Homework Help: Chemistry

Posted by Amy on Thursday, May 8, 2008 at 8:54pm.

For the following electrochemical cell:
Fe(s) / Fe2+(aq) // MnO4–(aq) Mn2+(aq) / Pt(s)
Which letter corresponds to the correct balanced chemical equation in an acidic solution?

A. 5Fe(s) + 16H+(aq) + 2MnO4–(aq)--> 2Mn2+(aq) + 8H2O(l) + 5Fe2+(aq)
B. Fe(s)+8H+(aq)+MnO4–(aq)+Pt2+(aq)---> Mn2+(aq)+4H2O(l)+Fe2+(aq)+Pt(s)
C. Fe(s) + 8H+(aq) + MnO4–(aq)--->Mn2+(aq) + 4H2O(l) + Fe2+(aq)
D. 2Mn2+(aq) + 8H2O(l) + 5Fe2+(aq)--> 5Fe(s) + 16H+(aq) + 2MnO4–(aq)
E. Mn2+(aq)+4H2O(l)+Fe2+(aq)+Pt(s)----> Fe(s)+8H+(aq)+MnO4–(aq)+Pt2+(aq)
What reaction is occurring at the cathode?

A. 5e– + 8H+(aq)+MnO4–(aq)---> Mn2+(aq) +4H2O(l)
B. Fe(s)---> Fe2+(aq) + 2e–
C. Mn2+(aq) + 4H2O(l)----> 5e– + 8H+(aq) + MnO4–(aq)
D. Fe2+(aq) + 2e– --->Fe(s)
E. Pt(s)---> Pt2+ + 2e

-----------------------
First Problem: ; would be C?
Second problem: would that be A because positively charged ions are coming from it

Chemistry - DrBob222, Thursday, May 8, 2008 at 9:41pm
Did you type the question correctly? You have one question and two set of answers.
- Chemistry - Amy, Thursday, May 8, 2008 at 9:46pm
  yea i just forgot the space in between sorry
  
  For the following electrochemical cell:
  Fe(s) / Fe2+(aq) // MnO4–(aq) Mn2+(aq) / Pt(s)
  Which letter corresponds to the correct balanced chemical equation in an acidic solution?
  
  A. 5Fe(s) + 16H+(aq) + 2MnO4–(aq)--> 2Mn2+(aq) + 8H2O(l) + 5Fe2+(aq)
  B. Fe(s)+8H+(aq)+MnO4–(aq)+Pt2+(aq)---> Mn2+(aq)+4H2O(l)+Fe2+(aq)+Pt(s)
  C. Fe(s) + 8H+(aq) + MnO4–(aq)--->Mn2+(aq) + 4H2O(l) + Fe2+(aq)
  D. 2Mn2+(aq) + 8H2O(l) + 5Fe2+(aq)--> 5Fe(s) + 16H+(aq) + 2MnO4–(aq)
  E. Mn2+(aq)+4H2O(l)+Fe2+(aq)+Pt(s)----> Fe(s)+8H+(aq)+MnO4–(aq)+Pt2+(aq)
  
  What reaction is occurring at the cathode?
  
  A. 5e– + 8H+(aq)+MnO4–(aq)---> Mn2+(aq) +4H2O(l)
  B. Fe(s)---> Fe2+(aq) + 2e–
  C. Mn2+(aq) + 4H2O(l)----> 5e– + 8H+(aq) + MnO4–(aq)
  D. Fe2+(aq) + 2e– --->Fe(s)
  E. Pt(s)---> Pt2+ + 2e
Chemistry - DrBob222, Thursday, May 8, 2008 at 10:00pm
I see that now. I just didn't look closely enough.
Write the two half reactions. I don't think you can come up with C. In fact, I think C is and OLD OLD trick. All the atoms balance, don't they? Yes, Very good. BUT, do the charges balance? No, I don't think so. Is the electron transfer lost equal to those gained? No, I don't think so. The charge is +7 on the left and +4 on the right. No good. The electron transfer is 2 for Fe going to Fe^+2 and 5 for MnO4^- at +7 going to Mn^+2 at +2. So the electrons don't balance either. Try this.
Fe==> Fe^+2 + 2e
MnO4^- + 8H^+ 5e ==> Mn^+2 + 4H2O
===================================
Now make the electrons lost equal those gained and add the two equations. You will get the correct balanced chemical equation. (This takes care of electrons transfer and charge balance at the same time.)

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