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Vacuum Fractional Distillation
#1 Posted : Thursday, June 17, 2021 10:20:14 PM
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In the AAMC chemistry question pack, vacuum fractional distillation is used to separate (+)-carvone and (-)-carvone. There is a question in which a theoretical leak is sprung in the apparatus and this increases the surface pressure, thus increasing the boiling points of the 2 compounds. However, I thought that anything with a vacuum would keep things under high pressure and if a leak appeared, would decrease the surface pressure? Can someone help me out here to explain why my logic is flawed?
#2 Posted : Saturday, June 19, 2021 7:43:19 PM
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Hey Sophia, great question!

When we are considering a vacuum apparatus, we are looking at something that is approaching zero in pressure. A way to visualize a vacuum, is when you have a syringe, and you plug the part where the needle goes in and pull the plunger. When you do that, sometimes the plunger will snap back. The reason its happening is because you have created a vacuum inside of the syringe, which is now approaching zero in pressure, and the higher pressure outside of the syringe (atmospheric pressure) will push the plunger back so that the pressure of the inner syringe will be equal to that outside of the syringe.

Lets now put some water in the syringe, and pull back on that plunger. Remember that water can exist in three phases, solid/liquid/gas. When you create a vacuum, or, decrease pressure in a system, you are decreasing the surface pressure. Since realistically only gases contribute meaningfully to pressure, the water inside the syringe will attempt to enter gas phase to contribute to increasing the pressure inside of that vacuum. So now, it will take less heat to induce liquid water to enter gas phase, therefore resulting in a decrease in the boiling point of a material.

Now if a leak occurs in a vacuum situation, it increases the surface pressure, which based on our thought experiment with water, would also now increase the boiling point compared to when it is under vacuum.

Hope that helps Sophia, don't hesitate to reach out if you have more questions! :)

Cheers and happy studying,

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