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

a few more
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
lets look at some reactions
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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