Raoult's law

The Raoult law of François Marie Raoult is a linear approximation for the vapor pressure of liquid mixtures . According to this, the partial pressure in the gas phase for each component of the mixture is given by the product of the mole fraction of the component and the vapor pressure that the substance would have in its pure form.

Vapor pressures p _A and p _B (green) and total pressure (black) over different mixtures of two chemically similar substances A and B. In the horizontal direction, the molar proportions are plotted: left is x _A = 1 and x _B = 0, right is the other way around. The dashed courses correspond to Raoult's law.

In the case shown on the right, the total pressure is only slightly higher than calculated according to Raoult's law. Most real mixtures show a behavior that deviates more strongly upwards or downwards.

If all components except one (the solvent ) have a negligible vapor pressure, the vapor pressure drops proportionally to the total amount of substance of all dissolved substances, regardless of their type. The vapor pressure of the solvent is called a colligative property .

Formula representation

${\displaystyle p_{i}=x_{i}p_{i}^{0}}$

with partial pressures , vapor pressures of the pure substances and substance quantities for ${\displaystyle p_{i}}$ ${\displaystyle p_{i}^{0}}$ ${\displaystyle x_{i}}$ ${\displaystyle i=\mathrm {A,B,\dotsc } }$

See also

• Henry's law (linear approximations at the ends of the curve, for a little ) ${\displaystyle x_{i}}$
• Duhem-Margules equation (allows the curve to be curved for a better approximation in the transition area)

Web links

• Video: Phase Equilibria of Ideal Mixtures: Raoult's Law and the Colligative Consequences . Jakob Günter Lauth (SciFox) 2013, made available by the Technical Information Library (TIB), doi : 10.5446 / 15230 .

literature

• Erich Meister: Basic practical physical chemistry. Theory and experiments (= UTB . 8329). 2nd, revised and expanded edition. vdf - Hochschul-Verlag at the ETH, Zurich 2012, ISBN 978-3-8252-8489-3 ( limited preview in Google book search).
• Karl Stephan , Franz Mayinger : Thermodynamics. Volume 2: Multicomponent Systems and Chemical Reactions. Basics and technical applications. Springer, Berlin et al. 1999, ISBN 3-540-64481-4 , ( limited preview in the Google book search).