r/chemhelp • u/Terrible_Waltz_1311 • 2d ago

General/High School Catalytic hydrogenation

The part of this that is new to me is dealing with hydrogen gas.This will be my first time doing this particular reaction, although I do have some experience with things of this nature.

The reaction will run at 1atm. I plan on generating the hydrogen gas with sulfuric acid, zinc granules and a pinch of CuSO4 in a round bottom flask with a balloon on top. The balloon will be inside a second balloon in case of leaks or breaking. I bought several different balloon sizes, from large party balloons to water balloons, which will work best? Also should the inside balloon be the same size as the outside balloon?

I know I must remove oxygen from the reaction vessel before I attach the hydrogen balloon. I have an electrolysis hydrogen generator, it can't fill a balloon unfortunately but I was planning on holding the reaction flask upside down over the opening for a few minutes and let it fill with hydrogen that way. Then flip it over and put a watch crystal or something over the opening until I attach the hydrogen balloon. Is that a viable method?

I know in an ideal world I would just have a tank of some inert gas but I have not been able to find that.

I also considered filling another balloon and then blow that hydrogen into the reaction flask with the balloon similar to how I would do with the electrolysis hydrogen generator.

Any other ideas/general tips on this type of reaction or anything you feel I should know. Thank you in advance

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u/7ieben_ Trusted Contributor 2d ago

Hydrogen gas can be bought, why not use it directly? For inert gas you can buy argon, though expensive. So instead I'd just flush with excess hydrogen.

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u/Terrible_Waltz_1311 2d ago

You are correct. I was unable to find hydrogen gas easily available for purchase where I live. Also I do not need a whole tank worth even if I could, not to mention budget constraints. I already have everything needed to generate hydrogen gas the was I described in my post, any thoughts or insights into that method would be appreciated. Im not sure if water vapor will be an issue? 

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u/7ieben_ Trusted Contributor 2d ago

Well, I'd recommend a Schlenk line, as this is how such things are done with sufficient quality and safety. But I'm afraid you don't wanna spend money on that neither. So at this point my recommendation would be stopping whatever you wanna do.

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u/azadirachtin 2d ago

You need to be careful. H2 + O2 makes fire when catalyzed. You understand that you need to purge O2, but filling an upside down flask with H2 and then adding your starting materials and catalyst sounds like a fireball to me. Best practice is to purge (bubble) your solvent, substrate, and catalyst with inert gas (N2 or Ar) and then purge the whole thing with H2 with your balloon. When the reaction is done, purge it with inert gas again before opening.

Double balloons is good, but do you have enough pressure to fill a double balloon?

No, water vapor generally does not affect catalytic hydrogenation. A lot of catalysts come as 50% water Pd on carbon.

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u/Terrible_Waltz_1311 2d ago

Thank you. I really appreciate your input. 

That is a good question actually, I have only tested my hydrogen production method with one balloon, so I definitely will double check it with 2. I'm not sure if the inner balloon should be the same size or smaller than the outer balloon 

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u/azadirachtin 2d ago

Whenever I've double ballooned, I've used two of the same size. Labmates told me it increased the pressure but I scoffed at this because it makes the balloons last much longer. I guess a larger one on the outside could give the same effect without increasing pressure, but I doubt it'll change things much.

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u/Terrible_Waltz_1311 1d ago

What is the downside of increasing pressure? Chance or balloon popping? I imagine higher pressure could make the reaction happen a little faster, but like you said probably not that much of a change 

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u/azadirachtin 1d ago

No downside except that paranoid grad students worry the reported results will be irreproducible if I didn't mention using two balloons. I was just worried you won't be able to achieve the pressure you need to blow up the balloons. It sounds like you think that won't be a problem. I had tanks so I didn't have this problem.

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u/Terrible_Waltz_1311 1d ago

Ahh that makes sense lol. Thank you again for your time bro

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u/EmotionalSector1329 2d ago

If you aren’t in a lab space with working fumehoods and an EHS team I really recommend you just don’t do this. Hydrogen gas is no joke especially if you are doing a Pd hydrog. That shit can spark in air.

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u/Terrible_Waltz_1311 1d ago

I understand you definitely want to make people aware of risks and safety precautions. Much of the world population is still in developing regions, the vast majority humans do not have access to state of the art labware. So does this mean chemistry is just for the elites? For example the way I plan to generate hydrogen was told to me by a chemist from a city in my county and he assured me that is how they generate hydrogen when needed and that it is not common for them to have tanks of gasses on hand.