Question

what happens when an iron and copper are connected to a power pack with the negative wire on iron and positive on copper in a copper sulphate solution? (what one is protected and what one rusts?)

Answer 1

because it's in contact with the negative terminal of the power source, the iron metal becomes the cathode. making the iron negative (relative to the other metal) prevents iron from getting oxidized (rusting).

conversely, making the copper positive relative to the iron makes the copper metal the anode. assuming there's O2 in the solution (dissolved O2 from air), the O2 could get reduced by combining with water:

O2 + 2H2O + 4e- -> 4 OH-

the standard reduction potential for the above half reaction is slightly more positive than the Cu/Cu2+ couple. this tells us that O2 can oxidize copper in standard conditions, at least in principle.

so if we make the iron negative enough (relative to copper), my guess is that O2 gets reduced at the iron cathode, and the copper becomes oxidized (and dissolves into the copper sulfate solution). i am sure what actually happens is more complicated than this.

Answer 2

i am sorry, on second thought, the half reaction at the cathode (iron) is just the reduction of copper ions:

Cu(II) + 2 e- -> Cu(s)

we don't need O2 at all because there are plenty of copper ions to accept electrons at the iron cathode.

notice the cathodic reaction is the reverse of the anodic reaction. the coupled redox reaction:

Cu(s, on copper) + Cu(II) -> Cu(II) + Cu(s, on iron)

while the overall reaction seems to indicate that nothing happened, the reduced copper forms a deposit on the iron cathode. it's really a net transfer of copper from the copper anode to the iron cathode.

Answer 3

well, the reduction of copper ions (at the cathode) should be more favorable than the reduction of water.

according to the standard reduction potentials:
2H2O + 4e- -> H2 + 2 OH- -0.83 Volt
Cu(II) + 2e- -> Cu +0.34 Volt

The reduction potential for copper is more positive, indicating that it's more favorable for the electrons to go to copper ions than to the water molecules.

i am ignoring the reduction of iron cations because these aren't initially present in the solution and we've protected the iron by turning it into the cathode.