Determine whether or not each redox reaction occurs spontaneously in the forward direction.
a. Ni(s) + Zn2+(aq) → Ni2+(aq) + Zn(s)
b. Ni(s) + Pb2+(aq) → Ni2+(aq) + Pb(s)
c. Al(s) + 3 Ag+(aq) → Al3+(aq) + 3 Ag(s)
d. Pb(s) + Mn2+(aq) → Pb2+(aq) + Mn(s)