Question

Briefly describe the theory of simple voltaic cell.

Answer 1

Theory of simple voltaic cell: Amongst the zinc and copper plates, zinc is more electro-positive (ionization potential – 0.76 V) as compared to copper
(ionization potential + 0.34 V) in electrochemical series. The dilute sulphuric acid is used as an electrolyte in the ionized state.

\[\ce{H2SO4⇌ 2H^+ + 2e^-}\]

When the zinc plate comes in contact with (H⁺) hydrogen ions, it being more electro-positive, ionizes to form zinc ions and free electrons.

\[\ce{Zn -> Zn^2+ + 2e^-}\]

The free electrons so formed, take the passage of least resistance, and hence, move out in the external circuit through the copper wires. The zinc ions, however, enter in the dilute sulphuric acid. Since Zn²⁺ ions are positively charged, they repel H⁺ ions in the acid solution, with the result that H⁺ ions start crowding at the copper plate.
The copper plate in turn loses its free electrons to H⁺ ions, which . form nascent hydrogen.

\[\ce{2H^+ +2E^- -> 2H}\]

The nascent hydrogen atoms so formed unite to form molecular hydrogen.

\[\ce{2H ->H2}\]

From the above explanation, it is clear that free electrons actually drift from zinc to copper in external circuit, and hence, the current should flow from zinc to copper.
However, we still continue saying the electric charge flows from copper to zinc and this current is called conventional current, whereas the actual flow of electrons is from zinc to copper which constitutes electronic current.
The emf between the zinc copper plate in the external circuit is 0.34 V – (-0.75 V)= 1.10 V.