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Multiple Oxidation States

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

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  1. Multiple Oxidation States Truman Chemistry Dept.

  2. 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….

  3. 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

  4. Rules continued… • 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

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

  6. Lets try some: • 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

  7. A few more? • 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

  8. Lets look at some reactions: • 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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