calculate largest E cell

[Katherine Wu 1H]

Please write the two species that would give the largest E°cell if assembled in an electrochemical cell.
(a)
Pt^2+(aq)+2e-→ Pb(s): E°=1.20V
Cu^2+(aq)+2e-→ Cu(s) E°=0.34V
Pt^2+(aq)+2e-→ Pt(s) E°=1.20V

(b)
Ti^2+(aq)+2e-→ Ti(s) E°=-1.63V
Sn^2+(aq)+2e-→ Sn(s) E°=-0.14V
Mg^2+(aq)+2e-→ Mg(s) E°=-2.36V
I don't understand why the answer would be (a) Pt and Cu and (b) Mg^2+ and Ti^2+?

[Rachel Yu 1G]

To find the largest possible E°cell you would want the strongest reduction half-reaction and strongest oxidation half-reaction. The strongest reduction half-reaction will have the largest E°cell potential while the strongest oxidation reaction will have the smallest E°cell potential since a weaker reduction means stronger oxidation. In both parts, you'll try to find the largest and smallest half-reactions because this will have the greatest difference, forming the largest E°cell potential.