1. Some reactions are Oxidation-Reduction Reactions
Oxidation-Reduction, also known as Redox, reactions are reactions that involve the transfer of electrons
Ex: 2Mg + O2  2MgO
2Mg Mg2+ + 4e-
Oxidation half-reaction (lose e-)
O2 + 4e O2-
Reduction half-reaction (gain e-)
2Mg + O2 + 4e Mg2+ + 2O2- + 4e-
2Mg + O MgO

2. Rules for Oxidation numbers
The oxidation number is the charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred.
Free elements (uncombined state) have an oxidation number of zero.
Na, Be, K, Pb, H2, O2, P4 = 0
In monatomic ions, the oxidation number is equal to the charge on the ion.
Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2
The oxidation number of oxygen is usually –2. For peroxide, O22- it is – Ex: H2O2

3. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. (Ex: NaH, sodium hydride)
Group IA metals are +1, IIA metals are +2 and fluorine is always –1.
The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion.
Example: AlCl3
Example: SO42-
The sum of the oxidation numbers of is 0.
Al = +3 , and 3Cl’s = -1
The sum of the oxidation numbers of is -2.
4O’s = -2 and since the sum is -2, the S must =+6
Just FYI, Oxidation numbers do not have to be integers. The oxidation number of oxygen in the superoxide ion,
O2-, is –½.

4. The Oxidation Numbers of Elements in their Compounds

5. Assign oxidation numbers to all the elements in the following compounds or ion:
Li2O
HNO3

6. Predicting special reactions

7. 1. metal + nonmetal  binary salt
A piece of lithium metal is dropped into a container of nitrogen gas.
6Li + N2
 2Li3N

8. Reverse Reaction binary salt  metal + nonmetal
Aluminum chloride decomposes in a vacuum.
2AlCl3→  
2Al + 3Cl2

9. 2. Metal oxide + water  metal hydroxide (a base!)
Solid sodium oxide is added to water
Na2O + HOH  2NaOH
Solid magnesium oxide is added to water
MgO(s)  +  H2O(l)  →  Mg(OH)2(s)

10. Metal hydroxides  metal oxide + H2O(g)
Reverse Reaction
Metal hydroxides  metal oxide + H2O(g)
Solid calcium hydroxide decomposes.
Ca(OH)2(s)  →    CaO(s)  +  H2O(g)

11. 3. Nonmetal oxide + water → HX (an acid!)
Carbon Dioxide is bubbled into water
CO2(g)  +  H2O(l)   →  H2CO3(aq)
Dinitrogen pentoxide is bubbled into water
N2O5(g) +  H2O(l)  →  HNO3(aq)

12. Reverse Reaction
HX  Nonmetal oxide  +  water (some acids)
H2SO4  →    SO3(g) + H2O(l)
H2SO3  →    SO2(g) + H2O(l)

13. 4. Metal oxide + nonmetal oxide  salt (ionic compound!)
Solid sodium oxide is added to carbon dioxide
Na2O + CO2  Na2CO3
Solid calcium oxide is added to sulfur trioxide
CaO + SO3  CaSO4

14. Salt  Metal oxide + nonmetal oxide
Reverse Reaction
Salt  Metal oxide + nonmetal oxide
(ionic compound!)
Calcium carbonate is heated.
CaCO3(s)  →    CaO(s)  +  CO2(g)

15. Synthesis of oxides
*Every element, metal or nonmetal, can form an oxide if reacted in oxygen (air!)
Metal  +  oxygen  →   metal oxide.
Magnesium burns in the presence of oxygen
2Mg(s)  +  O2(g)  →    2MgO(s)
Nonmetal  +  oxygen  →    nonmetallic oxide
Solid carbon burns in the presence of oxygen
C(s)  +  O2(g)  →    CO2(g)

16. Various Decompositions to know.
Metal chlorate  metal chloride + O2(g) 2KClO3(s)  →    2KCl(s)  +  3O2(g) Carbonic acid CO2(g) + H2O H2CO3  CO2 + H2O Ammonium Hydroxide NH3(g) + H2O NH4OH  NH3 + H2O Ammonium carbonate NH3(g) + CO2(g) + H2O (NH4)2CO3  NH3 + CO2 + H2O Nitrous acid NO(g) + NO2(g) + H2O HNO2  NO+ NO2 + H2O Make an educated guess – go for common!

17. Single Replacement reactions
Most metals (or metal hydrides) + water 
metal hydroxide and hydrogen gas
Ex: Li + H2O  LiOH + H2
Ex: CaH2 + H2O  Ca(OH)2 + H2

18. Helpful Hint # 1: Ions in solids heated in a vacuum NO3- NO2- CO32-
Gas that is evolved
NO2 NO
CO2
SO2
SO3
Example: CaCO3(s)  CaO(s) + CO2(g)

19. Helpful Hint 2: Certain salts react with acids to produce a gas.
Common Gas
H2S
Any salt sulfide(S-2) plus any acid form
H2S + salt
Ex: K2S + HClKCl+H2S
CO2
Any salt carbonate(CO3-2) plus any acid form HOH + CO2 + salt
Ex: HCl + CaCO3  H2O + CO2 + CaCl2
SO2
Any salt sulfite(SO3-2) plus any acid form
HOH + SO2 + salt
Ex: Na2SO3 + HClH2O + SO2 + NaCl
NH3
Any ammonium salt (NH4+) plus any strong hydroxide form NH3 + HOH + salt
Ex: NH4Cl + NaOH  NH HOH + NaCl