[pfnm]

These are Prac questions. I want to be prepared before the Prac. I'm certainly not fishing for answers. Rather I want to see my understanding or approach is flawed. I'll show the problems, and how I've tried to work them. Any help is appreciated.

BTW - the prac asks that we explain reactions by using chemical equations, and le Chatelier's principle, so that's what I've tried to do. Also, we need to note where reactions are substitution or redox.


Problem 1
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To 1mL of a solution of copper(II) ions, add 0.5mL of 0.5M KI solution.



My Attempt
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2CU2+(aq) + 4I-(aq) <----> 2CuI(s) + I2(aq)

cu2+ ions oxidised iodide ions to I2, and were reduced to CuI.

I'm wondering about the amounts specified: obviously in a lab they need to give us clear amounts. But have they chosen those numbers, just to show what is in excess and what limits? Would the reaction go ahead with less KI, for example?



Problem 2
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2. To a 1mL solution of copper(II) ions, add 1mL of ETDA solution and then 0.5 mL of 0.5M KI solution.

My Attempt
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I'm not sure how to approach this.  Since multidentate ligands have higher stability constants than unidentate ligands (such as I-), would no reaction occur after the addition of the KI?



Problem 3
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Place a small quantity of CuCl2.4H2O in a test tube, and heat until the compound changes colour.

My Attempt
-----------

I don't know what complex CuCl2.4H2O is. Would it have two Cl- ligands and 4 H2O ligands?

The equilibrium will move to the right, but I don't know what product is formed here.




Problem 4
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To a 1mL of 0.05M Ag+ solution, add successively the following reagents:

(a) 1M NaCl
(b) 5M NH3
(c) 1M KBr
(d) 1M Na2S2O3
(e) 0.5M KI

In each case where a precipitate forms, decant the supernatent liquid and add the next reagent to the precipitate.





My Attempt
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(a) 1M NaCl

I figure AgCl(s) precipitate is formed, based on excess Cl- ions reacting with Ag+.

So decant liquid,

(b) add 5M NH3(aq) to precipitate

AgCl(s) + 2NH3(aq) <---> Ag[(NH3)2]+(aq) + Cl-(aq)

Or is the NH3 reacting with small amount of Ag+ ion left in solution (ie that didn't precipitate)?

(c) Add 1M KBr

I'm stumped here. Br- shouldn't replace the multidentate, more stable NH3 ligands. All I can think of is, Br- reacts with Ag+ remaining in solution (Ag+ that didn't precipitate), forming a very small AgBr(s) precipitate.

I don't know how KBr would react with Ag[(NH3)2]+(aq) if no Ag+ ion were present in small amounts from the initial solution.
If no Ag+ ion is present, then it appears to me that no reaction would occur with KBr, Na2S2O3, nor KI concurrently.

Have I got this wrong?


Thanks for any input - if any clarification is necessary let me know. Cheers.