Unit Fourteen: Redox Reactions

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

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

Sayings to help… OIL RIG LEO the lion says GER

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+

Practice Problems 2 K(s) + Cl2(g) → 2 KCl(s) Oxidation: Reduction: Reducing Agent: Oxidizing Agent: 2 Al(s) + 3 Sn2+(aq) → 2 Al3+(aq) + 3 Sn(s) C(s) + O2(g) → CO2(g) K Cl2 K Cl2 Al Sn2+ Al Sn2+ C O2 C O2

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

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

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

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

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

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

Practice Problems Ca(s) + 2 H2O(l) → Ca(OH)2(aq) + H2(g) Sn(s) + 4 HNO3(aq) → SnO2(s) + 4 NO2(g) + 2 H2O(l) 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g) CO2(g) + 2 H2O(l) → CH4(g) + 2 O2(g)