MnO−4+8H++5e−→Mn2++4H2O;E∘=1.51 volt
MnO2+4H++2e−→Mn2++2H2O;E∘=1.23 volt
Subtracting
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯MnO−4+4H++3e−→MnO2+2H2O;E∘=?
E∘=5×1.51−2×1.233=7.55−2.463=5.093=1.70 volt.