Please help me with DP1.2 (1 Viewer)

[dwayne95]

I'm totally confused by fractional distillation. How does it work? In a distillation tower are compounds with the highest or lowest boiling points at the top? And how does fractional distillation work in a lab?

As you can see I'm pretty confused, and my grand plans to do dot point summaries these holidays are failing because I just don't get it.

Any help is very much appreciated! Thank you!

 

[theind1996]

Well hey, as far as I know, this is my explanation of fractional distillation for crude oil (and no, I did not copy and paste) - although my explanation MAY have some discrepancies. Here you go:

Fractional Distillation is generally used to separate liquids that have similar boiling points.
During the fractional distillation of crude oil, the fractionating column separates the different oils according to their molecular weight. The process commences by the crude oil going through a vaporiser (where it is obviously vaporised) and it changes from a liquid to a gas. The gas now starts rising up the fractionating column as it starts condensing. The higher up the gas is, the lower the temperature of the fractionating column. Thus, the components of the gas with the highest boiling points condense at the bottom of the fractionating column, while the components with the lower boiling points (more volatile), do vice-versa. As each component condenses, it separates away from the fractionating column. The substance with the lowest boiling point is re-condensed and collected.

Since each component becomes a liquid (condenses) at different temperatures, the vapour divides into liquids at different parts of the fractionating column.

Since these gases have now condensed, they can be used individually for whatever industrial purpose that calls them.


Forgive me if I'm wrong, I'm only just starting Year 11

 

[dwayne95]

Thank you so much! This makes a lot more sense now.

Actually, I'm still just a tad confused about why the highest boiling points condense at the bottom. In my mind, it should go lowest to highest. Does anyone know why?

 

[theind1996]

Thank you so much! This makes a lot more sense now.

No problems , good luck with year 12!

 

[Nooblet94]

And how does fractional distillation work in a lab?

As far as I know, fractional distillation is only carried out industrially - you wouldn't do it in a lab.

Actually, I'm still just a tad confused about why the highest boiling points condense at the bottom. In my mind, it should go lowest to highest. Does anyone know why?

Think of it this way, let's say you've got 3 compounds with boiling points 60C, 80C and 100C. It's important to remember that the distillation tower is heated from the bottom - the temperature decreases as you move up.

Now, suppose at the base of the tower it's 110C. All 3 of your compounds are gaseous because the temperature is greater than their B.P.

At the first level, the temperature is 90C - greater than the B.P of the first 2 compounds, but less than the B.P of the third. Therefore, the first 2 will be gaseous and the 3rd will condense to form a liquid.

At the next level the temperature is 70C - only higher than the B.P of your first compound. The last 2 compounds will be liquids at this point. However, your 3rd compound's already condensed at the level below, so you're left with only the second.

I think you get the picture. Also, these values are entirely made up, they'd be much, much higher and the temperature difference between levels of the tower would be much lower if i recall correctly.

 

[dwayne95]

Oh that's so logical! Silly me. Thanks fo the help!

I seem to remember in a lab once we set up a distillation to make alcohol. It had a condeser and water going in and out. I have googles it but I'm still not entirely sure how it works.

 

[dwayne95]

Haha I meant *condenser and *googled