Electrochemistry Preliminary

Four (Easy) Steps to Balance Red-Ox Reactions!! (Review of CHE3)

Step 1

  • Identify which molecules are oxidized and which are reduced

  • In order for you to identify, you must know the oxidation numbers of each atom in all molecules involved in the reaction. The following is the rules used to deduce the oxidation number of each atoms in a molecule.
    PriorityItemOxidation #Examples
    1Free atoms in elemental composition0Br 2, Br = 0
    2Total of oxidation # for
    a) neutral molecule
    b) ion

    charge of ion

    CH3OH, charge = 0
    CrO42-, charge = -2
    3Group IA
    Group IIA
    NaCl, Na = +1
    MgCl2, Mg = +2
    4Fluorine-1HF, SF6, F = -1
    5Hydrogen+1HCl, CH4, H = +1
    6Oxygen-2H2O, CO2, O = -2
    7Group VIIA
    Group VIA
    MgBr2 , Br = -1
    H2S, S = -2

    Step 2

  • Separate chemical equation into two half-reactions

  • By knowing oxidation numbers of each atom in molecules, oxidation half-reaction and reduction half-reaction can be defined. By clicking "ox#" botton you will see how the oxidation numbers are calculated. By clicking "ID" botton, you can identify two half-reactions.

    Now, you are ready to balance half-reactions.

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    March, 2000, Nikita Matsunaga