1. Unit Fourteen: Redox Reactions

2. Oxidation-reduction reactions a.k.a. Redox
Reactions involving the transfer of electrons
Redox Reactions are those in which:
A substance reactants with elemental oxygen (O2)
A metal reacts with a nonmetal
One substance transfer electrons to another

3. Some Terms… Oxidation – loss of electrons
Reduction – gain of electrons
Reducing agent – substance being oxidized
Oxidizing agent – substance being reduced
Oxidation and reduction MUST occur together

4. Sayings to help…
OIL RIG
LEO the lion says GER

5. Practice Problems 2 Mg(s) + O2 (g) → 2 MgO(s)
Oxidation: Reduction:
Reducing Agent: Oxidizing Agent:
Fe(s) + Cl2(g) → FeCl2(s)
Zn­(s) + Fe2+(aq) → Zn2+(aq) + Fe(s) Mg O2 Mg O2 Fe
Cl2 Fe
Cl2 Zn
Fe2+ Zn
Fe2+

6. Practice Problems 2 K(s) + Cl2(g) → 2 KCl(s)
Oxidation: Reduction:
Reducing Agent: Oxidizing Agent:
2 Al(s) Sn2+(aq) → 2 Al3+(aq) Sn(s)
C(s) + O2(g) → CO2(g) K
Cl2 K
Cl2 Al
Sn2+ Al
Sn2+ C O2 C O2

7. Oxidation Numbers
The number of electrons involved in bonding (a.k.a. oxidation state)
CO2: C = O =
Oxidation numbers help determine what is being reduced and oxidized. +4 -2

8. Oxidation Number Rules
Elements
All elements, whether atomic or molecular, have an oxidation number equal to zero
Examples: Cu = 0 or Cl2 = 0
Monatomic Ions
The oxidation number is equal to the charge on the ion
Examples: Cl- = -1 or Ca2+ = +2

9. Oxidation Number Rules
Common atoms in compounds or polyatomic ions
Certain atoms almost always have the same oxidation number
Hydrogen is almost always +1 (except in metal halides (i.e. NaH) where it is -1)
Oxygen is almost always -2 (except in peroxides where it is -1)
Group I metals are almost always +1
Group II metals are almost always +2
Halogens are almost always -1

10. Oxidation Number Rules
Summation
Compounds
All of the oxidation numbers in a compound must add up to zero
Example: H2O
H =
O =
Polyatomic Ions
All of the oxidation numbers in a polyatomic ion must add up to the charge of the ion
Example: CO32-
C = +1 -2 +4 -2

11. Practice Problems K2Cr2O7 NaMnO4 H2 Mn2+ HClO4 K = Cr = O = Na = Mn =+1 +6 +2 -2 +1 +1 +7 +7 -2 -2

12. Oxidation Numbers in Action
Oxidation is loss of electrons so the oxidation number will increase
Reduction is gain of electrons so the oxidation number will decrease

13. Practice Problems Ca(s) + 2 H2O(l) → Ca(OH)2(aq) + H2(g)
Sn(s) HNO3(aq) → SnO2(s) NO2(g) H2O(l)
2 Fe2O3(s) → 4 Fe(s) O2(g)
CO2(g) H2O(l) → CH4(g) O2(g)