Question

I need help with these questions, I just dont get it

Consider the equilibrium between chromate (CrO42-) and dichromate (Cr2O72-) in aqueous acid.

2CrO4^2-(aq) + 2H^+(aq) ⇌ Cr2O7^2-(aq) + H2O(l)

Answer the following questions about the above equilibrium.
1.To an orange colored equilibrium mixture, concentrated sulfuric acid is added. What do you expect to observe for the color of the reaction mixture? Explain your answer and give reasons.

2. To an orange colored equilibrium mixture, a few drops of ammonia (a base) is added. What do you expect to observe for the color of the reaction mixture? Explain your answer and give reasons.

2CrO42-(aq) is yellow

Cr2O72-(aq) is orange

Answer 1

First off, you have to consider that the CrO₄^2- ion tends towards a yellow color, while the Cr₂O₇^2- ion tends more towards an orange color.

1. If sulfuric acid is added to the equilibrium, the orange color should be more noticeable since the equilibrium would shift to the right (products). The addition of the acid increases the H⁺ concentration in the reagents side, so the equilibrium shifts towards the right.

2. This time the color of the reaction should tend more towards the yellow. This is because you are adding a base to the mixture, which is going to counter against the acid and therefore diminish the concentration of the latter in the mixture. This would make the equilibrium to shift towards the left (reagents), and therefore to the yellow color.