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 2Mg2+ + 4e- Oxidation half-reaction (lose e-) O2 + 4e- 2O2- Reduction half-reaction (gain e-) 2Mg + O2 + 4e- 2Mg2+ + 2O2- + 4e- 2Mg + O2 2MgO

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 –1. Ex: H2O2

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.

The Oxidation Numbers of Elements in their Compounds

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

Predicting special reactions

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

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

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)

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)

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)

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

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

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

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)

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!

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

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)

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  NH3 + HOH + NaCl