Balancing Redox Reactions
Balancing Redox Reactions
• We balance Redox reactions by making sure that the number of
electrons lost by the reducing agent equals the number of electrons
gained by the oxidizing agent.
• Two methods are typically used the Oxidation Number Method and
the Half-Reaction Method
Oxidation Number Method
5?
Step 1: Assign oxidation numbers to all atoms
0
+5
+1 -2
e
+2
+5
-2
+4 -2
+1 -2
Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
Step 2: Identify the oxidized and reduced reactants and determine how
many electrons each lost or gained
Copper went from 0 to +2, therefore it was oxidized and lost 2 eNitrogen went from +5 to +4, therefore it was reduced and gained 1 e-
Oxidation Number Method
Step 3: Multiply one/both pairs by a factor which causes an equal
number of electrons to be transferred
Lost 2 e-
Cu + HNO3 → Cu(NO3)2 + NO2 + H2O
Gained 1 e- Therefore multiply by 2
Step 4: Complete balancing by inspection
Cu + 2 HNO3 → Cu(NO3)2 + 2 NO2 + H2O
Cu + 42 HNO3 → Cu(NO3)2 + 2 NO2 + 2 H2O
Tracking electrons transferred per atom
• You must ensure that you are tracking the number of electrons
transferred per atom:
Cu(s) + H2SO4(aq) → Cu2SO4(aq) + SO2(g) + H2O(l)
Copper loses one electron per copper to form Cu(I) sulfate. Since there
are two copper ions in each copper (I) sulfate, two copper atoms are
required and 2 electrons total are transferred in the oxidation process.
2 Cu(s) + 2 H2SO4(aq) → Cu2SO4(aq) + SO2(g) + 2 H2O(l)
Class/Homework
• Complete questions 1-4 ONLY on the handout.
• Read Textbook pp. 664-673
• Text Q’s p. 668, 671, 673.