Raoult's law
Raoult's law concerns the vapour pressure of the solvent above a solution. This is always lower than the vapour pressure of the pure solvent, and the form of the law first proposed by François-Marie Raoult in 1887[1][2] was that the fractional lowering of the vapour pressure of the solvent is proportional to the amount fraction of solute.[note 1] A more general statement, which allows for the dissociation of the solute, is that the vapour pressure is proportional to the amount fraction of solvent.[4]
Both of these statements hold only for dilute solutions, and a solution which obeys these simple forms of Raoult's law is termed an ideal solution. Limited non-ideal behaviour can be accounted for by taking fugacities instead of vapour pressures, and this approach is used to define the activity of the solvent in a solution.[5]
An extended form of Raoult's law considers a mixture of volatile liquids, and states that the vapour pressure of each component is proportional to its amount fraction over the whole concentration range.[6] This is only (approximately) true for mixtures of chemically similar liquids: in general, the vapour pressure of a volatile solute is given by Henry's law rather than Raoult's law.
Notes and references
Notes
- ↑ As Raoult pointed out, this statement is only strictly true if the solute does not dissociate in solution. A formal treatment of the effects of dissociation was provided by van 't Hoff.[3]
References
- ↑ Raoult, F.-M. Loi générale des tensions de vapeur des dissolvants. C. R. Hebd. Seances Acad. Sci. 1887, 104, 1430–33, <http://gallica.bnf.fr/ark:/12148/bpt6k30607.image.f1429.langEN>. English translation
- ↑ Raoult, F.-M. Remarques sur un calcul de M. Van t' Hoff relatif à la tension de vapeur des dissolutions. C. R. Hebd. Seances Acad. Sci. 1887, 105, 1857–59, <http://gallica.bnf.fr/ark:/12148/bpt6k3061j/f857.image.langEN>.
- ↑ Hoff, J. H. van 't Lois de l'équilibre chimique dans l'état dilué, gazeux ou dissous. K. Sven. Vetenskapsakad. Handl. 1886, 21, No. 17 (58 pp).
- ↑ Atkins, P. W. Physical Chemistry, 4th ed.; University Press: Oxford, 1990; pp 161–62. ISBN 0-19-855283-1.
- ↑ Standard quantities in chemical thermodynamics. Fugacities, activities and equilibrium constants for pure and mixed phases (IUPAC Recommendations 1994). Pure Appl. Chem. 1994, 66 (3), 533–52. DOI: 10.1351/pac199466030533.
- ↑ Atkins, P. W. Physical Chemistry, 4th ed.; University Press: Oxford, 1990; p 173. ISBN 0-19-855283-1.
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