The Chromate - Dichromate Equilibrium:1

The Chromate - Dichromate Equilibrium

What happens if we add some hydrochloric acid?

For simplicity, the symbol H⁺ is being used for the acid proton, rather than the Brönsted-Lowry hydronium ion H₃O⁺. If you wish to use the symbol H₃O⁺, then you will have to add another H₂O to the right hand side of each equation for each acid proton. All ions in these equations are aqueous, but the (aq) symbol has been omitted for brevity.

Hydrochloric acid is a source of H⁺ ions. If there was no chemical reaction then adding HCl should increase the amount of H⁺(aq). The original H⁺in the water would increase to become H⁺

If we added H⁺ to the chromate solution and there was no chemical reaction:

2 CrO₄^2- + 2 H⁺ Cr₂O₇²- + H₂O (l) would become 2 CrO₄^2- + 2H⁺ Cr₂O₇²- + H₂O(l)

If we added H⁺ to the dichromate solution and there was no chemical reaction:

2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O (l) would become 2 CrO₄^2-+ 2H⁺ Cr₂O₇^2- + H₂O (l)

We wouldn't see any color change since H⁺ is colorless. But there is going to be a chemical reaction. From Le Chatellier's principle, we know the reaction will try to remove some of the H⁺we have added. Some of the added H⁺ reacts with the CrO₄^2-, removing some of each, and making more Cr₂O₇^2-.

For the chromate solution:

add some H⁺ (aq)
and it becomes

a much more orange solution

Original condition	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O (l)
Immediately after some H⁺is added	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O(l)
New equilibrium is established	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O(l)

Notice, that the H⁺(aq) is less than it would be if there was no reaction, but greater than it was at the beginning.

For the dichromate solution:

add some H⁺ (aq)
and it becomes

a slightly more orange solution

Original condition	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O (l)
Immediately after some H⁺is added	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O (l)
New equilibrium is established	2 CrO₄^2- + 2 H⁺ Cr₂O₇^2- + H₂O(l)

Notice, that the H⁺ is less than it would be if there was no reaction, but greater than it was at the beginning, and there has been a slight increase in the dichromate ion concentration.