[W-Unit]

Hello there

Alright, so we've been studying redox reactions and cell potential, and I understand them each individually just fine, but now it appears we have to put them together and I have no idea how to do this.

In a lab, we formed a voltaic cell with Cu as the anode and Ag as the cathode and were told to find the potential (the Eº thing) for it.  We found this to be .42V

Here was the setup of the lab:
A strip of one metal (I believe Cu, sorry I didn't think it was important at the time!) was placed in a porous cup which was then filled with a solution (I believe the solution was CuSO₄, but I'm not sure; my teacher didn't seem to place much importance on what the solutions were and they're not written anywhere on our sheets).  The porous cup containing the Cu strip was then submerged in a larger plastic cup which had been filled with a different solution (I assume a silver solution of some kind), and then a strip of Ag was submerged in this same solution (not in the porous cup).  The leads were then connected to the metal strips (positive went on Ag, negative on Cu), and that's how we took the reading.

Each lab group used Cu against a different metal and then we shared our data afterward.. so questions 2 and 3 are in regards to the data from the group that tested Cu / Al

Now, the questions for this lab that I'm having trouble with are:
1. Write balanced half reactions and a balanced overall reaction for the cell made at your lab table

2. Based on the percent error values calculated above, which, if any, of the metal electrodes seemed to cause errors.  Give possible explanations.
(The answer to this problem is Al, which gave a 141.3% error (the accepted value was -.24V and the experimental was .58V), but I don't know why)

3. Explain how the cell potential data for the cells containing Al supports the trend of the half reaction potentials at the top

I dont have ANY idea how to do any of these problems!!
#1 I have no idea where to start because we weren't given any charges on the electrodes to begin with and I also don't know what charge they end up with
#2 I frankly just don't know why Al would give such bad readings
#3 I don't even know what it's talking about.  What trend of half reaction potentials???

Thanks so much for anything you can help with!