Raoult

provocative opinion
Raoult's Law Is a Deception
Stephen J. Hawkes Oregon State University, Cowallis, OR 97331

Raoult's law should not be in the introductory chemistry curriculum. I t is unlikely that a student ever will need to know the vapor pressure of a solution and, if they do, Raoult's unreliable guidance will deceive more often than i t helps. I t works only for dilute solutions (andthen only for the solvent, not the solute, and not for polymers) or for solutions in which the intermolecular forces within the pure solute and solvent are very similar to those between the solvent and solute in the solution. This is the case, for example, with n-hexaneln-hexadecane ( 1 ) or dextroselwater (2). t fails completely when either component i s a polymer, I even a t infinite dilution (3,4). I n many cases i t is not even the best guess. For polymers, the volume fraction gives better guidance than the mole fraction (3, 4). Solutions of perfluorobutane in n-butane conform better to a mass fraction relation than to the mole fraction a t most concentrations (calculated from the data in (5)). I t is not even pedagogically useful. The law illustrates no wider principle. I t is not thefoundation for any later teaching. It cannot be proved or even made to appear reasonable by any argument that can be followed by undergraduates (the statistical proof is the subject of a graduate course). Intuitive argument based on the area available for evaporation suggests the volume fraction rather than the mole fraction, and is correct in the case of polymers. For the substantial number ofstudents who are "mathophobic", the calculation of the mole fraction is another algorithm to be uncomprehendingly memorized. I t is never used again in the introductory course so their education is not furthered by learning it. I do not know that i t was ever popular for the determination of approximate molecular weights, but i t is seldom used for that purpose now. If a need should arise tocalculate the vapor pressure of a solution, a chemical engineer should be consulted because their texts, e.g. (6, 7), show how to oerform the calculation with more reliable akot:il~lt,as in r~thrnsEven so, it is better to jerk :I pul~lishcd r7. .Y . T h ( w tal1l6:i usunlh arc not to he found in reaular chemical literature but a r e i n t h e referenceworks of chemical engineers. This must mean t h a tchemists and most other people who use chemistry do not use the data so the law and its corrections are not worth learning. Its unreliability does not resemble the unreliability of the ideal gas law. Most gases behave nearly ideally in situations that students are likely to meet; whereas, most solutions behave non-ideally. Deviations from Raoult's law are occasionally used to illustrate the effectof intermolecular interaction between solute and solvent. The discussion always i s held to small deviations t h a t cause only curvature of the tie line on graphs of vapor pressure against composition thus conveyinn the imoression that Raoult's law is usuallv a reasonnhle approximotion. lntermoleculnr intrrn~.tions il.usare tr:it(d more usefullv hv their t+fi!ct on wluhilit\. and the practicalproblem ofehoosing a solvent, abandoning the ar-

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Activitv coefficients -1 for "reaular solutions at 25 "C assumino molar . vo Jme = 100 cr'? a q nsl .o ..me 1rar.I on o o' sc ~ cTne n moer aga nsl eacn I ne s me o Herence oelneen lnc so .u i y paramelers S for solute and solvent chaic "like dissolves like" in favor of a discussion of dipoledipole,dipole-induced dipole, London, H-bond and electron donor-acceptor interactions.
Some Theory The deviation from Raoult's law may be expressed by an y activity coeff~cient thus
P=YxP
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For Raoult's law to aoolv. r.m u s t be unitv. I t mav be several .. .. orders of mabmitudt: in rce1 sdutionj. For solutions \vhere'.remlar solut~on"h c o n 4 ; t ~ ~ l i w . t

where r$ i s the volume fraction, u is the...