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Redox Reactions


Oxidation                Reduction                

combination with O2      loss of O2               

loss of electrons        gain of electrons        

gain in oxidation        loss in oxidation        
number                   number                   



An oxidation agent is a donor of electrons. A reduction agent is the absorber of electrons.

Oxidation Number Rules

  1. Lone elements are zero.
  2. Neutral compounds must add to zero.
  3. Polyatomic ions must add to the charge of the ion.
  4. Oxygen is -2 except for H2O2.
  5. Hydrogen is +1.
  6. Label the alkali earth metals and the halogens +1 and -1, then procede toward the center of the periodic table, making sure to observe all of the rules above.

Oxidation Number Reaction Calculation

Fe2O3 + CO  Fe + CO2                                                             

+3 -2  +2 -2  0  +4 -2                  Write the oxidation numbers down         
Fe2O3 + CO  Fe + CO2                                                             

+3 -2  +2 -2  0  +4 -2                  Notice how the carbon's number changes   
Fe2O3 + CO  Fe + CO2                    and how the iron's number changes.       
                                        Carbon gains two oxidation numbers,      
                                        thus oxidizing. Iron is reducing as it   
                                        loses three oxidation numbers.           

+3 -2  +2 -2  0  +4 -2                  Write in coefficients so that the        
Fe2O3 + 3CO  2Fe + CO2                  oxidation number changes cancel out.