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Refluxing

Refluxing

What is refluxing and why do we do it?
We're making esters at school and i'm not sure which part of it is refluxing...

Thanks for explaining!
9 REPLIES 9

Refluxing

Refluxing is part of the process of speeding up the reaction by heating it, but since heating causes evaporation of reactants, the refluxing recondenses the vapor and allows it to go back into the reaction, thus maximizing the efficiency of the reaction.

TTFN

Refluxing

The process described by Eden_Mei is accomplished by using a refluxing column. When I've had my students perform this lab in the past, I had them attach a long large-bore glass tube to the rubber stopper of the reaction vessel (flask). The trick is to heat the sample hot enough to make the edge of the heated solution rise about to about 3/4 of the vertical tube's height to maximize the refluxing surface area. That's a good height for the beginning student who is just learning the technique. Any hotter, and the liquid might rise too high and spill over the top of the tube.
The lab skills you learn in school may serve you well someday. When I was an analytical chemist, I was glad that I had the skills needed to maximize reflux height without spilling any of the sample. Chemistry is both a science and an art.

Refluxing

Why is it that by recondensesing the solution, the efficiency of the reaction is maxmised?

Are there any other reason why we use the refluxing process?

Also, when we made the ester at school, we had water going in at the bottom and running out at the top. Does it really matter which end the water goes in and out?

Refluxing

>Why is it that by
>recondensesing the solution,
>the efficiency of the reaction
>is maxmised?

I did a process similar to this a few weeks ago (also with the hopes of making an ester). The process is sped up (or made spontaneous) by heating up the solution. (In my case, we actually boiled it). However, we needed to keep the solution at this boiling temperature for 90 minutes - all of the solvent would have evaporated in less than 10 minutes. By applying cold to it (I used a test tube with ice, refreshed every 10 minutes), the evaporated solvent condensed and dropped back into the solution. This allowed me to sustain the mixture at boiling temperature for the necessary 90 minutes with little loss of solvent.

>Are there any other reason why
>we use the refluxing process?
>
>Also, when we made the ester
>at school, we had water going
>in at the bottom and running
>out at the top. Does it really
>matter which end the water
>goes in and out?

Heat rises, so venting the hot water out of the top makes sense.

Craig

Refluxing

On 5/10/2002 3:28:00 AM, chikorita wrote:
>Why is it that by
>recondensesing the solution,
>the efficiency of the reaction
>is maxmised?
>
>Are there any other reason why
>we use the refluxing process?
> Separation of liquids with similar boiling points. The liquid with the lower B.P. escapes first, and the maximized surface area encourages this process. I once worked in an extraction lab where pollutants from a soil sample had to be "dissolved out" with an organic liquid solvent. But also worked in the instrumental analysis lab, where a concentrated form of the sample had to be injected into a machine (gas chromatograph). To boil down the sample carefully, without losing sample, I used refluxing to provide the surface area for the solvent (hexane) to escape. The professional refluxing column had about 8 chambers, each with a little glass ball that would dance around.

Some liquids are extremely hard to separate, and are called azeotropic mixtures. Ethyl alcohol not meant for drinking purposes (and therefore taxfree) is purposefully "contaminated" with a chemical that is extremely hard to remove by refluxing. This prevents people from selling it (or drinking it) without having to pay the alcohol tax.

So you see, refluxing can be used not just for enhancing chemical reactions, but also as part of fractional distillation.

>Also, when we made the ester
>at school, we had water going
>in at the bottom and running
>out at the top. Does it really
>matter which end the water
>goes in and out?

Yes. There's a very real practical consideration. The simple condensers used at the high school level would experience a giant air gap on the upper portion of the condenser if water enters from the top connection. Any region surrounding the core tube that is filled with air instead of water will lower the efficiency of the cooling because of less contact with cold water. This is why the top water exit connection should always point up and not down. (Water then fill all condenser jacket regions before exiting.)


Refluxing

Sorry, when I asked "Are there any other reason why we use the refluxing process?" I mean for making ester.

Refluxing

Many esters have commersial value, and others are made as a step in creating some other chemical. In your case, I figure the only reason there was is to give you some chemistry practice, which is a noble cause in itself.

Craig

Re: Refluxing

The ancient Gilliland correlation of reflux ratio to number of plates required in multi component distillation is still popular.

Re: Refluxing

Salut Theodore, you are back !

Thanks for your e-mail, most appreciated.

There is a check valve in the wire, can read all e-mails, can't send !

An interesting Mc Cabe-Thiele work sheet [Nam Sun Wang]

Backward compatibility .

Jean

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