The Daniell Cell: Electrodes and Overall Equation

The Daniell Cell, Step by Step

The Daniell cell combines two half-cells:

• a zinc strip dipped in a zinc sulfate solution (Zn2+ + SO4^2-);
• a copper strip dipped in a copper sulfate solution (Cu2+ + SO4^2-);
• connected by a salt bridge, and by an external conducting wire between the two strips.

The Two Half-Equations

At the anode (zinc strip), oxidation:

Zn = Zn2+ + 2 e-

Check: charges on the left 0; on the right (+2) + 2×(-1) = 0. Balanced.

At the cathode (copper strip), reduction:

Cu2+ + 2 e- = Cu

Check: charges on the left (+2) + 2×(-1) = 0; on the right, Cu neutral, 0. Balanced.

The Overall Equation

The number of electrons exchanged is already identical (2 on each side), so we can directly add the two half-equations and cancel out the electrons:

Zn + Cu2+ -> Zn2+ + Cu

Final check: elements, Zn 1=1, Cu 1=1. Charges, on the left 0 + (+2) = +2; on the right (+2) + 0 = +2. The overall equation is indeed balanced.

What Happens Physically

The zinc strip gradually wears away (it loses matter, passing into solution as Zn2+ ions), while the copper strip grows (metallic copper deposits onto it). The electrons released by the zinc travel through the external wire to the copper strip: it is this flow of electrons that makes up the current usable by a device connected to the cell.

Common Mistake

Don't say that "the zinc receives electrons from the copper": it's the opposite. The zinc, the reducing agent, gives up its electrons; the Cu2+ ion, the oxidizing agent, captures them. The direction of electron transfer always follows the same logic: from the reducing agent to the oxidizing agent.