EYDRAZISIUM BROMIDE AS A SOLVEXT
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HYDRAZINIUM .BROMIDE ,4S A SOLVENT BYRALPHP. SEWARD Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvaniala)b Receiaed December 23, 1961
From the lowering of the freezing point of hydrazinium bromide by hydrazinium nitrate, hydrazinium iodide, and animonium bromide a cryoscopic constant of 8.20 f 0.1 degree per mole solute per kg. solvent was established. The molal lowerings for the solutes ammonium nitrate and lead iodide were, respectively, twice and three times 8.20. A molal lowering of half of 8.20 for the solute hydrazinium chloride is attributed to solid solution formation. Silver, lead, and mercury(I1) bromides cause abnormally large freezing point depressions. Mercury( I) bromide disproportionates. The heat of fusion of hydrazinium bromide calculated from the cryoscopic constant is 3540 cal. mole-', while a calorimetric measurement gave 3800 cal. m3le-1. Hydrazinium bromide is a poor solvent for non-ionic solutes. Water, however, gives the normal freezing point lowering. Hydrazinium bromide and tetiabutylammonium bromide form a system of two liquid layers a t temperatures above the melting point of the latter salt.
Observation of solubilities in and the cryoscopic behavior of solutions in the solvent hydrazinium bromide have been made with two objectives. One of these was a comparison of the lowering of the freezing point of this fused salt solvent on addition of a non-ionic solute with the lowerings produced by typical salts. Observations of this type are precluded for simple binary salts by their high melting points. As only for the ammonium nitrattl-tvater2 and potassium thiocyanate-water3 systems are data available of adequate precision for such a comparison, further observations seemed desirable. Chrelien and Sessius4 made cryoscopic measurements with hydrazinium chloride as solvent but their only non-ionic solute was urea, for which the measurements are somewhat uncertain since the freezing points varied with time. Hydrazinium bromide has the conveniently low melting point of 86.5', but the disadvantage that most organic compounds are notflsoluble in it. Urea and acetamide q-ere found to be soluble but the solutions appeared unstable, confirming the observations of Chretien and Nessius* in hydrazinium chloride, and the only satisfactory non-ionic solute found has been water. A second objective of the investigations was that of finding more about the mutual solubilities of fused salts, in particular, whether two salts with a common ion could differ sufficiently that two liquid phases would result. The considerable difference in the nature of the hydrazinium ion and quaternary substituted ammonium ions suggested that hydrasinium bromide and tetrabutyl ~mrnonium bromide might be immiscible in the liquid state and this proved to be the case. The bromide rather than the chloride system was chosen because of the relative ease of preparing the quaternary ammonium bromide compared to the chloride. In order to establish the normal cryoscopic behavior of hydrazinium bromide with ionic solutes, measurements were made with 1'2HbN03, NZH&I, N2HJ, II-I-14Br, 1\TH4NOs,and Pb12, and, in order to check certain observations of Chretien and (1) (a) Done in part a t Bristol, England. The author is indebted to Professor D. 13. Everett for making the facilities of the University chemical laboratories available to him: (b) supported in part b y the U. S. Atomio Energy Commission under Contract AT(30-1)-1881. (2) A. G. Keenan, J. Phus. Chem., 60, 1356 (1956); 61,781 (1957). (3) F. C. Ktacek. J . Wash. h a d . Sac., 26, 307 (1936). (4) A. Chretien and A. Nessius, Bull. aoc. cham. France, 7 , 258 (1940).
Nessius,* wiih AgBr, PbBr2, HgBr2, and IIg2Br2as solutes. Finally, as a check on the cryoscopic value, the heat of fusion of hydrasinium bromide was measured calorimetrically. Experimental Materials-Hydrazinium bromide was prepared by adding aqueous hydrobromic acid to aqueous hydrazine until the methyl orange end-point was reached. After removing most of water by heating until the temperature reached about 100 , the salt was recrystallized twice from methanol. Final drying was accomplished by bubbling dry nitrogen through the molten material or more quickly by melting under a vacuum. Hydrazinium bromide is deliquescent only in a quite humid atmosphere. Its melting point was found to be 86.5'. Gilbert and Cobbs reported the m.p. as 84'. The other hydrazine salts were prepared in a similar fashion and had melting points in agreement with previously re orted values. Tetrabutylamrnonium bromide was made from tetrabutylammonium hydroxide and hydrobromic acid. The ammonium salts were commercial "Reagent" chemicals while silver bromide and the lead and mercury salts were prepared by precipitation from aqueous solution. Freezing Point Measurements.-These were made by the Beckmann technique, in part with a Beckmann thermometer and manual stirring, and in part with a thermistor and d.c. bridge for temperatures and mechanical stirring. The reproducibility was of the order of zt0.0lo in the most dilute solutions, up to 10.03O a t higher concentrations, with both methods but the latter method was more convenient. Super cooling was kept at a fairly constant value of about 0.5' by addition of solvent cr stals. As was found by Chretien and Nessius with K d C 1 , contact with the atmosphere resulted in a progresEive decrease in freezing point. Hence a stream of dry nitrogen was passed over the solutions during measurements. When the solute was water, the solution was covered by a layer of paraffin oil t o prevent loss by vaporization. With water as solute the concentrations of solute were extended to higher values and since the Beckmann technique becomes less reliable as the freezing points depart further from that of pure solvent, solutions over 5 molal in water were stirred with solid solvent in a large constant temperature bath and filtered samples of liquid taken. The composition of these samples was determined gravimetrically by precipitating silver bromide. Calorimetric Measurements.-These measurements, of only moderate precision, were made by heating samples of hydrazinium bromide, sealed in thin walled glass tubes, to various temperatures in the constant temperature bath and dropping them into the calorimeter. The calorimeter consisted of a 500-ml. dewar jar containing 400 ml. of water and equipped with a mechanical stirrer. Temperatures were read with the aid of a thermistor which was sensitive to better than 0.001". To reduce heat loss in transfer of the sample to the calorimeter the sample was removed from the bath in a glabs sleeve which surrounded it until it was dropped into the calorimeter. The heat capacity of the calorimeter wab determined by similar operations with a piece of
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( 5 ) E. C. Gilbert and A. W. Cobb, J. Am. Chem. Soc., 87, 39 (1935).
RALPHP.S i : w . ~ 1 ~ 1 ~
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aluminum metal. From several observations with two different samples of the salt the mean heat capacity from room temperature up to the melting point was found t o be 25.2 cal. mole-' deg.-l and the heat of fusion to be 3800 cal. mole-'. Individual values differed by no more than 2% from thme figures.
Results and Discussion The Cryoscopic Heat of Fusion.-When the freezing point lowerings produced by the solutes N2HjC1, N2H5S03, KzHJ, KH4Br, S H 4 S 0 3 ,and PbI2 a t various molal concentrations were plotted against solute molality, the points for NzH&03, NzHJ, and SH4Br all lay close to a straight line which had a slope of 8.20 deg. (moles solute)-l (kg. solvent)-l. There was no systematic curvature and individual deviations from the line were of magnitudes comparable to the uncertainties in individual determinations. The freezing point lowerings due to NH4N03and PbI, were observed to lie close to straight lines drawn to have, respectively, slopes of twice and three times 8.20. It may be concluded that solutions in the solvent K2H5Br behave in the normal manner for molten salts. This behavior is illustrated by the experiments of Van Artsdalena with NaK03 as solvent in which the introduction of solutes having two and three ions foreign to the solvent produced twice and three times the lowering in freezing point produced by equal molalities of solutes having only one ion foreign to the solvent. A line drawn through the points for the solute NzH&l would have a slope of about 4. To confirm the suspicion that this difference from the other solutes is due to the formation of solid solutions, freezing points mere observed over the complete composition range in the N2HjBr-K2H&1 system. With the bromide freezing at 86' and the chloride at 92' a minimum freezing point of 72' was observed. If the system behaved ideally with no solid solution formation, a eutectic point should exist at about 46'. Furthermore, no sign of a eutectic halt could be found in cooling any of the mixtures. A molal cryoscopic constant of 8.20 for a solvent freezing a t 359.7'K., and having a formula weight of 113, leads t o a heat of fusion of 3540 cal. mole-I. Although there is a 7% discrepancy between the cryoscopic 3540 and the calorimetric 3800 heat of fusion, the agreement is good enough to indicate that 8.20, rather than some multiple, is the normal cryoscopic constant. Silver, Lead, and Mercury Bromides in Hydrazinium Bromide.-These bromides as solutes give more than the normal freezing point lowerings, increasingly so with rising concentration in the case of lead and silver as shown in Table I. With these solutes the freezing points were less reproducible and the numerical values of AT/m more uncertain. They are sufficiently consistent t o show that the deviations from normal behavior are in the direction which would be expected if the formation of complex ions has decreased the amount of solvent present. There is no sign that a t lower concentrations ATlm approaches double its normal value as was reported by Chretien and Nessius4 for PbClz and HgClz in their solvent, (6, E.
R. Van Artsdalen, J. Phgs.
Chem., 60, 172 (1966)
T'ol. fiG
TABLE I LOWERIXG OF THE FREEZISG POINT OF S2H6BrBY SILVER, LEAD,AND MERCURY BROMIDES AT VARIOUSMOLAL CONCENTRATIONS
AgBr
m
PbBrz
nz
HgBrz
ATjm m
ATlm
aT/m Hg2Br2 m Al'/m
0 081 0 169 0 275 0 377 8 40 8 92 8 95 9 17 0.080 0 172 0.267 0 377 8.45 8 80 9.41 9 52 0 021 0 121 0 221 0 347 9 3 10 5 10 7 10 2 0 047 0 128 0 214 0 301 10 1 9 6 10 0 10 7
X2HsC1. The observations with mercury(1) bromide were made to see if the Hg2++ion could exist in the salt melt. Immediately on addition the mercury(1) bromide turned black and, after stirring, a drop of liquid mercury formed in the tube. From a melt to which 0.00663 formula weight of HgzBr2had been added there was recovered a drop of mercury amounting to 0.00630 g.-atom. It is apparent that the freezing point lowerings recorded for the Hg2Br2are actually those for the HgBrz formed by disproportionation. This behavior agrees with that of HgZCl2in K2H5C1as reported by Chretien and N e ~ s i u s . ~ The equilibrium Hgz++ Hg++ Hg could well be essentially complete to the right owing to the formation of more stable Hg(I1) halide complexes. Water in Hydrazinium Bromide.---Up to a water mole fraction of 0.50, values of minus log of NzHjBr mole fraction plotted against the reciprocal of absolute temperature give points lying on a straight line. The straight line corresponds to the equation
+
Since the slope of the line depends on the heat of fusion calculated from the freezing point lowerings produced by typical salts, it is seen that solutions with water as solute do not differ significantly from those with salts as solutes. It appears that a small amount of water does not alter the heat of fusion of the solvent salt and that the water is randomly dispersed among the ions. If this is so, water molecules would have the same effect in dilute solution as the addition of an equal number of ions which are foreign to the solvent salt. As this may not be obvious, consider a mixture of n moles of HzO in nomoles K2HsBr. If the salt is completely dissociated, the mole fraction of each ion is no/(n 2no). Taking the activity of each ion as the ratio of this mole fraction to the ion mole fraction in the standard state of pure liquid N2€IsBr where it is no/2no, each ion activity becomes 2no/(n 2n0). The solvent activity, taken as the product of the ~ ) ~-log (solvent, ion activities, is 1/(1 n / 2 ~ 2and activity) = log [I n/no 1/4(n/n~)2]which differs from -log (solvent mole fraction) = -log [no/(no n ) ]by only 2.4% for a 0.10 solute mole fraction. It is perhaps surprising that the linear relation is followed so well in solutions having a high water concentration. Above 0.50 mole fraction of the water the measured solubilities become slightly smaller than those predicted by the straight
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+
+ +
+
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June, 1962
HYDRAZINIUM BROMIDE AS A SOLVENT
line. Even a t the lowest temperature, 1 6 O , although this is 70' below the melting point and the salt mole fraction is only 0.285, the solubility predicted by the straighl line differs by only 3% from the measured value. The only salts for which comparable measurements have been made are potassium thiocyanate and ammonium nitrate. For potassium thiocyanate Plester, Rogers, and Ubbelohde' found the heat of fusion to be 3390 cal. per mole by calorimetric measurements. From cryoscopic observations Dingmans8 found heats of fusion of 3130 cal. with KC1 as solute and 3185 with KBr as solute, while Kordes, Bergmann, and Vogelg report freezing point lowerings by KBr corresponding to a heat oi fusion of 2250 cal. Kracek3 found the solubility of KSCN in water near the melting point of the salt to vary with temperature in accordance with a heat of fusion of 2960 cal. I n view of the disagreement in the cryoscopic measurements it is not clear whether solutions of water in KSCN behave differently from solutions of salts in KSCN. In ammonium nitrate as solvent, Keenan2 found no significant difference between the freezing point lowerings produced by water and those produced by salts. The Hydrazinium Bromide-Tetrabutylammonium Bromide System.-When tetrabutylammonium bromide was added to hydrazinium bromide no decrease in freezing point could be detected. The solubility of tetrabutylammonium bromide thus must be less than 0.003 molal at a temperature only about 30' below its melting point. At temperatures above the melting point of tetrabutylammonium bromide, two liquid layers were found. Although a number of instances of two liquid layer salt systems have been reported, these involve systems without a common ion. The currently (7) D. K. Pleater, S.E. Rogers, and A. R. Ubbelohde, J . SCL Instr., 33, 211 (1956). 18) P. Dingmans, Ree. trat. chm., 58, 559 (1939).
(9) E. Kordes, Tir. Bergmann, and G.W. Vogel, 2. Elektrochem., 55, 600 (1951).
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accepted model of a molten salt is that in which the nearest neighbors of cations are anions and the nearest neighbors of anions are cations, without the long range order existing in the crystalline state and with lower average ion coordination numbers than in the solid form. This picture of the state of the liquid suggests that only the considerable difference in cation size causes liquid-liquid immiscibility in the abore system. The Displacement of Iodide from Solution in Hydrazinium Bromide by Bromide.-On addition of 5.97 mmoles of N2H51,the freezing point of 19.54 g. of N2H5Brwas lowered by 2.41'. When 2.01 mmoles of (C4H9)4X.Brwas added the freezing point of the solution was higher. Ejight successive observations of the freezing point showed increasing temperatures, an apparently limiting value 1.60' below the initial freezing point of the solvent being reached. This can be accounted for by the removal of 2.01 mmoles of iodide ion from the solution and its replacement by 2.01 mmoles of bromide ion from the tetrabutylammonium bromide. The slomness of the exchange can be attributed to the fact that two phases were involved. This experiment was done to find further evidence on the effect of ion size on solubility. Coulomb's law predicts a lower energy for a system where the larger cation is associated with the larger anion and the smaller cation with the smaller anion compared to that of the reciprocal salt system. I n the system described above the favored pairing would be SzH5.Br and (C4H9)4Y.I. A qualitative observation with hydrazinium picrate added to the two phase iY2H5Br-(C4H9)4n'.Brsystem was found to be consistent with the observations on the system containing iodide. In this case the (C:4Hg)qBrphase acquired a deeper yellow color than did the NzH6Br phase which, however, must have retained some of the picrate. As the solubility of hydrazinium picrate in hydrazinium bromide is quite small, freezing point lowering experiments were not attempted.
