Urgent: Interpreting experiment (1 Viewer)

[Aerlinn]

We did an experiment with the equliibrium: Co2+(aq) + 4Cl-(aq) <---> CoCl4(2-) (aq)
Co2+(aq) is pink (I'm guessing it's hydrated crystals in solution, 'cause anhydrous CoCl2 is blue?), Cl-(aq) is colourless, CoCl4(2-)(aq) is blue.
Basically, we did 3 tests.

Test A: Temperature change. Test tube, with 5ml CoCl2 mixed with 4ml NaCl, a watermelon pink solution
Results: Placed in HOT water: Dark pink
In COLD water: Light red

I deduced that the equilibrium above was endothermic... is it?
Do the Co2+ ions come from CoCl2 and the Cl- ions come from both CoCl2 and NaCl, and the Na+ just float in solution?

Test B: Change in Chloride ion concentration.
5ml CoCl2: red pink
Add 5ml concentrated HCl: purple
Add 5ml AgNO3: white pink (milky pink)

I have trouble deciphering what happens in this one. Is the milky pink because AgCl precipitate forms in the third one? Do all Chloride ions get removed from the equilibrium, or just some? I don't know, 'cause if this is what happens, I imagine there would be a white precipitate in pink solution, not milky white...

Test C: Effects of dilution
5ml 1-propanol (clear), add 1 ricegrain amount of CoCl2: blue colour
Water added drop by drop till colour change: White precipitate forms at bottom, pink froth at the top.

What's happening in this last test? ie. Why CoCl2 was added to 1-propanol before diluting (there has to be a reason =S), why only a ricegrain amount of CoCl2 was added, what the blue colour is (Random guess: CoCl2 undergoing the above equilibrium, and something causes it to have a net forward reaction?! (if so, what causes it to favour the forward reaction?)), what the identity of the precipitate is, what the pink froth is... =S

Just having trouble interpreting the results of the experiment, so hoping someone can help me with it today or tomorrow? :wave:

 

[xiao1985]

hmm....

before any thing happens... how'd you get Co2+ is pink and CoCl4 2- is blue?!

edit:

q1) i guess your observation is that the dark pink in hot water is closer to the blue of CoCl4 2- comparing to Co2+ pink eh? if that's true, then you are right...

re: ions: yes, Na+ serves as counter ions for CoCl4 2- anion

q2) white ppt in pink soln... so it looked milky pink... if you let the ppt settle, you will see white ppt settle to the bottom, and you have a clear, pink soln...

q3) blue stuff initially i am guessing is CoCl2 solid (since it's not in water, it cannot be dissolved)... adding water, i am THINKING gives Co2+ and 2Cl-... honestly not too sure the white ppt at bottom...

 

[Aerlinn]

well, that's what the experiment sheet said, so, it's according to my teacher =S
I don't know about the latter, but Co2+, I think CoCl2 is blue, but CoCl2 6H20 (the hydrated crystals) are magenta/pink, so CoCl2 6H20 might be what it's talking about?

 

[Aerlinn]

1) yay ^^ Hm, counter ions, what do they do? (keep solutions neutral?) Are those kinda like spectator ions?
So would the NaCl affect the [Cl-] too?
My teacher was all 'normally you should avoid using ions which are present in the equilibrium expression. Huh? If that should be avoided, wonder why NaCl (with Cl-) was added to CoCl2(aq), couldn't we have just had CoCl2(aq) and that would have formed ions Co2+(aq) and Cl-(aq) and established the equilibrium?

2) That sounds reasonable Since the precipitate's the only thing I can think of, it's probably right Would we call this: Ag+(aq) + Cl-(aq) <---> AgCl(s) the completing equilibria?

3) So are you saying that maybe, adding water has made CoCl2 become Co2+ and 2Cl-, which has then gone and established Co2+(aq) + 4Cl-(aq) <---> CoCl4(2-) And then, I guess, since we've added water, it would push equilibrium left to produce more particles, and Co2+ is what the 'pink' froth is... would ya say that's plausible? (Now just gotta figure out the precipitate...)
I'm not sure why we used 1-propanol either, is it just because it can dissolve CoCl2? (is it possible to dissolve something without water? Because aren't ions aqueous...?)

~~~
Update:
Someone came up with the fact that the precipitate might be C3H7C1 Think water might react to form it too? Beats me how they came up with it, but maybe you might know... after all, you're more of a chem expert than i am

 

[xiao1985]

1) you need to add two mole equivalence of NaCl, as the reaction you are doing need 4 mole of Cl-, whilst initially, you only have 2... avoiding use reagents present in the reaction probably you don't want to influence the reaction equilibrium....

2) yup

3) C3H7C1?! you mean C3H7Cl?!

i doubt the substitution can occur... but regardless, addition of water is going to push equilibrium to the left (the side with Co2+ and 4Cl-), much like in a gas to gas reaction, increasing volume will favor the side with more gas molecules.... (which is why you see pink froth)

though i am still not too certain of the nature of the white ppt unfortunately