Ch. 20 OxidationReduction Reactions
20.1 Oxidation and
Reduction
I. Oxidation
• A. Originally meant adding Oxygen,
ex. CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
• B. Involves burning, bleaching, rusting, corrosion
• C. Corrosion is breaking down
metal by oxidation (rusting)
• D. Process sped up by salts or
acids
II. Reduction
•
•
A. Originally meant loss of Oxygen from
compound
B. Ox. and Red. always occur together, called
“REDOX” reactions
Cu + HNO3  Cu(NO3)2 + H2O +NO2
III. Electron Transfer
•
•
•
•
A. More general definition of Ox. and Red.
B. Lose Electrons: Oxidation
C. Gain Electrons: Reduction
D. LEO goes GER
LEO
IV. REDOX Rxn
0
0
+2
-2
Mg + S  MgS
Oxidation
Reduction
• 1. Write charges of each element/ion
• 2. Determine whether gain or loss of electrons
(become more + = lose e-, become more - = gain e-)
• 3. Reducing Agent: does reducing (reactant with
oxidized element: Mg), Oxidizing Agent: does oxidizing
(reactant with reduced element: S)
20.2 Oxidation Numbers
I. Oxidation #
• A. + or - number assigned to atoms to determine if
Oxidized or Reduced
• B. Useful for covalent compounds (no natural
charge)
• C. Based on electronegativity
II. Ox. # Rules
• 1. Ion ox. number = charge (ex. Na+ has ox. # of +1)
• 2. Hydrogen in compound is +1, except in metal
hydrides it’s –1 (ex. NaH)
• 3. Oxygen in compound is –2, except in peroxide
(O22-) it’s –1 (ex. H2O2 is hydrogen peroxide)
• 4. Elements or diatomic molecules are 0 (ex. O2, Fe)
• 5. For compounds, sum of ox. #s must equal charge
III. Assign Ox. # to the Following:
1. N2
2. MgO
3. Na2O2
4. NO3-
5. S2O3
6. NaCl
7. NH4+
8. H3O+
9. NaH
20.3 Balancing REDOX
Rxns
I. Half-Reaction Method (Acidic)
• A. Make a reaction with just particles being oxidized
and another with particles just being reduced (“half
rxns”)
• B. S + H+ + NO3-  SO2 + NO + H2O
S  SO2
NO3-  NO
• C. Balance everything except Oxygen or Hydrogen
• D. Balance Oxygens by adding H2O
62 H2O +3 S  3 SO2 +12
4 H+ +12
4 e-  NO + 8
+
2 H2O
NO
12
3 e +16
4 H +4
4
3
• E. Balance Hydrogen by adding H+
• F. Balance charges by adding electrons (e-)
• G. Multiply reactions by numbers to balance
electrons
• H. Cross out common parts and combine
3 S + 4 H+ + 4 NO3-  2 H2O + 3 SO2 + 4 NO
II. Example
• Balance using half rxns for acidic rxn:
Fe2O3 + CO  Fe + CO2
III. Half-Rxn Method (Basic)
• A. At end, add as many OH- to both sides as you
have H+
4 OH- + 3 S + 4 H+ + 4 NO3-  2 H2O + 3 SO2 + 4 NO + 4 OH-
• B. Combine H+ and OH- to water and cancel them
4 H2O + 3 S + 4 NO3-  2 H2O + 3 SO2 + 4 NO + 4 OH-
2 H2O + 3 S + 4 NO3-  3 SO2 + 4 NO + 4 OH-