急20點 redox reaction

First, you have to memorize some principles:

1. MnO4-(aq) ---> Mn2+(aq)
Cr2O72-(aq) ---> Cr3+(aq)

2. Halide ion undergoes REDuction
Halide element undergoes OXduction

3. Fe3+ undergoes REDuction
Fe2+ undergoes OXduction

4. ( Half eqt )
Electrons in LHS of the eqt --- REDuction gain of e-
Electrons in RHS of the eqt --- OXduction loss of e-

Then, you can balance redox chemical eqts:

** + e- ----> balance charge / no . of e-
+ H2O -----> balance no. of O
+ H+ -----> balance no. of H
+ OH- -----> ( alkaline medium , out of dse syl )



MnO4-(aq) ---> Mn2+(aq) - not sufficient no. of oxygen in RHS
MnO4-(aq) ----> Mn2+(aq) + 4H2O(I) - not sufficient no. of H in LHS
8H+(aq) + MnO4-(aq) ----> Mn2+(aq) + 4H2O(I) - unbalanced charge
5e- +...............................1

Fe3+(aq) ---> Fe2+(aq) - unbalanced charge
e- + Fe3+(aq) ----> Fe2+(aq) ..................2

2x5,
overall eqt:
MnO4-(aq) + 5Fe3+(aq) ---> Mn2+(aq) + 5Fe2+(aq)
the solution turns very pale green.