How Does Adding Sulfuric Acid Prevent Hydrolysis in CuSO4 Crystal Preparation?

[takando12]

We did this experiment in the lab.We had to prepare crystals of pure copper sulfate from a given impure sample of blue vitriol(powder). So my teacher started with shaking the sample with water and then added a few drops of dilute sulfuric acid to it. It was then heated to crystallization point and then cooled and the pure crystals of CuSO4 separated out.
1) Why are we adding Sulfuric acid?
my teacher told me it is to prevent the hydrolysis of CuSO4. But how does that happen?
2) The crystallization point:
I did a little reading about this but I'm not very clear. So when it's heated the solution will form stable clusters and this is called nucleation. But i am unable to understand how this happens. And then we just cool it and the crystal keeps building? Can someone please explain? And what is the liquid that's finally left behind after cooling when we separate the crystals out?

 

[Borek]

What is hydrolysis? What do you know about solubility of copper hydroxide? How does acidifying the solution change the picture?

Nucleation doesn't happen on heating (even if it requires some activation energy), it can happen at any moment. Start with a simpler question: how does the solubility of copper sulfate change with temperature? Do you know what saturated solution is?

 

[takando12]

What is hydrolysis? What do you know about solubility of copper hydroxide? How does acidifying the solution change the picture?

Nucleation doesn't happen on heating (even if it requires some activation energy), it can happen at any moment. Start with a simpler question: how does the solubility of copper sulfate change with temperature? Do you know what saturated solution is?

Hydrolysis is when the anion and cation in the salt salt form an acid and a base in water, the reverse of neutralization. So i suppose if we added the powder to water it would form H2SO4 and CuOH. So if we add H2SO4...we get CuSO4 and H2O and that is the copper sulfate solution?

I know the solubility increases with rise in temperature. So we're heating it only so it dissolves better? I don't understand.

 

[Borek]

So i suppose if we added the powder to water it would form H2SO4 and CuOH.

Cu(OH)₂ to be precise. But yes, we acidify the solution to avoid this reaction

Cu²⁺ + 2H₂O → Cu(OH)₂(s) + 2H⁺

as the precipitated copper hydroxide will contaminate the sulfate.

I know the solubility increases with rise in temperature. So we're heating it only so it dissolves better?

Yes. When you heat the solution you dissolve more solid. What will happen when you cool the solution down? Just think about the solubility and how it changes.

 

[takando12]

Yes. When you heat the solution you dissolve more solid. What will happen when you cool the solution down? Just think about the solubility and how it changes.

Ok so we heat it so that it dissolves better, but we stop the heating at the "crystallization point" and then we cool it and when you reduce the temperature , the solubility decreases? How does that connect to the crystals formed? I'm sorry, I don't know.

 

[Borek]

but we stop the heating at the "crystallization point"

No idea what is the "crystallization point". I can only guess that it is intended to mean the moment at which all the solid is dissolved. Please note that it depends on the initial amounts of water and solid, and if there is too much solid (or not enough water) it is not guaranteed to happen.

However, assuming you heated the solution till everything got dissolved, you got a saturated solution, containing as much salt dissolved as possible. Then you cool the solution and solubility decreases. As the solution can't hold that much salt, solid must precipitate, period.

Technical details of the crystallization, nucleation, crystal growth kinetics and so on are irrelevant here and they are only confusing if you don't get the basic things right first.

 

[takando12]

No idea what is the "crystallization point". I can only guess that it is intended to mean the moment at which all the solid is dissolved. Please note that it depends on the initial amounts of water and solid, and if there is too much solid (or not enough water) it is not guaranteed to happen.

However, assuming you heated the solution till everything got dissolved, you got a saturated solution, containing as much salt dissolved as possible. Then you cool the solution and solubility decreases. As the solution can't hold that much salt, solid must precipitate, period.

Technical details of the crystallization, nucleation, crystal growth kinetics and so on are irrelevant here and they are only confusing if you don't get the basic things right first.

This makes so much more sense than the crystal stuff. My lab book says that we must take a drop of the solution at the end of a glass rod and cool it by blowing. If I can see a crust as tiny crystals on the glass rod, the lab book says the crystallization point has been reached. So what they meant is it's fully saturated and then can't hold it and so it crystallizes at that point?

 

[Borek]

Looks like that's what they mean.