Mg+2fecl3 = MgCl2+2fecl2mg+FeCl2 = MgCl2+Fe (reduction first, then replacement)
The main reaction is
Fe3 ++ 3 H2O = Fe(OH)3+3h+Mg+2h+= Mg2 ++ H2↑
Total 3 mg+2fecl3+6H2O = 3mgcl2+2fe (OH) 3+3H2 =
Mainly because Mg is too active, FeCl3 is relatively acidic (pH value is about 2 ~ 3).