Four successive members of the first series of the transition metals are listed below. For which one of them the standard potential ( E M2+/M ) value has a positive sign?
- Co (Z = 27)
- Ni (Z = 28)
- Cu (Z = 29)
- Fe (Z = 26)
Answer
(3) Cu (Z = 29)
Solution
As per NCERT, E M2+/M values (V): Fe −0.44, Co −0.28, Ni −0.25, Cu +0.34.
Copper alone has a positive value because its high enthalpy of atomisation and high ionisation enthalpies (to form Cu2+) are not balanced by its hydration enthalpy.
∴ Cu does not liberate H2 from acids.
Answer: (3)