[Solais]

Okay, the question is:
Cl2 + 2e- -> 2Cl- ; Eo= 1.36V
Tl1+ +e- -> Tl  ;Eo= -0.34V

I keep getting postive 1.70, when the answer is -1.70.  I'm subtracting the oxi. from the red.  What am I doing wrong?

[PopcornMuffin]

Something like this happened to me as well lately. In my case however the question showed the short-hand galvanic cell diagram. So in your case I'm guessing they asked about: Cl2|Cl- || Tl+|Tl

So even though the Ecell values suggest that Cl2 should be reduced, the diagram is telling you explicitely that the cathode is Tl+/Tl and should therefore be reduced.

[Solais]

My practice test didn't have the diagram and that was probably throwing me off.

[Solais]

I forgot to add the equation given was Tl1+ + 2Cl- -> 2Tl +Cl2

[english]

Are you using a table of reduction potentials?  Then your formal reduction potential is the cathode potential minus the anode potential...

Ecat - Ean = E⁰

[Solais]

Yes, I am using a table of reduction potentials.  So, if I'm using one it's always cat-an?

[Rabn]

You know, I've heard all the little help-me-outs in regards to how to figure out what is what with electrochemistry.  There is one method that never fails. If there is no reaction given, i.e. nature is allowed to take its course, the reduction half reaction with the lowest value is the reducing agent.  You then solve using the Oxidizing Agent minus reducing agent formula.  In this case there is a reaction given. The reaction shows that the chlorine is being oxidized, and is the reducing agent.  Using the same formula, oxidizing agent minus reducing agent, the E of the solution is then -1.7 volts.  If this is unclear, please ask questions.  It isn't easy to wrap your head around the first time through.

[english]

Yes, I am using a table of reduction potentials.  So, if I'm using one it's always cat-an?

Yes.  As long as you determine the anode and cathode correctly.