phases. a