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!
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.
When water is added to the solution, it disrupts the equilibrium by diluting the concentration of the chloride ions and increasing the concentration of water molecules. This shift occurs because the forward reaction of the equilibrium is exothermic, meaning it releases heat.
As water molecules are added, they provide a medium for the released heat to be absorbed and dispersed. This leads to a decrease in the temperature of the solution, which favors the endothermic reverse reaction. The forward reaction rate is decreased as a result of the decrease in temperature.
Moreover, the increase in the concentration of water molecules on the right side of the equilibrium promotes the reverse reaction according to Le Chatelier's principle. The equilibrium shifts to the left to counteract the increase in concentration of water molecules and restore the balance.
In terms of reaction rates, the addition of water slows down the forward reaction by reducing the concentration of chloride ions and cooling down the solution. This shift to the left ultimately decreases the rate at which the dark blue complex forms, resulting in a color change to pink.