4 N-IIIH3 + Cl20 N2-IIH4 + 2 N-IIIH4Cl-I

This is an oxidation-reduction (redox) reaction:

2 Cl0 + 2 e- 2 Cl-I (reduction)

2 N-III - 2 e- 2 N-II (oxidation)

Change the oxidation states