The Chromate - Dichromate Equilibrium

What happens if we add some barium nitrate?

The NO^3- part of the barium nitrate has no effect in this reaction -- it is just a spectator ion.  However, the Ba²⁺ can react with the CrO₄^2- to form a solid precipitate of BaCrO₄.  The solubility of BaCr₂O₇ is much higher, and it does not form a precipitate.

The primary role of the Ba²⁺ ions in this reaction is to show the amount of CrO₄^2- present.  The more CrO₄^2- there is, the greater the quantity of precipitate that will be formed.

When Ba²⁺ ions are added to a chromate solution, a lot of BaCrO₄ precipitate forms.
 

Ba2+ ions are added  and a lot of precipitate of BaCrO4 forms.

When Ba²⁺ ions are added to a dichromate solution, a small amount of BaCrO₄ precipitate forms.
 

Ba2+ ions are added and a small amount of precipitate of BaCrO4 forms. Most of the Ba2+ ions remain  unreacted.

What would happen if you added acid to the dichromate solution?

Because the reaction of the H⁺ ions will cause the small amount of CrO₄^2- ions present to become even smaller, the small amount of precipitate will dissolve.  There are no longer enough CrO₄^2- ions for the Ba²⁺ to react with.
 
 

When H+ is added  the equilibrium  shifts to produce  even more Cr2O72-

So, when Ba2+ is added  there is not enough CrO42-  for a precipitate to form. The Ba2+ remains unreacted.

 
What would happen if you added sodium hydroxide to the dichromate solution?

Because the reaction of the OH^- ions will remove some of the H+, this will cause a large amount of CrO₄^2- ions to form in the solution.  Much more precipitate will form since there are now enough CrO₄^2- ions for the Ba²⁺ to react with.
 

When OH- is added  the equilibrium  shifts to produce  even more CrO42-

So, when Ba2+ is added  there is now enough CrO42-  for a precipitate to form.