Predicting Redox Reactions in Solution

in solutions, molecules and ions act approximately independently of one another – so there are a series of steps involved in predicting redox reactions

1. List all entities that are present
2. Next using your knowledge of oxidizing and reducing agents (an table C11) label all possible oxidizing and reducing agents
3. Choose the strongest oxidizing agent (SOA) present - read from the top of the table down
4. Choose the strongest reducing agent (SRA) – read from the bottom up
5. Reduction half reactions are read from left to right (following the forward arrow)
6. Oxidation half reactions are read from right to left (following the reverse arrow)
7. Any substance not present in the redox table is a spectator ion
8. Multiply the two half reactions be factors with result in the number of electrons gained in the reduction reaction being equal to the number of electrons lost in the
         in the oxidation reaction
9. Combine the two half reactions and reduce (if in basic conditions add one OH- (aq) for each H+ (aq))

           Hints for listing and labelling entities        
 

Aqueous solutions contain H2O (l) molecules
Acidic solutions contain H+ (aq) ions
Basic solutions contain OH-(aq) ions
Some oxidizing and reducing agents are combinations

 

For example MnO4-(aq) and H+(aq)

   H2O (l), Fe2+(aq), Cu+(aq), Sn2+(aq) and Cr2+ (aq) may act as either oxidizing or reducing agents.

                Label both possibilities in your list.

Example: Nitric acid is painted onto a copper sheet to etch a design.

 
Step 1 NO3- (aq) H+ (aq) H2O (l) Cu (s)
    OA   OA  
Step 2 NO3- (aq) H+ (aq) H2O (l) Cu (s)
      RA RA
Step 3   SOA   OA  
  NO3- (aq) H+ (aq) H2O (l) Cu (s)
Step 4     RA SRA
Step 5 NO3-(aq) + 2 H+(aq) + e- NO2 (g) + H2O (l) reduction rxn
Step 6 Cu (s) Cu2+ (aq) + 2 e- oxidation rxn
Step 8 NO3-(aq) + 2 H+(aq) + e- 
Cu (s)

NO2 (g) + H2O (l) 
Cu2+ (aq) + 2 e-
x 2
x1
 
  2 NO3-(aq) + 4 H+(aq) + 2 e-
Cu (s)

NO2 (g) + H2O (l) 
Cu2+ (aq) + 2 e-
 
 
Step 9 2 NO3-(aq) + 4 H+(aq)+ Cu (s) → NO2 (g) + H2O (l) +  Cu2+ (aq