Lesson 3 of 11 · 11 min
Competitive electron transfer
NCERT §7.2.1
Leave a strip of zinc in blue copper sulphate for an hour and it comes out coated in copper, while the blue fades. Try it the other way round and nothing happens.
The lesson in notes
In short
A zinc strip dipped in copper nitrate solution for about an hour gets a reddish copper coating, and the blue of the solution fades: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). Zinc loses electrons to Cu²⁺.
The Zn²⁺ formed can be shown by passing H₂S through the colourless solution after making it ammoniacal: a white precipitate of ZnS appears.
A copper rod in zinc sulphate solution shows no change, and H₂S gives no black CuS. So Cu does not hand electrons to Zn²⁺; the reverse transfer does not occur.
Copper in silver nitrate solution does react: the solution turns blue as Cu²⁺ forms and silver is deposited, Cu(s) + 2Ag⁺(aq) → Cu²⁺(aq) + 2Ag(s).
The tendency to release electrons therefore falls in the order Zn > Cu > Ag. Arranging metals by this competition for electrons gives the metal activity series (electrochemical series).
In some pairs neither side wins completely. Cobalt in nickel sulphate solution reaches a state where both Co²⁺ and Ni²⁺ are present at moderate concentrations: the reaction reaches equilibrium.
The same competition, when the two half reactions are kept in separate beakers joined by a wire, can drive an electric current. This is the basis of a galvanic cell.