What is the difference between the oxidation number method and the ion-electron method?

1 Answer
Sep 9, 2014

They are just different methods of keeping track of electrons during redox reactions.

Suppose you had to balance the equation: Cu + AgNO₃ → Ag + Cu(NO₃)₂.


You determine the changes in oxidation number and balance the changes.

The oxidation number of Cu goes from 0 to +2, a change of +2.
The oxidation number of Ag goes from +1 to 0, a change of -1.

To balance the changes, you need 2 Ag for every 1 Cu.
1 Cu + 2 AgNO₃ → 2 Ag + 1 Cu(NO₃)₂


You write the net ionic equation and separate it into half-reactions. Then you equalize the electrons transferred in each half-reaction.

The net ionic equation is
Cu + Ag⁺ → Cu²⁺ + Ag

The half-reactions are
Cu → Cu²⁺ + 2e⁻
Ag⁺ + e⁻ → Ag

To equalize the electrons transferred, you multiply the second half-reaction by 2.
Cu → Cu²⁺ + 2e⁻
2Ag⁺ + 2e⁻ → 2Ag

Add the two half-reactions:
Cu + 2Ag⁺ → Cu²⁺ + 2Ag

If necessary, re-insert the spectator ions.
Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag