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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.     

Fe2O3 + 3CO  2Fe + 3CO2                 Now balence the atoms                    


Half Reaction

S + HNO3  SO2 + NO + H2O                                                         

S + H+ + NO3-  SO2 + NO + H2O                          Write the reaction in     
                                                       ionic form                

S  SO2                     NO3-  NO                     Separate the oxidation    
Oxidation                  Reduction                    and reduction processes   

2H2O + S  SO2              NO3-  NO + 2H2O              Assuming this reaction    
                                                        is happening in an acid   
                                                        solution, use H2O to      
                                                        balence the oxygen        

2H2O + S  SO2 + 4H+        4H+ + NO3-  NO + 2H2O        Balence the hydrogens     

2H2O + S  SO2 + 4H+ + 4e-  3e- + 4H+ + NO3-  NO + 2H2O  Add electrons to balence  
                                                        the charges               

6H2O + 3S  3SO2 + 12H+ +   12e- + 16H+ + 4NO3-  4NO +   Multiply the equations    
12e-                       8H2O                         to make the electrons     
                                                        match                     

6H2O + 3S + 12e- + 16 4H+ + 4NO3-  3SO2 + 12H+ + 12e-  Add the equations and     
+ 4NO + 8 2H2O                                         cancel uninvolved         
                                                       molecules                 

3S + 4HNO3  3SO2 + 4NO + 2H2O                          If necessary, add         
                                                       spectator ions like K+    
                                                       and Cl-                   


Problems

  • 20.h. H2O(l) + SO3(g) H2SO4(aq)
  • 27.a. MnO2 + HCl MnCl2 + Cl2 + H2O
  • 23.a. MnO4-(aq) + I-(aq) MnO2(s) + I2(aq)
  • 18.a. SbCl5 + KI SbCl3 + KCl + I2
  • Vocabulary terms copied from Kang, Kenneth. Chemistry Notes Semester 1. [http://members.aol.com/ksykang/chemnot2.htm].

    Information and problems from Wilbraham, et al. Chemistry. 3rd ed. Addition Wesley, 1993.