Question

Distillation postlab questions

1.)As a liquid begins to distill, a student notes that the boiling chips are missing. The student removes the thermometer and drops a few chips into the distillation flask. What will probably happen? What is the correct procedure?

2.)Why is better separation of two liquids achieved by a slow rather than a fast fractional distillation?

Answer 1

Dear riend your answer is

1.) Liquids often boil in an uneven fashion, or "bump" as it's called in chemistry. Bumping occurs frequently when you are distilling solvents or mixtures and there aren't any scratches or irregularities on the walls of the boiling pot that would help boiling bubbles to form. If this happens, a thing called superheating occurs- because the walls of your boiling pot are smooth and bubbles of vaporized liquid cannot easily form, the temperature of the liquid can actually rise above it's boiling point without boiling- the solution is now a superheated liquid. This can be dangerous because bubbles eventually will form, and when finally do, they usually erupt violently because so much of the liquid is just itching to boil but hasn't been able to, that it all goes bloooop and releases all of it's superheated boiling energy at once!
The result is that the entire solution turns into a mass of gas and bubbles, and with a resounding "thump" it can send a shockwave of undistilled boiling solution shooting up and through the entire distillation system- at the very least contaminating everything you've just distilled, and at worst it can actually cause the glassware to come apart or even break due to the sudden pressure wave!!
An example of this happening at home- there are many reports of people heating coffee in a microwave- and when the oven goes "ding" and the door is opened, there sits the nice cup of coffee... until they touch it and it unexpectedly goes "SPLAT"- half the cup of scalding coffee is suddenly all over the place inside the oven, on their hand and on the floor in a big scalding mess- what happened? the coffee was too still to boil and it just sat there superheated until they moved it, and at that point it bumped!!

Bumping is easily prevented by adding a few things called "boiling chips" to the liquid. Boiling chips are small, insoluble, stones (the size of grains of rice or sand) made of calcium carbonate, silicon carbide, or carbon (crushed coal) just to name a few. These stones contain many microscopic pores and/or or sharp points which provide a rough surface upon which boiling bubbles can form. When boiling chips are used, essentially all of the bubbles that rise through the solution will have formed on the surface of these chips. Always use a few boiling chips when boiling liquids, especially solvents.

Never add boiling chips to a hot liquid, because it can cause immediate boiling over of the solution. If you forget to add boiling chips before you begin heating a solution, you must cool the solution before adding one to prevent product loss. "Porous" boiling chips cannot be re-used since the pores inside these stones become filled with liquid on cooling. "Sharp" boiling chips like silicon carbide or coal can often be reused until they become coated with guk and become innefective.

2.) When you're attempting to separate say, a 50-50 mixture of ethanol and water, what you'll find is that the initial boiling point is going to be somewhere closer to that of pure ethanol. But instead of sticking there througout the time period that you're boiling the ethanol off, you will find that the vapor temperature smoothly and constantly rises up and up until it's 100 degrees c.
What's happening is that the "starting" temperature is determined by the RATIO of alcohol to water in the boiling pot, and hence the ratio of ethanol to water that is evaporating- and as the alcohol goes away the ratio changes, thus the vapor temperature constantly rises, as does the amount of water that's being distilled along with the ethanol. Thats why it is better separation of two liquids achieved by a slow rather than a fast fractional distillation.

Thank you, all the best.