I^- +IO4^- ----> I3^- + IO3^-
-
Let's do it by half-reactions. But before we begin, let's recognize that the question writer is funnin' us. The real first half reaction should be: 2I- ===> I2 + 2e-. But because I2 + I- ===> I3-, we have to write:
3I- ===> I3- + 2e-
H2O +IO4- +2e- ===> IO3- + 2OH-
____________________________
H2O + 3I- + IO4- ===> I3- + IO3- + 2OH-
3I- ===> I3- + 2e-
H2O +IO4- +2e- ===> IO3- + 2OH-
____________________________
H2O + 3I- + IO4- ===> I3- + IO3- + 2OH-