Experiment on Negative Deviation of Mixtures

Topics: Distillation, Solution, Physical chemistry Pages: 2 (563 words) Published: November 28, 2005
This experiment aims to introduce the negative deviation of a mixture from the ideal behavior due to the dominance of the heterogeneous molecular interactions over the homogenous ones. In this experiment, a calibration curve for the refractometer was made using known percentages of cyclohexanone and 1,1',2,2'-tetrachloroethane (TCE). Simple distillation of solutions of cyclohexanone and TCE was followed and the refractive indices of different fractions from the vapor and the liquid were measured. The system in hand showed a strong negative deviation from Raoult's law, forming an azeotropic mixture at T = 157.5 0C (refer to plot.2).


This experiment aims to introduce the deviation from the ideal behavior of a mixture due to the non-homogenous interactions between its components. By definition, a solution which behaves ideally is one in which the vapor pressure of a particular component is proportional to the mole fraction of that component in the liquid phase over the entire range of mole fractions. Note that no distinction is made between solute and solvent. Empirically it has been found that in very dilute solutions the vapor pressure of solvent (major component) is proportional to the mole fraction X of the solvent where the proportionality constant is the vapor pressure, po, of the pure solvent. This rule is called Raoult's law:

psolvent = posolvent Xsolvent for Xsolvent = 1 (1)2

For a truly ideal solution, this law should apply over the entire range of compositions. However, as Xsolvent decreases, a point will generally be reached where the vapor pressure no longer follows the ideal relationship described by raoult's law. We should note that under very dilute conditions of a real solution, there is a distinction between the solvent and the solute; if we similarly consider the solute of a dilute real solution, Eq(1) doesn't apply and the following relationship is obeyed:

psolute = K...

References: 1. Shoemaker, D.P.; Garland, C.W.; Nibler J.W. In Experiments in Physical Chemistry. Binary Liquid-Vapor Phase Diagram. The McGraw-Hill Companies, INC. USA. 1996; 6th ed., p 207

2. John M. White, Physical Chemistry Laboratory Experiments, Prentice-Hall, Englewood Cliffs, NJ, 1975, Experiment 5.5.
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