NH4HC03: A Stimulant For Learning R. Ronald Richards Greenville College, Greenville, IL 62246 Typically, we assume that the equilihrium vapor pressure of a pure solid or liquid depends on temperature only. Vapor pressure is not expected to change on addition of another coexistent phase or with a volume change of the vapor phase. For example, the vapor pressure of pure liquid bromine is identical with the equilihrium pressure of bromine ahove a two phase mixture of liquid and aqueous bromine, and the pressures are independent of volume. (Negligible solubility of water in Br;l(l) is assumed.) The existence of Br- and HOBr in the aaueous ohase, due to reaction of Br2with water, does not alter the result. which dissociates into Ammonium hvdropencarbonate, . . gaseous ammonia, water, and carbon dioxide, is one of a family of com~oundsthat behaves differently. The pressure of a three phase system containing pure solid, aqueous solution, and vanor differs from that of the solid-vapor system and varies with the relative volume of the aqueous and vapor phases. The vapor pressure of saturated aqueous NH4HC03 a t 25' with very small vapor volume (5.6 atm) ( I ) is about seventy-six times that of pure dry solid NH411COn (5.6 cm Hg) (2). ~emonstratio'of the large increase in pressure on addition of water to NH4HCOs(s) followed by classroom discussion of the apparent anomaly can stimulate chemistry students to think more carefully about equilibria. Most college chemistry students and possibly some professors cannot readily explain the anomaly. Yet the reason is simple. If it is not evident, you are in good company. Confusion has existed in the literature. Dihhits 131 in 1874 a .o.~ a r e n t l ywas the first to study quantitatively the vapor pressure of saturated ammonium hydrosencarbonate solutions. He found the pressure to be nearly L o atmospheres a t 14.5' and stated that the pressure is a function of the vapor-liquid volume ratio. Berthelot and Andre (4) in 1886 were interested primarily in the vapor pressure of the dry solid but noted that the
pressure is larger when the solid is slightly wet and increases on addition of more water. Much later Nishizawa (5)measured the vapor pressure of
(6).
In 1926 Bonnier (2) extensively studied the system. He determined the vapor pressures of dry NH4HCOa(s) and aqueous solutions. For the latter he studied the effects of concentration and vapor-phase volume. Hutchison (7) determined the equilihrium constant for reaction (1) in a system containing both solid and aqueous phases. His values were about 40% smaller than those of Bonnier who obtained data for the system without added water. NH4HCOds)
-
NH&)
+ HzOM + CO&)
(1)
Zernke ( I ) reviewed the vapor pressure data, clearly discussed the system, and reported his data in the limit of zero vapor volume. One more paper of significance has appeared. But before it is discussed, let us examine the system. At 25'C the vaporization reaction of dry solid NH4HCOs is straightforward. If solid is present, the total gaseous pressure is fixed and independent of gas phase volume. If water is added to form an aqueous phase, the total pressure of the system increases dramatically. A major shift of reaction (1) toward ~ r o d u c t soccurs due to the large solubility of NHz(g) in the newly formed aqueous phase.' Joseph Priestly (81,who discovered not only oxygen hut also ammonia, experimented with ammonia, carbon dioxide, and ammonium salts and writes concerning the solubilities of NH:I and C02, ". . . I I The ammonia fountain is an excellent demonstration of the effect. See reference (12).
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Number 7
July 1983
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presently found to be the fact, that this alkaline air (NH3) would be readily imbibed by water.. ." and "Water, a t least we know, cannot he made to contain much more than its own bulk of fixed air (COz)." oressure As reaction (1) moves to the right a much hieher ... . oiCO2 is produced. I ~ ~ S P < !water U . < cvl~dcnse.l10 remain in eauilihrium with r he auuwus otkise while am~mn:ad i s s d v e ~ . ~ &om a mathematical viewpoint, the equilihrium relationhip
~ requires that (P~,oPco,)becomes very large as P N Hbecomes Q very small. Since experimental data indicates that P H ~does not change by a large factor on addit~onof the aqueous phase, Pco2 must he large, resulting in a large PT. PT = PH%O + PNH.+ PCO? A. W. Fralwis (91i11mmuri7esrhr rxplanutiun in nnal~otract uf Bonnier's work. "It 15 s l ~ , t a nthat H . 0 d o e nut a,.t IIIIOII NH4HCO3 merely as a solvent, hut as a;eagent with high'affinity forNHs liberating CO2." I t is interesting to note that on heating dry NH4HC03(s) above about 33OC the partial pressure of water exceeds that of an aqueous phase, water condenses, and apparent anomalous behavior begins. Variance of the vapor pressure of the saturated solution with the relative volume of the aqueous andvapor phases presents the second part of the anomaly. Consider a saturated solution with no vapor phase. The predominant species in solution will be NH4+ and HCO3-, but NH, and CO2 will also be present with concentrations fixed through equilibria. If a vapor phase is allowed to form, COn will preferentially vaporize into it. Growth of the vapor phase will shift the concenand ~ Pco,. For trations of species in solution and hence P N H example, evaporation of CO2 will shift reaction (2) to the right which reduces [H']. HC0,-(aq) + Ht(aq) e COdaq) ~1 COAg) (2) NHlt(aq) e Htiaq) + (NHdaq) * NH3iq)I Reduced acidity slightly represses the further escape of C02 and enhances vaporization of NH3. From another viewpoint, the solution will contain more (NH4)2C03a8 C02 vaporizes.
2NHdHCOds) t 2NH1HC0daq) s (NH&COj(aq) + COdg) Thus, as the vapor volume increases, for a fixed aqueous phase becomes ~ larger which volume, more CO2 vaporizes, and P N H results in a lower PT. Further insight and pedagogical usefulness can he ohtained by application of the phase rule F=C-P+2 where F, C, and P are the degrees of freedom, number of components, and number of phases, respectively. For a dry solid in equilibrium with vapor, each phase consists of one
556
Journal of Chemical Education
component, NHdHCO3. One degree of freedom exists, and typical vapor pressure behavior is observed. A solid and saturated solution with no vapor phase consists of two components, NH4HC03 and HzO. But if the solid and saturated solution coexist with a vapor phase, then three components are needed to characterize the solution. A convenient choice is N H ~ H C O JH20, , plus (NH&C03 formed on evaporation of CO2. Since the phase rule predicts two decrees of freedom. the pressure is not fixed when only one variable such as temperature is given. Two variables, such as the ratio of solution to vapor volume and temperature, must he specified before the pressure is fixed. Data given by Ernst Janecke in 1929 (10) presents a most interesting puzzle. Janecke extensively studied the NH3H20-C02system. His principal interest was condensed phase equilibria, hut he does give some pressure data and indicates that saturated NH4HC03(aq) exerts a pressure of one atmosphere a t 60'. He does not state how or where he ohtained the data. The misleading information is surprising for several reasons. First, Janecke was an authority who contributed much to the phase literature of the early twentieth century. A. Finley (11) writes in the preface to the fifth edition of "The Phase Rule and Its Applications," "The opportunity has also been taken of explaining more fully . . . the graphical methods suggested by Janecke which are now very widely employed." Second, a pressure of more than one atmosphere is easily observed even a t room temperature. The continual formation of bubbles during solution implies a pressure larger than atmospheric. Third, a literature search would have revealed his mistake. Solid NH4HC03 without additional water exerts a pressure of about one atmosphere a t 60".He apparently did not realize that the pressure increases significantly on addition of water. Sometimes principles of equilibria are elusive. Smelling salts, which historicallv have contained so-called "ammonium
stimulant for student learning. Literature Cited ill Zernike, J . , R r c u o d 70.711 119511. 121 Bonnier.C.. Ann. Chim.. 5. 37 11926). 131 Dihhiti. H. C., J. PmkL C h o m . 10,417 118741. 141 Berthelof and Andre. Compl. R e n d , 103,668 (1886). 15) Nishiaaivs. K J Chem. I n d . IJaoani.23. , R10 ~ 11920).~ ~ , . 161 Washburn,EdwardW.,"lnternationai~rirical~abl'hs~~c~ra~-~illBookCo.,N~iv York. 1928.Vol 1II.o. 366. 171 Hutchison. W. K.. J . t h e m S u r . 410 119111. 181 Priertly. J.. "Expenrnents and Ohservaiionson Different Kindsof Air."Krsus Reprint C o , New York. 1970 ioriginal, 1774),Vul It. pp. 370, 312. 191 "Chemical Abstracts," American Chemical Society. 20,1572 (18261. (101 daneike.Z..Elrktiochrm., 35.716 (19291. (11) nndlai~ A,, "ThePhaw R u l e a n d l h A p ~ I i c ~ f i n n r , " S fEd..Longmnnr, h Green and
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