I'm having trouble understanding what an azeotrope is. Any clarification is appreciated.
Sheryl
Two liquids with different boiling points normally can be separated by fractional distillation; i.e., the lower boiling point material boils first and when all of it has been evaporated and condensed (and separated into another container), then the temperature is raised until the second material begins to boil. This is another way of saying that the liquid phase and the vapor phase of the two compounds is very different. The vapor phase consists primarily of the lower boiling point material while the liquid phase consists of say equal parts of the two materials. However, some materials form azeotropes; that is, the composition in the vapor phase and the composition of the liquid phase is the same. An ethyl alcohol-water mixture forms an azeotrope which is about 95% EtOH and 5% H2O. When this occurs, there is no separation because the liquid phase is 95/5 and the vapor phase is 95/5; therefore, the 95/5 mixture is what boils and the same mixture then is condensed. HCl/H2O forms an azeotrope, also, and is so reliable that it can be used as a standard. It is approximately 22%HCl/78%H2O.
You said the composition in the vapor phase and the composition of the liquid phase is the same. How about if the liquid is 40% water and 60% ethanol. Does this mean that the vapor is 40% water? How can that be if the azeotrope is 95% ethanol? It sounds like the vapor phase is steady at a ratio of 95:5.
Thanks for any clarification.
Sheryl
Hi Sheryl,
I understand that azeotropes can be a bit confusing. In the case of an ethanol-water mixture, when it forms an azeotrope, the composition in the vapor phase and the composition of the liquid phase are indeed the same. However, this does not mean that the vapor phase is steady at a specific ratio like 95% ethanol and 5% water.
Let me explain it further. Azeotropes are special mixtures where the boiling point of the mixture is different from the boiling points of its individual components. In the case of ethanol-water, because of the molecular interactions between the two compounds, the azeotrope forms at a specific composition, which in this case is about 95% ethanol and 5% water.
When you have a liquid mixture containing 40% water and 60% ethanol, and you try to distill it, the vapor phase that is formed will have a different composition than the original liquid. The vapor phase will contain more ethanol and less water compared to the liquid phase. However, as the distillation continues, the composition of the vapor phase will gradually approach that of the azeotropic composition of 95% ethanol and 5% water.
So, during distillation, the vapor phase will contain a higher percentage of ethanol compared to the liquid phase until it reaches the azeotropic composition. At that point, further distillation will not change the composition of the mixture, as the vapor phase and the liquid phase will have the same composition, and the azeotrope will continue to boil and condense at that specific composition.
I hope this clarifies how the composition of the vapor phase and liquid phase relates to azeotropes. If you have any more questions, feel free to ask!
Best,
Explain Bot