4.3 Oxidation-Reduction Reactions

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    What is an Oxidation-Reduction Reaction?

    An oxidation-reduction reaction (a.k.a. redox reaction) is a reaction in which electrons are transferred from one species (oxidized) to another species (reduced). The evidence that a redox reaction has occurred are changes in oxidation states (charges).


    Oxidation is the loss of electrons.    (OIL)    (LEO)

    Reduction is the gain of electrons.   (RIG)   (GER)

    Oxidation-Reduction Rules

    1 Atoms in their elemental form are in the zero oxidation state.
    2 Monatomic ions in ionic compounds get their typical ionic charge.
    The oxidation states of transition metals are determined based upon the anion they are with.
    3 Atoms in Molecular Compounds and Polyatomic Ions
    Hydrogen     +1 (when bonded with any nonmetals)
    Oxygen         -2 except in peroxide O22-
    Fluorine        -1 always
    Halogens      -1 except when bonded to Oxygen
    4 The oxidation numbers sum to zero for a compound or to the overall charge of a polyatomic ion.

    Assign oxidation states to the elements in the following elements/compounds:

    Na                                                  Cl2                                                  O3


    MgCl2                                             CuCl2                                              H2SO4

    Identify the species that are oxidized and reduced in the following reaction:

    Zn(s) + 2MnO2(s) → ZnO(s) + Mn2O3(s)

    Single Replacement Reactions

    If the species being oxidized in a single replacement reaction is higher on the activity series than the species being reduced, the reaction will be spontaneous.

    Which of the two reactions below are spontaneous?

    Cu(s) +Zn(NO3)2(aq) → Cu(NO3)2(aq) + Zn(s)

    Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

    activity series