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Multiple Oxidation States. Truman Chemistry Dept. Oxidation States can change during a reaction. Example: Oxidation state of oxygen CH 4 + 2O 2  CO 2 + 2H 2 O In O 2 the oxidation state of O is zero In CO 2 and H 2 O the oxidation state of O is -2

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Multiple oxidation states

Multiple Oxidation States

Truman Chemistry Dept.


Oxidation states can change during a reaction
Oxidation States can change during a reaction

  • Example: Oxidation state of oxygen

    • CH4 + 2O2 CO2 + 2H2O

      • In O2 the oxidation state of O is zero

      • In CO2 and H2O the oxidation state of O is -2

    • So the main question here is how can we predict how the oxidation state will change?

    • There are rules to predict this….


Rules

All elements in a diatomic molecule have an oxidation state of zero (they are neutral) ie. Cl2

If an element is monatomic (by itself) the charge is only as specified (ie. Cl-). Free elements have a charge of zero (ie. Fe)

All neutral compounds must have a net (total) oxidation charge of zero and the element with the highest electronegativity (furthest right on table) is the negative element.

Rules


Rules continued
Rules continued… of zero (they are neutral) ie. Cl

  • Hydrogen always has a +1 charge except in hydrides where it is negative one (ie. LiH or CaH2)

  • Oxygen always has a -2 charge except in peroxides (H2O2) where it is -1

  • Group 1 elements are normally +1

  • Group 2 elements are normally +2

  • Group 17 elements are often -1


Polyatomic ion rules
Polyatomic ion rules… of zero (they are neutral) ie. Cl

  • Polyatomic ions have a net charge that is equal to the sum of all the charges.


Lets try some
Lets try some: of zero (they are neutral) ie. Cl

  • 1. The oxidation number of nitrogen in N2 is (1) +1    (2) 0    (3) +3      (4) -3

  • What is the oxidation number of hydrogen in CaH2? 

    (1) +1     (2) +2      (3) -1     (4) -2

  • What is the oxidation number of carbon in NaHCO3? 

    (1) -2     (2) +2      (3) -4   (4) +4


A few more
A few more? of zero (they are neutral) ie. Cl

  • What is the oxidation number of chlorine in HClO4? (1) +1      (2) +5    (3) +3     (4) +7

  • What is the oxidation number of sulfur in H2SO4? (1) 0     (2) -2     (3) +6   (4) +4

  • What is the oxidation number of chromium in K2Cr2O7? (1) +12     (2) +2   (3) +3    (4) +6


Lets look at some reactions
Lets look at some reactions: of zero (they are neutral) ie. Cl

  • Al + O2 Al2O3

    • What happened to the oxidation states?

    • Half reactions?

  • S + NO3- SO2 + NO

    • What happened to the oxidation states?


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