When doing a simple distillation of ethanol-water, is the vapor pure ethanol or does it also contain water? (knowing that ethanol has lower boiling point than water)
If it is not pure ethanol, why is it not possible to separate water & ethanol completely?
Thanks in advance.
Water/ethanol form an azeotrope which is a combination (in this case) of 95.63% ethanol/4.37% H2O and it has a LOWER boiling point (78.2 C) than either H2O (at 100 C) or pure ethanol (78.4 C). Therefore, in a simple distillation, you get the azeotrope(lower boiling point) coming over. Anhydrous ethanol is made by adding benzene, cyclohexane, toluene, (or some others) to the azeotrope and distilling. The added component modifies the boiling point such that pure ethanol can be obtained. Ethanol/water is a combination with a lower boiling point; HCl/H2O form an azeotrope that is a higher boliling point. HCl 20.2%/79.8% H2O boils at 110 C and not -84(HCl) or 100(H2O).
During a simple distillation of ethanol-water, the vapor produced is not pure ethanol but rather a mixture of ethanol and water. This occurs because, even though ethanol has a lower boiling point than water (78.4°C for ethanol compared to 100°C for water at atmospheric pressure), the two compounds form an azeotropic mixture.
An azeotropic mixture is a liquid mixture that boils at a constant temperature and composition, similar to a pure substance. In the case of ethanol-water, the azeotropic mixture has a boiling point of 78.2°C and a composition of approximately 95.6% ethanol and 4.4% water. This means that when the mixture is heated and starts to vaporize, the vapor produced maintains the same composition as the liquid.
The formation of an azeotropic mixture occurs due to the molecular interactions between ethanol and water molecules. Ethanol and water are capable of forming hydrogen bonds with each other. As a result, as the temperature rises, the ethanol molecules tend to evaporate along with some water molecules. This intermolecular bonding prevents a complete separation of the two compounds during distillation.
To obtain pure ethanol, additional techniques such as fractional distillation or using azeotropic distillation agents can be employed. Fractional distillation involves using a fractionating column, which provides multiple condensation and vaporization stages. This helps to separate the components of the azeotropic mixture more effectively. Azeotropic distillation agents, such as benzene or cyclohexane, can also be added to form a ternary azeotrope that has a different boiling point from the ethanol-water azeotrope.
In summary, during a simple distillation of ethanol-water, the vapor produced is a mixture of ethanol and water due to the formation of an azeotropic mixture. Complete separation of water and ethanol is challenging without additional techniques like fractional distillation or azeotropic distillation agents.