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r/chemhelp•
reenigne

Precipitation lab (BaCl2 + Na2SO4 --> yadda yadda) question

I'm doing a % yield lab, where I mix specific amounts of BaCl2 & Na2SO4 to make insoluble BaSO4 + NaCl. This all makes sense.

However, there's a portion of the lab where we add concentrated HCl to the mixture. I'm not sure what the purpose of the HCl is here. My guess is that it drives the reaction in some way (increases yield), but mechanistically I don't know why.

Any ideas why?

Thanks!

Edit for clarity.

  • mix Na2SO4 + BaCl2
  • add small amount of concentrated HCl
  • boil (also not 100% certain of the purpose of the boiling steps)...are we simply maximizing ionization of the salts and maximizing the collisions/formation of insoluble BaSO4?
  • cool
  • aspirate liquid
  • add water
  • boil again
  • cool
  • aspirate liquid
  • dry insoluble salt

Posted Oct 29, 2013, 6:05 PM UTC · edited

26
Don_Tom•

The HCl is to prevent BaCO3 from precipitating, the boiling is (I think) to expel the CO2 from the solution.

2
Brisbanealchemist•

You are correct. The use of concentrated HCl is supposed to push the pH past about 2 so that carbonate breaks down into carbon dioxide and water. The boiling then reduces the solubility of the carbon dioxide to near zero to prevent carbonate forming as carbonic acid.

2
reenigneOP•

Weird. I guess that makes sense. so, we're just minimizing an unintended reaction.

1
[deleted]•

It sounds like BaCl2 is either the limiting reagent or you're just trying to increase solubility by adding HCl to the mixture. The purpose of adding HCl is to dissociate the Hydrogen and the Chlorine to make a Chhloride ion (Cl^-), much in the same way BaCl2 dissociates and forms Ba^2+ and 2Cl^- .

1
reenigneOP•

I'm not sure I follow.

you're saying that HCl is somehow increasing the solubility of the BaCl2? So, if BaCl2 is limiting, you want to maximize its dissolution and hence its dissociation, right?

If so, can you explain how the HCl increases solubility of BaCl2? Or is this just something that's been determined empirically?

1
[deleted]•

Sorry, that was probably badly worded.

So, if BaCl2 is the limiting reagent (I'm not sure if you're adding specific quantities or just drops where it wouldn't matter), then adding HCl would increase the amount of Cl^- ions in the solution since you're essentially just adding more Chloride once HCl dissociates.

I think the solubility argument I was thinking of was along the same lines, actually. If you don't have enough BaCl2 to make all of the insoluble material, HCl is there to increase your yield. It seems unlikely though because NaCl is not insoluble, which is my issue.

1