fair enough, but then whats causing the aluminum from getting oxidized (it actually has a higher ox potential i believe) and also, do you use the aluminum reduction potential to calculate the minimum voltage required to run the rxn? how do you figure that out?
Do you mean why doesn't the aluminum get oxidized and plate the gold bar as well? Well, there are two reasons. First, the way the external voltage source is hooked up will determine which metal gets plated. You attach the metal you want to coat (i.e. the aluminum) to the negative terminal of your battery/voltage source so that the dissolved metal ions (like gold) can get reduced there and coat the surface of the aluminum. The gold is attached to the positive terminal of the battery so that the gold will be oxidized there. In this configuration, since the aluminum is hooked up to the negative terminal, it cannot be oxidized. If you reversed the terminals of the battery, you'd get the opposite plating. The gold would get coated with aluminum.
The second reason is that, typically, the solution that the gold and aluminum are both dipped in contains gold ions already (in other words, you use something like gold nitrate to make the solution) so that there will already be a significant concentration of ions in solution to be reduced. Thus, even if you accidentally switched the battery terminals around, there wouldn't be any aluminum ions in solution to be reduced.
related question...how do you figure out the amount of time it takes to plate X grams of gold? do you have to have a current given? kaplan gave the question without a current and then in the explanation proceeded to say "since 1 amp is" ...
I would have to see that question to know for sure if current is necessary. However, in general, if you have X grams of gold, you can convert that to moles of gold. Now I can't remember what the oxidation state of gold is, but I vaguely remember the most common one is +3. If that's the case, then one of the half reactions is Au---->Au(3+) + 3 e-. So, if I'm right about this oxidation state (even if I'm wrong you'll get the idea from this), that means that for every mole of gold you want to plate, you'll need to move 3 moles of electrons. So you multiply the number of moles you want to plate by 3, and multiply by avogadro's number. That'll give you the number of electrons. Now, one amp is 1 coulomb per second. 1.6*10^-19 coulombs is the charge of one electron, so multiply 1.6*10^-19 by the number of electrons you've calculated and that'll give you the number of coulombs you require to plate that gold. Now divide that number of coulombs by your current to give you the time it'll take in seconds.
Sorry bout the long explanation, but if you just follow the units you can calculate the time it'll take.