Inorganic Chemistry Help: Solubility Transformation and Equilibrium Constants

[broegger]

Hi,

I'm doing this chemistry report, and I have absolutely no experience in chemistry.

We have this solution containg (among some other stuff) Ba^2+, Sr^2+ and Ca^2+ ions along with SO4^2-ions in excess. Now we add some Na2CO3 to transform the sulphates to carbonates. They ask me these questions:

1) Write the reaction equations for this transformation. My guess is:

BaSO4(s) + CO3^2-(aq) --> BaCO3(s) + SO4^2-(aq)​

and so on.

2) Is the solution acid or basic in this reaction? Since CO3^2- is a weaker base than SO4^2-, I would say that the answer is basic.

3) Calculate the equilibrium constants by using the solubility products. Ok, here I start by writing the equilibrium equation:

K=\frac{[{\text{SO}_4}^{2-}]_{eq}}{[{\text{CO}_3}^{2-}]_{eq}}​

This should be valid for both the Ba-, Sr- and Ca-case. But this doesn't make sense, since the constants should be different. What am I missing here?

4) Explain why BaSO4 reacts to form BaCO3 even though BaSO4 is sparingly soluble (I hope this is the phrase) to a higher degree than BaCO3. I'm lost at sea here.

 

[Bystander]

1) Write the reaction equations for this transformation. My guess is:

BaSO4(s) + CO3^2-(aq) --> BaCO3(s) + SO4^2-(aq)​

and so on.

2) Is the solution acid or basic in this reaction? Since CO3^2- is a weaker base than SO4^2-, I would say that the answer is basic.

Good so far.

3) Calculate the equilibrium constants by using the solubility products. Ok, here I start by writing the equilibrium equation:

K=[SO42−]eq[CO32−]eqK=[SO42−]eq[CO32−]eq
K=\frac{[{\text{SO}_4}^{2-}]_{eq}}{[{\text{CO}_3}^{2-}]_{eq}}​

This should be valid for both the Ba-, Sr- and Ca-case. But this doesn't make sense, since the constants should be different. What am I missing here?

You are missing/skipping/forgetting the cation concentrations.

4) Explain why BaSO4 reacts to form BaCO3 even though BaSO4 is sparingly soluble (I hope this is the phrase) to a higher degree than BaCO3. I'm lost at sea here.

The system(s) will eventually reach equilibrium.