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Equilibrium chemistry Question

  1. May 7, 2005 #1
    (Excuse spelling of thread name).
    1.) If i originally had CaCl2 . 6H20 + HCl solution (which is deep purple in color), then added distilled water which caused a rose pink color, then:

    Is there a source of "stress"? That is, is adding water also adding stress to a system? Or is the change in color from purple to pink simply a matter of dilution (water causes the solution to dilute, thus making the purple color faint and hence pink)?

    Also, if there is in fact a stress, what is the direction of the equilibrium shift?

    2.) If i have a mixture of 0.2 M FeCl3 and 0.2 M KSCN (mixed together) in four beakers, and did the following to each:
    a.) Add KCl.
    b.) Add Fe(No3)3
    c.) Add KSCN.
    d.) NaOH
    Then what would be the stress in each and the direction of the equilibrium shift? For a.), the color was changed from light amber to an even lighter amber. For b.) the color was changed to red. For c.) darker red. And for d.) Lemon-yellow.

    I guess my second question is a more general one. I don't seem to understand how to investigate the effects of the reagents on the equilibrium reaction. The only one i seem to understand is KSCN, because KSCN is in fact part of the original reactants; thus, adding KSCN will contribute to the forward reaction.

    Thank you.
  2. jcsd
  3. May 7, 2005 #2


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    Staff: Mentor

    Are you sure it was CaCl2, not CoCl2?

    Second question is not an easy one..

    a - probably just dilution, although Cl- is a complexing agent too
    b & c - changes in Fe3+/SCN- equilibrium, there are six different complexes in this solution, all colored.
    d - Fe3+ reacts with OH-, as long as Fe(OH)3 doesn't precipitate I will suspect some mixed complexes Fe(SCN)n(OH)m are created.

    To really understand what is going on in the solutions you should compare all stability constants to see what side reactions are taking place. This is not a thing you want to do by hand...

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  4. May 7, 2005 #3
    Yes, i meant CoCl2. Sorry for the confusion.

    Your second answer regarding KCl and its dilution of the solution seems to be in accord with my lab results. However, i don't understand how you determined that it will only dilute the solution and not truly effect the equilibrium. Would you mind sharing?

    Also, how did you determine that Fe3+ will bond with OH- in d.) ?
  5. May 7, 2005 #4


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    Science Advisor
    Homework Helper

    ah, La Chatelier lab.

    [tex]Co(OH_2)_6^{2+}_{(aq)} + 4Cl^{-} \rightleftharpoon Co(Cl)_4^{2-}_{(aq)} + 6H_2O [/tex]

    adding water to the system, towards what direction do you think it will shift towards, the left or the right? Which rate will increase further, the forward or reverse? It's not dilution in this case, because the water will actually react, it's consumed.

    you can think of disturbance in terms of there influence on the forward and reverse reactions.

    why don't you write out the equation first and do a similar analysis to the one I did above. It's always the first step, observe the equation.
    Last edited: May 7, 2005
  6. May 8, 2005 #5


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    Staff: Mentor

    Stability constants for iron thiocyanate complexes are much larger then for iron chloride complexes - log of first constant (for FeL(2-) complex, where L stands for ligand) is 3.02 for thiocyanate and 1.48 for chloride, log of second constants are 4.64 and 2.13 respectively.

    Call it chemical intuition :)

    More seriosuly - Fe(OH)3 precipitate easily from the solutions. If you don't see precipitate, but you see some color change there is only one explanation - thiocyanate are complexing strong enough to not allow precipitation (yet). But the color have changed, so the thiocyanate complex is not dominating in the solution. Some mixed complex with both OH- and SCN- ligands will be the easiest way to explain the situation. But that's only guess!

    Chemical calculators for labs and education

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    CASC - concentration conversions, solution preparation
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