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Homework Help: Balance Chemical Reactions

  1. Oct 27, 2014 #1
    1. The problem statement, all variables and given/known data

    Write the balanced chemical reactions in ionic and neutral forms for:
    i) Enargite leaching by sodium bisulfide in basic solution to form chalcocite and thioarsenate.
    ii) Oxygen pressure oxidation of As2S5 in acid solution to form scorodite. Assume all sulfide sulfur forms sulfate. (Assume that ferric ion is present in solution).

    2. Relevant equations

    3. The attempt at a solution
    i) Cu3AsS4 + HS- = Cu2S + AsS43-
    Cu3AsS4 + HS- = 3/2Cu2S + AsS43-
    Cu3AsS4 + HS- = 3/2Cu2S + AsS43- +H+
    Cu3AsS4 + HS- +e-= 3/2Cu2S + AsS43- +H+

    ??? I cannot seem to balance the last equation properly. I do know that the e- present in the equation means that there should be 2 half reactions, not one as above, but cannot figure those out either.

    ii) As2S5+O2 = AsO43-+SO42-

    Left side:
    As2S5 = AsO43-+SO42-
    As2S5 = 2AsO43-+5SO42-
    As2S5+28H2O= 2AsO43-+5SO42-
    As2S5+28H2O= 2AsO43-+5SO42-+56H+
    As2S5+28H2O= 2AsO43-+5SO42-+56H++40e-

    Right side:
    O2 = ?
    O2 = 2H2O
    O2 + 4H+= 2H2O
    O2 + 4H++4e-= 2H2O

    Add together (and cancel out e-):
    As2S5+28H2O= 2AsO43-+5SO42-+56H++40e-
    10*(O2 + 4H++4e-= 2H2O)

    Ionic Form ===> As2S5+8H2O+10O2 = 2AsO43-+5SO42-+16H+

    It is balanced (I checked).

    But, I get into trouble when I try to get the neutral form. Nothing on the right requires ferric (the cation I use to put all aq terms into neutral form), thus there is an imbalance of iron. Also, how do I deal with the 2H2O that appears as part of scorodite? Do I just assume that it has become part of the solution?

    EDIT: Figured out 1)ii) Use H+ (and NOT ferric, Fe3+) as the cation to turn products into neutral compounds (and thus the entire reaction into neutral form).
    i.e. As2S5+8H2O+10O2 = 2H3AsO4+5H2SO4
    Last edited: Oct 27, 2014
  2. jcsd
  3. Oct 27, 2014 #2


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    Staff: Mentor

    In basic solution means you should use water and OH- to balance hydrogen and oxygen. I don't see them in your equation.

    Have you tried to decide what is getting oxidized and what is getting reduced?
  4. Oct 27, 2014 #3
    I solved it:

    Cu3AsS4 + 3/2HS- = 3/2Cu2S + AsS43-

    I can't believe I didn't see that earlier. Thanks for the reply!

    (Why can't I edit my post after a given amount of time? It would be nice to be able to grab the formatted stuff from there and change it to suit my current needs...)
  5. Oct 28, 2014 #4


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    Staff: Mentor

    And where is the hydrogen on the right?

    I am afraid we had to do so because some people abuse the editing system. Locking the post means you can't edit the post once caught on posting nonsense.
  6. Oct 28, 2014 #5
    @Borek Yes, the 3/2H+ on the right exists, I just didn't put it on here, sorry.

    Ah, I see. Thanks.
  7. Oct 28, 2014 #6


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    Staff: Mentor

    What about balancing charge? -3/2 on the left, -3 on the right.
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