Chem 12, no laughing . . ..

 

 

Given two ions in a solution we can calculate the Trial Ion Product. My question here is, if one ion is disproportionately greater than the other will precipitate always form around when the Qsp == the Ksp value? Is this linear behaviour?

 

I am just not convinced of the whole idea here. It assumes that the saturation point is invariant given the concentrations of the individual ions. What I am having a problem with is if Ksp = [A] = (1)(1) = 1 and we have [A]=10000 and =.0001 we will have a precipitate because Qsp == Ksp. So where is anything less than .0001M we will not? I would assume with such disproportionate values we would have to take more points into consideration.

 

Am I missing something obvious that makes this assumption obvious?

 

Thx T.I.P.'s

Edited November 13, 201114 yr by Xittenn

I'm not sure if I understand what you are asking but if it's what I think it depends order of a reaction it it. You are describing a second order reaction, and since is the limiting reagent it will go to completion due to the excess of reagent A in accordance with Le Chatelier's principle.

Here is an example question . . . ..

 

Hebden Chemistry 12; Unit III, Question 57

 

What concentration of [math] S^{2-} [/math] is required to just start precipitation of [math] CuS [/math] from a 0.20 M solution of [math] CuCl_2 [/math]?

 

 

Answer:

 

[math] K_{sp} = [ Cu^{2+} ] [ S^{2-} ] = 6.0 \times 10^{-37} [/math]

 

[math] [ S^{2-} ] = \frac{K_{sp}}{ [ Cu^{2+} ] } = \frac{6.0 \times 10^{-37}}{.2} = 3 \times 10^{-36} [/math]

 

 

If this were [math] CuS [/math] dissociating we would have a ratio of 1M to 1M dissociated ions in solution. But in our trial we have .2 M of Copper and [math] 3 \times 10^{-36} [/math] of Sulphide. The comparative ratios are drastic but the saturation point based on the solubility product remains the same regardless of this fact.

 

If the Sulphide concentration were lower there would be no precipitate? According to Hebden this is the reality, but for me it doesn't make logical sense. Is this a case of the change would be so small as to be negligible so shut up and do your homework?

Edited November 13, 201114 yr by Xittenn