Oxidizing agents and reducing agents?

First, you must understand what a redox reaction is; it is a reaction in which the elements change charge. For H3O+--->H2O, hydrogen and oxygen to do not change charge since on both sides of the reaction hydrogen has a 1+ charge and oxygen has a 2- charge. So H30+ is not reduced or oxidized. MnO2 is an oxidizing agent because the charge on the Mn changes from 4+ to 2+. An oxidizing agent is the component that has it's charge reduced. Cl- is a reducing agent because the charge on Cl goes from 1- to 0. A reducing agent is the component that loses electrons, or gets a greater charge.