The reaction (NH4)2Cr2O7→N2+Cr2O3+4H2O, is an example of :
Intermolecular redox reactions: In which one substance is oxidised while the other is reduced.
Intramolecular redox reactions: In which one element of a compound is oxidised while the other is reduced.
(NH4)2Cr2O7→N2+Cr2O3+4H2O
-3 +6 0 +3
Hence, the reaction is an example of intramolecular redox reaction.