Barium Chromate chemical equilibria

[MeteorX]

1. Recently did a lab experiment involving Barium Chromate.
1) When BaCl2 and K2CrO4 were mixed, I obtained a colourless solution and a yellow ppt was formed. This solution and ppt were split into 3 test tubes.
2) When HCl was added to tube 1, i got an observation that an orangey yellow solution is formed and the yellow ppt dissolves.
3) When H2SO4 was added to tube 2, the colourless soluton remains the same and the yellow ppt does not dissolve.
4) When HNO3 was added to tube 3, a paler orangey yellow solution is formed and the yellow ppt dissolves.



Had to explain what happened in each tube



1) BaCl2 + K2CrO4 ---> BaCrO4 + 2KCl. the yellow ppt in this case would be BaCrO4. However, some of my friends got a yellowish solution instead of a colourless one. Is that because there was excess chromate ions left after the reaction which gave the yellow colour? while for mine, complete reaction occurs, so there are no more chromate ions present and i got a colourless solution of KCl?

2) Because HCl was added, there is a common ion effect for Cl-, therefore [KCl] is increased and equilibrium shifts to the left according to LCP and the ppt dissolves to give a yellow colour due to CrO42-. However, because HCl was added, there is also an increase in [H+], 2CrO42- + 2H+ <----> CrO72- + H2O . Eqm for this reaction would shift to the right to produce CrO72-. Is that why a orangey yellow solution was formed? Because of the CrO42- and CrO72- mixture?

3i)This is the main problem for me. For the ppt remaining the same part, is it because H2SO4 is a diprotic acid? Therefore for the 2nd dissociation step, HSO4- <---> SO42- + H+ , because HSO4- is a weak acid, only a minimal amount of SO42- is formed to react with Ba2+ to form Barium Sulphate and as such we do not notice white ppt forming and yellow ppt remains the same?

ii) But why does the solution remain colourless? Because the first dissociation step is, H2SO4 ---> HSO4- + H+. Wont the increase in [H+] cause the production of more CrO72-, leading to an orange solution? Or is it because all of the CrO42- from equation 1 has already reacted with Ba2+, therefore there is no CrO42- to react with the excess H+ to form CrO72- and the solution remains colourless?

4) HNO3 + BaCrO4 ---> Ba(NO3)2 + H2CrO4 . The ppt dissolves because it reacts with HNO3 to give barium nitrate, which is very soluble in water. For the solution, the yellow colour comes from the CrO42-, is it orangey yellow also because of the explanation of that in 2) ? but its slightly paler due to HNO3 being a slightly weaker acid compared to HCl, and thus, [H+] is lower and less CrO72- being formed?

Sorry for the long question

 

[Borek]

1. Yes, yellow solution is the effect of the excess chromate present.

2. Cl^- doesn't play any role here. It would be obvious if you will write net ionic equation. The most important part is chromate/dichromate equilibrium.

3. I don't understand the result. I would expect BaCrO₄ to be converted to BaSO₄ - that is, yellow solution and white precipitate. Precipitate can be yellowish, but lack of color change of the solution doesn't look correct to me.

4. This is not much different from the first reaction. Solubility of the barium nitrate doesn't play any role here, just like solubility of chloride wasn't important in the first case.

Try to describe all these reactions in terms of net ionic reactions.

And don't abuse bold in your posts, they are perfectly readable with standard text.

 

[MeteorX]

Sorry! Didnt bold on purpose, I must have accidentally done it!

All 3 will be using the same ionic equation right? And I have to talk about the chromate/dichromate equilibrium for everyone of them?

Ba++ + CrO4-- <----> BaCrO4 &
2CrO4-- + 2H+ <-----> Cr2O7-- + H2O

2) Oh, so Cl- does not play a part. This is because I did some research online and found many people talking how the presence of the Cl- will affect the experiment.

3) BaCrO4 + SO4-- ---> BaSO4 + CrO4--

Will the solution be purely yellow, or be slightly mixed with an orange colour because of the Cr2O7--present too?

Also, how can I explain the precipitate being yellowish instead of white? Can I use the explanation of H2SO4 being a diprotic acid as I did in my earlier post? That because HSO4- is a weak acid, only a small amount of SO4-- is formed and until more/excess H2SO4 is introduced into the solution, BaSO4 (white ppt) will not be be noticeable (as in more then BaCrO4)?

4) This is similar to the HCl reaction? as in the more important part would be the chromate/dichromate equilibrium also?

 

[Borek]

All 3 will be using the same ionic equation right?

Mostly - but the presence of sulfate changes the situation.

And I have to talk about the chromate/dichromate equilibrium for everyone of them?

Yes.

Ba++ + CrO4-- <----> BaCrO4 &
2CrO4-- + 2H+ <-----> Cr2O7-- + H2O

Correct.

3) BaCrO4 + SO4-- ---> BaSO4 + CrO4--

Will the solution be purely yellow, or be slightly mixed with an orange colour because of the Cr2O7--present too?

I would expect it to be orange, sulfuric acid is strong. Even the second dissociation step is not that weak (pKa2 = 2).

Also, how can I explain the precipitate being yellowish instead of white? Can I use the explanation of H2SO4 being a diprotic acid as I did in my earlier post? That because HSO4- is a weak acid, only a small amount of SO4-- is formed and until more/excess H2SO4 is introduced into the solution, BaSO4 (white ppt) will not be be noticeable (as in more then BaCrO4)?

This is more complicated, as BaSO₄ precipitation will remove SO₄^2- from the solution, effectively shifting second dissociation step to the right.

Could be the precipitate will remain partially yellow, if chromate can coprecipitate with sulfate.

4) This is similar to the HCl reaction? as in the more important part would be the chromate/dichromate equilibrium also?

Yes.

 

[MeteorX]

Ok. Thanks a lot for your guidance Mr.Borek! Finally could clear my doubts!