Identify electrodes, signs, charge carriers and half-equations in simple cells.
Method and worked examples
A galvanic cell converts energy from a spontaneous redox reaction into electrical energy by separating oxidation and reduction. Oxidation occurs at the anode and reduction at the cathode. In a galvanic cell the anode is negative and the cathode positive. Electrons travel through the external metal circuit from anode to cathode; they do not travel through the salt bridge.
The salt bridge allows ion movement that maintains electrical neutrality. For a Zn/Cu cell, Zn becomes Zn2+ at the anode while Cu2+ deposits as Cu at the cathode. Anions migrate towards the zinc side and cations towards the copper side in the standard simple model. An electrolytic cell instead uses an external electrical supply to drive a non-spontaneous reaction. Oxidation-at-anode and reduction-at-cathode remain valid in both types.
Example 1
For Zn+Cu2+→Zn2++ Cu, identify the galvanic-cell electrodes.
Zinc is oxidised at the negative anode: Zn →Zn2++2e−. Copper ions are reduced at the positive cathode: Cu2++2e−→ Cu.
Example 2
What happens to electrode masses in the ideal Zn/Cu cell?
Zinc mass decreases as atoms become ions. Copper mass increases as Cu2+ gains electrons and deposits as metal.
Watch out:Electrons flow through the external conductor; ions carry charge through the bridge and solutions.
Galvanic cells and charge flow | Year 11 Chemistry | Ascent Arcade