Electrochemistry and Redox
Electrochemistry and Redox
This domain tests oxidation numbers, redox identification, electrochemical cells, standard electrode potentials and electrolysis. You must track electrons and charge carefully; signs and electrode names are frequent sources of lost marks.
Core rules
- Oxidation is loss of electrons; reduction is gain. OIL RIG.
- Oxidation number rules: elements 0; monatomic ions equal charge; oxygen usually −2; hydrogen usually +1; sum of oxidation numbers equals overall charge.
- In a voltaic cell, oxidation occurs at the anode (negative) and reduction at the cathode (positive). Electrons flow through the external circuit from anode to cathode.
- E°cell = E°cathode − E°anode, using standard reduction potentials. A positive E°cell indicates a spontaneous reaction.
- In electrolysis, the power supply drives a non-spontaneous reaction; the anode is positive and the cathode is negative.
Worked example
For Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s), E°(Cu2+/Cu) = +0.34 V and E°(Zn2+/Zn) = −0.76 V. Copper is reduced at the cathode; zinc is oxidised at the anode. E°cell = +0.34 − (−0.76) = +1.10 V. The positive value confirms the reaction is spontaneous under standard conditions.
Traps
Do not reverse the sign of the anode potential a second time after using E°cathode − E°anode. Electrode potentials are not multiplied when an equation is multiplied. In electrolysis, electrode polarity is opposite to that in a voltaic cell. Balance atoms and charge in half-equations.