Question

What is the reduction half-reaction for the following unbalanced redox equation?

Cr2O2 2- + NH3 + → Cr2O3 + N2

A. Cr2O3 →Cr2O72-


B. NH4+ → N2


C. N2 → NH4+


D. Cr2O72- →Cr2O3

Answer 1

C. N2 → NH4+ would be the correct option

Answer 2

First find the oxidation states of the various atoms:
in Cr2O2 2- Cr @ +1; In NH3 N @ +3; in CrO3 Cr @ +3, N2 N @ 0
Note that N gained electrons, ie, was reduced; Cr was oxidized
Now there is a problem, because B has NH4+ which the problem did not, and is not balanced, showing e- in/out
B.NH4+ → N2

Which of the following is an oxidation half-reaction?
A.Sn 2+ →Sn 4+ + 2e-
Sn lost electrons so it got oxidized