What Happens Inside a Chemical Cell

In a Daniell cell a zinc rod is immersed in a zinc sulfate solution, and a copper rod in a copper sulfate solution; the two solutions communicate through a porous partition. Two redox reactions take place at the two electrodes.

The half-reactions at the two electrodes

At the anode (Zn) — the rod releases ions into solution and leaves electrons on the metal: $\text{Zn} \to \text{Zn}^{2+} + 2e^{-}$ At the cathode (Cu) — copper ions capture electrons from the metal and are deposited: $\text{Cu}^{2+} + 2e^{-} \to \text{Cu}$

The result is that electrons build up on the zinc rod (the $-$ pole), while they are drawn away from the copper rod (the $+$ pole). This separation of charge between the two electrodes is what makes the cell a source.

If we connect the two electrodes with an external wire, electrons flow from the anode to the cathode: the cell delivers current. The reactions continue as long as there is zinc to consume or $\text{Cu}^{2+}$ ions to reduce; once the reactants are exhausted, the cell stops working.

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Topics: Circuiti elettrici Concepts: Forza elettromotrice

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