Complete and balance:
ClO2–(aq) → ClO–(aq) + ClO4–(aq)
This is an example of a disproportionation reaction, where a single species is simultaneously oxidized and reduced.
Oxidation half–reaction (Cl goes from +3 to +7):
ClO2–(aq) + 2 H2O(l) → ClO4–(aq) + 4 H+(aq)
Cl is mass balanced
O balance
H balance