Galvanic Cell

Dipping a strip of zinc metal directly into copper sulphate solution causes the blue colour of the solution to fade as red-brown copper metal deposits on the zinc strip — a spontaneous redox reaction, $\text{Zn(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{Cu(s)}$, whose released energy is entirely lost to the surroundings as heat when the two reactants are simply mixed together. The two half-reactions underlying it are oxidation of zinc, $\text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^-$, and reduction of copper, $\text{Cu}^{2+}\text{(aq)} + 2e^- \rightarrow \text{Cu(s)}$.

The Daniel cell captures the energy of this same reaction usefully by physically separating the two half-reactions into two half-cells rather than letting zinc metal and Cu²⁺ ions meet directly:

• Oxidation half-cell: a zinc strip dipped in aqueous zinc sulphate.
• Reduction half-cell: a copper strip dipped in aqueous copper sulphate.

The two strips are connected externally through a wire, a switch, and a load such as a voltmeter, while the two electrolyte solutions are connected internally by a salt bridge — an inverted U-tube filled with agar-agar gel mixed with an inert electrolyte such as KCl or Na₂SO₄, whose ions do not react with anything in either half-cell and are never themselves oxidised or reduced.

When the switch closes, electrons flow from the zinc strip to the copper strip through the external wire, driven by the two half-reactions occurring simultaneously at the two electrodes:

• Anodic oxidation (zinc electrode, negative): zinc atoms lose electrons and enter solution as Zn²⁺ ions, while the released electrons flow into the external wire — this constant loss of electrons is exactly why the zinc electrode carries a negative charge.
• Cathodic reduction (copper electrode, positive): Cu²⁺ ions arriving from solution accept the electrons delivered through the external wire, are reduced to metallic copper, and deposit on the electrode — this constant consumption of electrons is exactly why the copper electrode carries a positive charge.

Role of the salt bridge. As zinc dissolves, the anodic compartment accumulates excess Zn²⁺ and becomes positively charged; as copper deposits, the cathodic compartment loses Cu²⁺ relative to its SO₄²⁻ and becomes negatively charged. Left unchecked, these charge imbalances would quickly stop the reaction. The salt bridge prevents this: Cl⁻ ions migrate out of it into the anodic compartment to neutralise the excess positive charge there, while K⁺ ions migrate out of it into the cathodic compartment to neutralise the excess negative charge there — completing the internal half of the circuit without ever letting the two electrolyte solutions mix directly. …