edited by
RALPH K. BIRDWHISTELL Universtiy of West Florida Pensawla. FL 32504
textbook forum The Water Solubility of 2-Butanol: A Widespread Error Donald 6. Alaer California State-university Chico, CA 95929 While using a currently popular organic chemistry text a s a source of data for comparing relative solubilities of some alcohols and ketones, i t was observed that the solubility of 2-butanol which was given as 12.5 g per 100 g of water was muchlower than expected. I n general, secondary alcohols have approximately the same solubility a s the corresponding ketones and the solubility of 2-butanone is about 26 g per 100 g of water. It was found that of 12 recentedition organic texts five gave quantitative solubility data for 2-butanol and they all had this same value. I n addition, the older editions of the CRC Handbook of Chemistry and Physics, which still listed quantitative solubilities, and Large's Handbook of Chemisty also gave this value. The Merck Index lists a n even lower solubility of one part in 12 parts of water (approximately 8 g per 100 g of water). However, solubility values of 26.0,22.5, and 18.0 g per 100 gof solution a t 20,25, and 30 "C, respectively, were reported by Alexejew in 1886, with similar results reported by Dolgolenko in 1907 and Dryer in 1913. This information was compiled and cited in 1941by Seidell.' Current industrial data provided in a compilation of material safety data sheets,' indicate a solubility of 20.00 lb per 100 lb of water a t 68.0 O F . This is equivalent to 20 g per 100 g of water a t 20 "C. The origin of this error is undoubtedly from Beilstein's Handbuch der Organischen Chemie. The main work (das Hauptwerk), which covers the literature through 1909, cites a solubility reported by Norris and Green3 of one part alcohol per eight parts of water. This corresponds to 12.5 g per 100 g of water. Interestingly no other citations regarding the solubility of 2-butanol appear there or in any subsequent supplement of Beilstein. As minor as this error might seem, it is worth bringing it to the attention of the readers of this Journal, because it is so widespread and errors of this type can make it appear that there exists a serious lack of predictability in chemistry for a s simple a property a s solubility. This frustrastes both students and instructors. I n addition, this provides a good example of the potential risk of using data from secondary sources without verifvin~ . the inform&ion in the primary iiterature. 'Seidell, A. Solubilities of Organic Compounds, 3rd ed.; D. Van Nostrand: New York. 1941; Vol. 2, p. 269. 'u.S. Department of Transportation;the U.S. Coast Guard Chemical Hazard Response information System (CHRIS);Superintendent of Documents, U.S. Government Printing Office:Washington, DC. orris, J. F.; Green, E. H. American Chemical Journai1901, 26, 305.
Intensity and Rate of a Photochemical Reaction John M. Simmie University College Galway, Ireland For a chemical reaction such a s
the rate of reaction per unit volume ( u , R, or R) is normally defined
This is only true for systems a t constant volume. Amore universal definition is given by the rate of conversion
where 5 is the extent or advancement of reaction, vi is the stoichiometric coefficient, and ni is the chemical amount of species i. The difference between these definitions (eqs 1 and 2) is partly responsible for the confusion that exists about the correct way to describe photochemical reactions. For example, for a photochemical reaction
the rate of reaction is quoted in various ways: "I." or the number of photans absorbed per unit time and unit volume ( I 1. "qZ/ with q as a quantum yield (2,3),and simply '%, ar the energy falling on a unit area in one second (4).
Part of the problem arises from the proper d e f i ~ t i o nof intensity, which, according to IUPAC, is a synonym (5)for irradiance with SI units of W.m2. Other workers use a more intuitive definition of intensity: the number of photons per unit time (6).However, I appears a s a ratio in the Beer-Lambert-Bouguer (BLB) law relating incident I, and transmitted light intensity I,, path length I , and concentration c.
Thus, the units of I are irrelevant computationally (in virtually all situations that a student might encounter) but not pedagogically. If we accept that the rate of reaction is given by the number of moles of photons absorbed per unit time per unit volume (91, and that "the intensity is the energy that falls on a unit moss-sectional area per unit time" (7)then
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where A is the cross-sectional area, NA is Avogadro's constant. and h is Planck's constant. Although eq 5 is technically correct, it is misleading: It suggests that the rate of reaction deoends on the reciprocal of the frequency of the radiation absorbed. A way out of these difficulties is to present the rate of conversion of eq 3 as
This includes two noteworthy differences:a dimensionless number q as a primary quantum yield that recognizes that not all absorptions lead to the same chemically distinct species; and an absorbed light flux Fa in moles of photons per unit time (mo1.s-'1. Using the alternative units e i n s t e i n s ~ 'is not helpful here, but simply adds another layer of terminology. Students are not likely to confuse the symbol F, with the Faraday constant. At constant volume the reaction rate per unit volume becomes
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Journal of Chemical Education
Since flux is proportional to intensity (F = 0 one can use eq 4 to show by F, = F, - F, that
remembering that the BLB relationship holds for collimated monochromatic radiation (8). As required by the rate law for eq 3, the term Fn contains both the incoming ohotons and the concentration of the absorbing species;.In other words, the rate of photochemical reaction should be seen as (mo1.s-') x (m-3) rather than (m01.m.~) x (s-'1, both conceptually and dimensionally Literature Cited 1.At!&$, P. W. Phya&olCh~mistry,4th ed.: OxfordUniversity: Oxford, 1990:p 822. 2. Noggle, J.H. Physiml Chemistry:Little, Bmwn: Boston, 1985:p 531. 8. A h * . ,.R. A. Phvsicai Chemistm. .. 7th ed.: Wilw: New York. 1987: 1,789. 4. Freifelder, D. physio! Chemalry for ~ k d w & ofBiology and C&mistry: Science Books: Boston, 1982. 5. Mills, I.; CStsr. T.:Hornann, K.; Kallay, N.: Kuchitsu, K Quoniit&s. Unita and Symbols in Phvsicol Ch~misiry:Blackwell:Oxford, 1988. 6. Riv& M.J. ~h.;. E d i r 198966,1049-1051. 7. LeSne. I. N. Physical Chsmrsfry, 2nd ed.:MeGraw-Hill: New York, 1983; p 729. 8. Logan, S. R. J. Chem. Educ 1990.67.872875. ~
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