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Reaction Mechanism

  1. Nov 21, 2003 #1
    Year 11 chemistry problem the whole class is stuck on and our regular teacher is away for a month:

    A strip of substance x(2+) is placed in a red solution of y(2+) (aq). After 3 hours the red colour becomes less intense. There is no change after this. Some x(2+) still remains.

    a) write a reaction mechanism

    b) explain why the reaction stopped



    No problems with part b I just need help with the reaction mechanism.

    Any help appreciated. Thanks.
     
  2. jcsd
  3. Nov 24, 2003 #2
    Any ideas anyone?
    I really need some help on this.
     
  4. Nov 24, 2003 #3

    Monique

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    Could it be a redox reaction?
     
  5. Nov 24, 2003 #4
    If X is a solid it is most likely that it is not in its ionic state. Therefore it is most likely the case that there is a redox reaction btw X and Y 2+ (the ion being stabilized by water). The red color disappears since it is being converted to Y. Thus Y 2+ gains two electrons from X. X becomes 2+. Better have someone else check on this, they might be able to give you a better explanation.
     
  6. Nov 24, 2003 #5

    ShawnD

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    The guy above is right. Having ions in solid form is not common at all. It is indeed a redox reaction.
    To make it look more familiar, just replace X2+ with Zn2+ and replace Y2+ with Cu2+. The reaction would be adding zinc to copper ions. The half reactions would look like this:
    Zn --> Zn2+ + 2e-
    Cu2+ + 2e- --> Cu

    I don't really know what "write a reaction mechanism" means so I can't help with that.

    Since the reaction ratio between the X2 and the Y2 is 1 to 1, the reaction stopped because the amount of X or X2 added was more than the amount of Y2 in the solution.
     
  7. Nov 24, 2003 #6

    Monique

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    But it says the red color becomes less intense, it doesn't say it disappears, leading me to think that an equilibrium has been reached.
     
  8. Nov 24, 2003 #7

    ShawnD

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    good point....but what kind of reaction would that make this? If not all of the red stuff reacts, would that mean this is NOT a redox reaction? What other kind of reaction would it be?
     
    Last edited: Nov 24, 2003
  9. Nov 24, 2003 #8
    Reaction mechanisms are primarily used in physical chemistry, ie kinetics and reaction rates. However if it is a redox reaction, I can't see how a mechanism can be proposed. After all, it is just a transfer of electrons, the species don't interact.

    Given the question is asking for a mechanism, this leads me to believe it is in fact not a redox reaction. Judging by the reaction halting, it does look as if equilibrium has been reached.

    You guys realise this is a year 11 question, so the answer is probably just staring at us :)

    To the original poster: Is that the entire question?
     
    Last edited: Nov 24, 2003
  10. Nov 25, 2003 #9
    A redox reaction can reach equilibrium. Remember that each half reaction would have a different EMF; judging by this we can calculate K from the different standard EMF potentials. Correct me if I am wrong.
     
  11. Nov 25, 2003 #10

    Monique

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    I think so too, if the reduction potentials are not too far from eachother..
     
  12. Nov 27, 2003 #11
    My humble opinion:

    It was most likely a strip of a metal. The reactions may appear to be an equal equation, however, due to the surface area of the metal strip, it may not react with the inner core of the metal. Therefore, even though the equation balances, it does not ract to conclusion.

    : I am trying to remember the old penny experiment from genchem. The surface area was an issue for this.
     
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