Distillation
AP CHEM solutions Azeotropes are mixtures of two or more liquids whose proportions cannot be altered by simple Distillation. This occurs because the vapor phase has the same composition as the liquid phase at a specific boiling point. In essence, the liquid and vapor behave as a single component, making separation by Distillation impossible.
Types of Azeotropes
There are two main types:
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Minimum-boiling azeotropes: These boil at a lower temperature than either of their pure components. The intermolecular forces between the components are weaker than those within the pure components, leading to a lower boiling point. An example is ethanol and water (95.6% ethanol).
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Maximum-boiling azeotropes: These boil at a higher temperature than either of their pure components. Stronger intermolecular forces between the components than within the pure components result in a higher boiling point. An example is hydrochloric acid and water.
vapor pressure and Raoults Law Deviation
Understanding azeotropes requires examining deviations from Raoults Law. Raoults Law states that the partial pressure of a component in an ideal solution is proportional to its mole fraction and its vapor pressure in the pure state:
$ P_i = x_i P_i^* $
where:
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$ P_i $ is the partial pressure of component $ i $
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$ x_i $ is the mole fraction of component $ i $
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$ P_i^* $ is the vapor pressure of pure component $ i $ Azeotropes form because of significant deviations from Raoults Law. These deviations can be:
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Positive deviations: The intermolecular forces between unlike molecules are weaker than those between like molecules. This leads to a higher total vapor pressure than predicted by Raoults Law, resulting in minimum-boiling azeotropes.
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Negative deviations: The intermolecular forces between unlike molecules are stronger than those between like molecules. This leads to a lower total vapor pressure than predicted by Raoults Law, resulting in maximum-boiling azeotropes.
Breaking Azeotropes
Since simple Distillation fails, other methods are needed to separate azeotropic mixtures:
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Pressure-swing Distillation: Changing the pressure alters the azeotropic composition, allowing for separation through multiple Distillation steps at different pressures.
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Adding a third component (entrainer): An entrainer is added to form a new azeotrope with one of the original components, making separation possible.
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Membrane separation: Utilizing selective membranes to separate components based on differences in their size or properties.