Question

Hi, we have done an experiment in class where we have an equalibrium between (Co(H2O)6)^+2 + 4Cl^- <-> (CoCl4)^-2 + 6H2O
where we added HCl dropwise until a colour change occured which changed the solution into a dark blue as the concentration of the Cholride ions increase.
The problem i am having is after adding HCl we were asked to add water dropwise until a colour change occurs turning the solution pink. The addition of water clearly shifts the equilibrium to the left but i am not sure how to explain the shift. This question is similar to another one that was posted but the reason for the shift was not explained, im not sure if what the water does or reacts with to decrease the reaction rate of the forward reaction as we need to explain in terms of reaction rates. Ty so much for your help on this website!

Answer 1

It's LeChatelier's Principle. If a reagent is added to a system at equilibrium the reaction will shift so as to undo what we've done to it. So if you add water, it will shift so as to use up the water you've added. Since H2O is on the right the rxn will shift to the left. That way the shift can help use up the water that's been added.