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Chemistry · Glossary

What is Azeotrope?

Also known as: positive azeotrope · negative azeotrope

Definition 7.10 University Chemistry — Year 2 · Chapter 7 — Binary Liquid–Vapour Diagrams and Distillation

An azeotrope is a liquid mixture that boils without changing composition: the vapour it gives has its own composition. A positive azeotrope (positive deviation) has a maximum of pressure on the isothermal diagram and a minimum of boiling temperature on the isobaric one; a negative azeotrope the reverse.

Left: ethanol–water at atmospheric pressure, from a one-parameter model fitted to the azeotrope composition (0.881); the azeotrope boils below both pure liquids. Right: a model pair with a strong negative deviation (A = -2.0, vapour pressures of benzene and toluene); the azeotrope boils above both. Left: ethanol–water at atmospheric pressure, from a one-parameter model fitted to the azeotrope composition (0.881); the azeotrope boils below both pure liquids. Right: a model pair with a strong negative deviation (A = -2.0, vapour pressures of benzene and toluene); the azeotrope boils above both.
Left: ethanol–water at atmospheric pressure, from a one-parameter model fitted to the azeotrope composition (0.881); the azeotrope boils below both pure liquids. Right: a model pair with a strong negative deviation (A=−2.0A = -2.0, vapour pressures of benzene and toluene); the azeotrope boils above both.

Examples

Example 7.12 (Ethanol and water)

Ethanol and water deviate positively from Raoult’s law and form an azeotrope of 95 % ethanol by mass at atmospheric pressure. With M=46.07M = 46.07 and 18.02 g/mol18.02\,\mathrm{g}/\mathrm{mol}, its mole fraction of ethanol is

xaz=95/46.0795/46.07+5/18.02=0.881.x_{\text{az}} = \frac{95/46.07}{95/46.07 + 5/18.02} = 0.881 .

It boils a little below pure ethanol (78.3∘C78.3{}^{\circ}\mathrm{C}). The diagram below is computed from a one-parameter activity model, ln⁡γ1=A(1−x)2\ln\gamma_1 = A(1 - x)^2, ln⁡γ2=Ax2\ln\gamma_2 = Ax^2, whose parameter (A=1.09A = 1.09) is fitted so that the azeotrope falls at that composition: the model reproduces the shape, and places the azeotrope at 77.9∘C77.9{}^{\circ}\mathrm{C}.

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