Question

High School Chemistry, Help!?
For my summer chem class, we did a lab for the equilibrium of reactions, the whole lab went well until I saw the post lab questions. Can someone help me with these two questions? Thanks so much! I already tried to do them, my answers are below the questions.

1. Write the chemical equation when excess chloride ion is added to aqueous solution of cobalt chloride. Note observed color of each complex ion.

CoCl42- (aq) <===========> Co2+ (Aq) + 4Cl1-
CoCl42- = blue Co2+ (aq) + 4Cl1- = pink

2. What is the likely composition of the solution when the intermediate or t

Answer 1

transition color is reached? How does this provide visual proof of the idea that not all reaction go to completion?

For this question, the transition color is a ppinkishpurple. The relative amounts of the two different complex ions are about the same, but only a certain amount of reaction can take place due to limiting reagent. This provides visual proof of incomplete reaction because if all reactions have to go through completion, this solution would either have to be pink or blue, which it is not.

3. Use LeChatelier's principle to explain the color changes observed upon addition of water and calcium chloride to an equilibrium mixture of the two complex ions in this reaction.
In the lab, we added water to the CoCl2 solution until the color became stable (pink). And in a separate test tube, we added calcium chloride to the CoCl2 solution. I can only answer a part of the question. The addition of Calcium chloride puts an excess chloride into the reaction, so it would try to balance out the excess chloride by making more Co2+ and 4Cl1-, thus resulting in a more pink solution. The addition of water I dont quite understand.

4. What was the effect of adding AgNO3 on the position of equilibrium for these two complex ions? Is this effect consistent with LeChatelier's Principle.
Completely clueless about this one. Help!

Long post, for anyone who read through this, thank you sooo much!

Answer 2

I think that the addition of water is just to dilute the calcium chloride in order for you too see that the less concentrated it is, the more you'll have to add so as to make the solution pink. (sorry, I can't work out a better solution, I think it is just a typical bad High School question )

When you add Ag+ to a chloride containing solution it is predictable a formation AgCl precipitate, that would increase the concentration of Co2+ but, the nitrate also complexes the cobalt to form a reddish complex. I don't know which reaction would proceed further (it will require some further investigation as I don't know which of the reactions has the biggest equilibrium constant).

Answer 3

Also: when you add water to a cobalt solution it can complex the cobalt to hydrate the cobalt chloride (pinkish).

http://en.wikipedia.org/wiki/Cobalt(II)_chloride

Hope it helps...