Edited by
textbook forum
GEORGEWIGER California state Unverslfy Carson. CA 90747
What is the Boiling Point and Heat of Vaporization of Sulfuric Acid? R. Thomas Myers Kent State University, Kent, OH 44242 This paper could well he subtitled "The Relation of Common Sense to Chemical Data," for if one looks in a handbook one can find rather implausihle data for the answer to the question in the title. One handhook ( 1 )gives: 3X°C (98.3701, no information on heat of vaporization. Another popular handbook ( 2 )gives: 330°C, and 122.1 callg a t :32fiiC (slightly helow the cited hoiling point). I began to douht this data when I noted the great discrepancy in hoiling points, and when I calculated the Trouton constant for H2S04. Using the ahove data the latter comes out to he 19.9 cal/mole/K, much too low for such a highly assuciated liquid. One reason for the discrepancy in hoiling points is that the hoiling point of sulfuric acid is undefined in the true sense hecause this compound decomposes on vaporizing, even a t room temnerature. The mhst complete study of the equilibrium between " ~ u r e "liauid sulfuric acid and the eases ahuve it is by Lumarks is that liquid ckinskii ( j ) .The reason for the sulfuric acid is itself dissociated, consisting mostly of H2SO4, hut also containing HSOn-, H:80.1+,H:iO+,HS~OT-, HzSa07, and H z 0 ( 4 ) . One way of defining the hoiling point of sulfuric acid is to give the temperature a t which the total vapor pressure of the gases above it reaches 760 mm of Hg (1.013 X 10" Pa). Luchinskii found this to he 280°C. At this temperature thevapor consists oE 76.6 mole % SO:,,22.6% HzS04,and 0.8% HaO. This temperature is the one which should he entered into handbooks. ( I t is analogous to the vaporization of, say, organic carhoxylic acids. For these the liquid phase consists mainly, hut not entirelv. " . of dimers. and there is partial dissociation into monomers in the vapor phase.) Upon hoiling, the sulfuric acid loses SO:, relative to H20, and continued distillation leads to a maximum-boiling azeutrupe, with 98.33% HzSOn, hoiling a t :138.8"C. This value is freauentlv given in handbooks. The rap^ ;aImve thi, azvlrz~lw h,i>lhv tl.tnt in: parI1,d ~m~sslirv~: I I
