Oxidation-Reduction and Electrochemistry
Core concepts
Oxidation is loss of electrons and reduction is gain (OIL RIG). Oxidation numbers track electron transfer; the oxidising agent is reduced, and the reducing agent is oxidised.
Electrochemical cells convert between chemical and electrical energy. In a galvanic cell, oxidation occurs at the anode and reduction at the cathode; electrons flow through the external circuit from anode to cathode. Standard electrode potentials predict cell voltage: E°cell = E°cathode − E°anode. Positive E°cell indicates a spontaneous reaction.
Electrolysis uses external power to drive non-spontaneous reactions. Candidates must write half-equations, balance them by charge and atoms, and identify products at each electrode.
Traps to watch
- Reversing the anode/cathode assignment between galvanic and electrolytic cells.
- Forgetting to balance electrons when combining half-equations.
- Assuming the more negative electrode potential means the stronger oxidising agent (it is the stronger reducing agent).
- Confusing oxidation number with charge on an ion.
- Ignoring spectator ions.
Worked example
A cell with Zn²⁺/Zn (E° = −0.76 V) and Cu²⁺/Cu (E° = +0.34 V) has E°cell = 0.34 − (−0.76) = 1.10 V. Rule of thumb: subtract the anode potential from the cathode potential; a positive result means the reaction is spontaneous.