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Homework Help: Standard reduction potential

  1. May 24, 2016 #1
    http://highered.mheducation.com/olcweb/cgi/pluginpop.cgi?it=jpg::::::/sites/dl/free/0023654666/650262/Standard_Reduction_Potential_19_01.jpg::Standard reduction potentials

    The voltage is slightly different in my textbook.

    The questions I am trying to answer involve getting both half reactions and adding up the volts. The problem I am not getting the half reactions.

    here are the questions s= solid e- = electrons

    Cr(s) + 3AgCl(s) CrCl3(aq) + 3Ag(s) <---> One of the half reactions I get and the other I don't.

    Cr>Cr3+ + 3e = -.744 (.744 in my text)

    Cr3+(aq) + 3e-<>Cr(s) = .744V

    Now this is where I have trouble. I think it would be Ag+ (aq) + e- <> Ag(s)

    but the problem is the half reaction when I write is this. 3Agcl(s)+e->3AG(s)

    Because of the nature of reaction I would eliminate the moles and the cl which gives

    Ag+ (s) + e- <> Ag(s).

    This is very similar but there solid where there is an "aq" How is this possible

    question 2) Cl2+ 2Br> 2Cl- +Br2 two half reactions e- + Cl2<> 2Cl- the closet half reaction in my book is 2e- + Cl2 <> 2Cl- clearly there is a "2e-" Did I assign oxidation numbers wrong. Again I get for the other half reaction I get 2Br(-)<>br2+e- and it should be br2+ 2e<>2Br

    Also why do sometimes you remove the moles like ex 1 and ex 2 you keep the moles in the equation?

    Can someone answer this by doing the question and posting a picture of the equation and the work so I can understand.

    Please tell me if this very messy and I will rewrite it,

    Thanks for your help.
    Last edited by a moderator: May 24, 2016
  2. jcsd
  3. May 24, 2016 #2


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    Staff: Mentor

    Yes, it is messy, but at least the first thing is easy to address.

    You are in a way right - the reaction is Ag → Ag+. However, in the presence of Cl- Ag+ immediately is removed from the solution, so the concentration of Ag+ is Ksp/[Cl-] (do you see why?). That means standard redox potentials for reactions Ag → Ag+ and Ag + Cl- → AgCl (electrons omitted for brevity) are different.
  4. May 24, 2016 #3
  5. May 24, 2016 #4


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    Staff: Mentor

    Sure you did, you just don't see it (which sadly means you have not learned the basics well).

    Assuming there is a solid AgCl in the equilibrium with the solution of chlorides of concentration [Cl-], what is the concentration of Ag+?

    It is like saying "I don't know how to calculate 3*5+6, multiplication was its own unit". These are not separated things, they all happen at the same time in the solution.
  6. May 24, 2016 #5
    I am almost certain my teacher didn't teach that. I will send you a private pm because I want to keep the class I am in private. So don't link if you find it, The teacher posts all the notes online.

    Please respond in a private pm.
  7. May 25, 2016 #6
    I may have accidentally picked the wrong page and it was not assigned. It was assigned sorry.
  8. May 25, 2016 #7


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    Staff: Mentor

    You were told what Ksp is, that's perfectly enough.

    Problem solving doesn't mean "copy the solution you were already shown in the past" but "apply your knowledge to find the solution". And you know everything that is needed here. Even if you were not shown exactly identical problem it is not more difficult than other problems shown in the notes.

    I once again suggest that you try to solve this question:

    assuming Ksp=10-10, and [Cl-] = 1 M.
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