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Homework Help: What is the significance of Mg(OH)2's solubility?-Chatlier's

  1. Mar 2, 2016 #1
    We did a lab on Le Chatlier's principle. A post-lab question asks wether or not Mg(OH)2 fits the pattern of solids being soluble in warm water.

    NaOH was added to MgCl2 to provide the OH. The phenolphthalein indicator within the solution changed color with NaOH's addition--magenta. When heated it turned pink. When cold it turned violet.


    Is the answer to my question that MgOH doesn't fit the pattern, because the indicator said so? because the forward dissociative reaction is exothermic?
     
  2. jcsd
  3. Mar 2, 2016 #2
    The equilibrium reaction:
    Mg(OH)2(s) <==> Mg++ + 2 OH-
    According to Le Chatlier's principle for an exothermic reaction, if you lower the temp., the equilibrium reaction will shift right (the dissociation gives off heat and if there is less heat in the outer environment (lower temp.), then the reaction wishes to produce more heat (via more products forming)).
    Does the indicator turn more pink (ex. turning from pink to violet) when you raise or lowered the temperature? What does this say about the amount of reactants/products that formed?
    So putting this all together, you will clearly see if Mg(OH)2 would be more or less soluble in warmer water.
     
  4. Mar 3, 2016 #3

    Borek

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    Indicator and its color change doesn't matter much here. You have not checked how it behaves when it is heated alone, without a presence of Mg(OH)2, so you can't be sure its color changes are related to Mg(OH)2, or not.

    Did the Mg(OH)2 dissolve on heating?
     
  5. Mar 3, 2016 #4
    The indicator goes from low (pink) to high (violet) depending on the presence of OH-s. Violet means more of the latter, so I think it means that cold water caused the production OH ions.

    MgOH<--> OH + Mg
     
  6. Mar 3, 2016 #5
    So no, it didn't o_O
     
  7. Mar 3, 2016 #6

    Borek

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    Not exactly. Yes, how deep is the color depends on the pH, but around pH 9 (which is 10-5 M strong base) the color gets saturated. If you added strong base in typical for a student lab amounts pH was not lower than 11, so even ±1 pH unit changes didn't matter at all.
     
  8. Mar 3, 2016 #7
    What do you mean by saturated?

    If the indicator isn't telling me anything useful, then how am I suppose to answer the question? It's the only way of telling wether or not MgOH is dissociating that I can think of.
     
  9. Mar 3, 2016 #8
    Violet, to my understanding means more OH ions. It turned violet when the temperature lowered. The presence of ions means that MgOH dissociated, which means that it's soluble in colder water.

    Unless I'm interpreting my lab results incorrectly.
     
  10. Mar 3, 2016 #9

    Borek

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    the color won't change any further when the pH goes up.

    Assuming there is no indicator in the test tube containing some salt, how would you test if it dissolves in a warm water?
     
  11. Mar 3, 2016 #10
    If you no longer see it's color/shape in the solution?
     
  12. Mar 3, 2016 #11

    Borek

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    Have you seen Mg(OH)2 after heating?
     
  13. Mar 3, 2016 #12
    no
     
  14. Mar 3, 2016 #13
    That's the way I thought of it, but I'm just learning Le Chatlier's principle right now in my own class, so I'm far from an expert on this.
     
  15. Mar 4, 2016 #14

    Borek

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    So does it dissolve on heating, or not?
     
  16. Mar 4, 2016 #15
    If you're saying that the indicator is unreliable, then I don't know how I would tell within these circumstances. The experiment was driven by recording the changes in color depending on the "stresses".
     
  17. Mar 4, 2016 #16

    Borek

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    I am not saying it is "unreliable" - I am saying its indications are not directly related to the solubility changes vs temperature. Could be it was there to help you control the amount of added NaOH - but you have never explained what was the experimental procedure.

    No matter what - you don't use a pH indicator to check whether solubility of a substance changes with a temperature. To do so you take a sample of the substance and you test if it dissolves in a cold and hot water.
     
  18. Mar 4, 2016 #17
    The indicator was there to tell us when a reaction has occurred. Since its color measures the Ph, I would think....
     
  19. Mar 4, 2016 #18

    Borek

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    No, it doesn't tell you whether the reaction occurred, it tells if pH is above 8.2. It happens that if the pH is high enough Mg(OH)2 will precipitate, so there is a correlation, but in general these two things are separate.
     
  20. Mar 4, 2016 #19
    It sounds like I'm back where I started.

    This was the college level version of the wells mixtures experiment.
     
  21. Mar 5, 2016 #20

    Borek

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    No idea what it is. Please describe the experimental procedure you have followed.
     
  22. Mar 5, 2016 #21
    The Initial colors of the mixtures in wells A-1 through A-3, A-4, and A-5 were high pink, medium pink, and high pink respectively. When Na2SO4 solution was added to well A-1, the color changed to a light orange. When NaHSO4 was added to well A-3, the color changed to magenta. When the mixture from well A-4 was heated, the color changed to a very light pink. When the pipet containing the mixture from well A-5 was placed in cold water, the color changed to a higher pink. When the pipet containing the mixture from well A-4 was placed in cold water, the color changed to a higher pink. When the pipet containing the mixture from well A-5 was placed in hot water, the color changed to a light orange.


    HSO4-(aq) + H2O(l) <—> H3O+(aq) + SO42-(aq)


    If Na2SO4 is added to the mixture in A-1, the reaction should shift to the left since Qc will be great than Kc. When Na2SO4 was added to well A-1 in the experiment, the color became lighter indicating the presence of acid, meaning the reaction shifted to the left.




    HSO4-(aq) + H2O(l) + Heat <—> H3O+(aq) + SO42-(aq)


    The initial colors and appearance in the mixtures of wells B-1and B-2 were both milky. After adding HCl to well B-1, and adding Na4EDTA to well B-2 the mixtures of both wells cleared. The initial colors of the mixtures in wells B-3 and B-4 were both magenta. After the mixture from well B-3 was heated, it turned light pink. After the mixture from well B-4 was cooled, it turned violet. When the mixtures from B-3 and B-4 had their temperatures reversed, they turned light pink and light violet respectively.


    Mg(OH)2(s) <—> Mg2+(aq) + 2OH-(aq)


    The initial color of the mixture in well C-1 was light purple. With the addition of water, HCl, and AgNO3, the color of the mixture in well C-1 turned light pink, purple blue, and into a very light pink. When adding water to the mixture in well C-2 the color turned blue. After the addition of ethyl alcohol, the mixture turned medium pink. When H2O was added to the mixture in well C-3, the color turned pink. When the mixture from well C-6 was heated and cooled, the color turned blue and pink respectively.


    [Cocl4]2-(alc, blue) + 6H2O(l, colorless) <—> [Co(H2O)6]2+(aq, pink) + 4Cl-(aq, colorless)
     
  23. Mar 5, 2016 #22

    Borek

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    It is hardly clear what indicators were used in all experiments, so guessing what is happening is quite problematic.
     
  24. Mar 6, 2016 #23
    Sorry!

    1st reaction--thymol blue indicator

    2nd reaction--phenolphthalein

    3rd--no one specific: D.I water, HCl, AgNO3, absolute ethyl alcohol.
     
  25. Mar 7, 2016 #24

    Borek

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    OK, I give up - I have no idea what the experiment aims at. I am not aware of any process that could change phenolphthalein color in a way described - I have never seen nor heard about phenolphthalein solutions becoming violet. It goes from colorless, though pink to magenta - actually the "pink" is just the diluted magenta, it is not a separate hue. In a very high pH it becomes colorless again, in a very low pH it becomes red, it never gets violet as far as I know.
     
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