Oxidation and Reduction Redox

1
Oxidation and Reduction (Redox)

• Lance S. Lund
• April 21, 2009

2
Assigning Oxidation Numbers

• The oxidation number of an element is zero.
• The oxidation number of an atom in a monoatomic
  ion equals the charge on the ion.
• The oxidation number of oxygen is -2 in most of
  its compounds. Exception In peroxides, the
  oxidation number of oxygen is -1.

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Assigning Oxidation Numbers

• The oxidation number of hydrogen is 1 in most of
  its compounds. Exception In hydrides, the
  oxidation number of hydrogen is -1.
• The oxidation number of fluorine is -1 in all of
  its compounds. Each of the halogens has an
  oxidation number of -1 in binary compounds,
  except when the other element is a halogen above
  it in the Periodic Table or the other element is
  oxygen.

4
Assigning Oxidation Numbers

• The sum of oxidation numbers of the atoms in a
  compound is zero. The sum of the oxidation
  numbers of the atoms in a polyatomic ion equals
  the charge on the ion.

5
Determine the Oxidation Numbers

• Determine the oxidation numbers of each element
  in the following species
• K2SO4 H3PO4
• Cr2O72- Br2

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Redox Reactions

• The Loss of Electrons is Oxidation
• An element that loses electrons is said to be
  oxidized. The species in which that element is
  present in a reaction is called the reducing
  agent.
• The Gain of Electrons is Reduction
• An element that gains electrons is said to be
  reduced. The species in which that element is
  present in a reaction is called the oxidizing
  agent.

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Balancing Redox Equations

• Assign oxidation numbers to each atom.
• Determine the elements that get oxidized and
  reduced.
• Split the equation into half-reactions.
• Balance all atoms in each half-reaction, except H
  and O.
• Balance O atoms using H2O.
• Balance H atoms using H.

8
Balancing Redox Equations

• Balance charge using electrons.
• Sum together the two half-reactions, so that
  e- lost e- gained
• If the solution is basic, add a number of OH-
  ions to each side of the equation equal to the
  number of H ions shown in the overall equation.
  Note that
• H OH- ? H2O