Barium chromate equilibria questions?

Question

Explain the following observations:

1. Barium chloride is added to potassium chromate(K2CrO4). Yellow ppt. formed.
(this mixture is separated into 3 test tubes for part 2 - 4)

2. 6M HCl is added to the mixture above, the ppt dissolves to form a yellow solution.

3. 3M H2SO4 is added to the mixture, no visible change, ie the ppt does not dissolve.

4. 6M HNO3 added to the mixture, ppt dissolves to form a yellow solution.




I need to explain the observations in terms of the equilibrium equations, Le Chatelier's Principle, etc. Also, im not sure of whats going on in the tube, and why doesnt sulfuric acid dissolve the ppt while the other two acids can? Thanks for your help!

Answer 1

First:

BaCl2 + K2CrO4 ---> BaCrO4 + 2KCl

gives the yellow precipitate of Barium Chromate.

Tube 2.- If you add HCl to the mixture above, the precipitate disolves due to "common ion effect" that tells us that on a solution of two dissolved solutes that contain the same ion (in this case Cl- ion), the presence of a common ion suppresses the ionization of a weak acid or a weak base. LeChatelier principle tells us that at the moment you add more Cl- ions equilibium displaces to the side of the reaction that suppress the formation of Chromate ion ready to combine with Barium ions avoiding the formation of the yellow precipitate.

Tube 3.- Adding Sulphuric Acid just changes the pH of solution but here is no common ion with the BaCl2 so there is no change about the pp. of BaCrO4

Tube 4. In the case of HNO3 (a strong acid) the reaction occurs as follows:

BaCrO4 + HNO3 ----> Ba(NO3)2 + H2CrO4

Barium Nitrate is highly soluble in water.

Hope it helps you.

Cheers!

Answer 2

Barium Chromate Precipitate

Answer 3

The barium has a greater affinity for the chromate than for sulfate