Based on the sign of E°cell, classify these reactions as spontaneous or nonspontaneous as written. Assume standard conditions.
a) Ni2+(aq) + Pb(s) ------------------- Ni(s) + Pb2+(aq)
b) Ni2(aq) + S2-(aq) ------------------- Ni(s) + S(s)
c) Al3+(aq) + 3Na(s) --------------------- Al(s) + 3Na+(aq)