Oxidation/Reduction Reactions REDOX REACTONS! All chemical reactions fall into two categories those that are redox and those that are not redox! Redox reactions involve the transfer of electrons. 2Zn(s) + O2(g)  2ZnO(s) is an example of a redox reaction, watch the following video to see what happens to the Zn metal and the oxygen gas. http://www.youtube.com/watch?v=e6Xxz-VBE6s Remember that ZnO(s) is an ionic compound composed of Zn2+ and O2- ions.

What is oxidation and reduction? Early chemists thought of oxidation as the combining of an element with oxygen, as in combustion. Now we refer to oxidation as the loss of electrons. In the previous example Zn loses two electrons, Zn(s)  Zn2+ + 2e- Reduction Early chemists thought of reduction as the loss of oxygen from a compound. Now we refer to reduction as the gain of electrons. In the previous example O must gain the electrons that Zn loses, O + 2e-  O2-

Redox reactions without oxygen: Mg(s) + S(s)  MgS(s) This reaction can be broken down into two steps: Mg  Mg2+ + 2e- (loss of electrons: oxidation) S + 2e-  S2- (gain of electrons: reduction) H2(g) + Cl2(g)  2HCl(g) Hydrogen and chlorine undergo the following changes to form a hydrogen chloride molecule: H  H+ + e- (loss of electrons: oxidation) Cl + e-  Cl- (gain of electrons: reduction)

“LEO the lion says GER” Elements can be oxidized or reduced depending on the reaction. Observe Iron in the following reactions: (remember that Iron III oxide is an ionic compound) Oxidation 4Fe(s) + 3O2(g)  2Fe2O3(s) - combustion or rusting Fe  Fe3+ + 3e- - Loss of Electrons - Oxidation Reduction 2Fe2O3(s) + 3C(s)  4Fe(s) + 3CO2(g)-loss of oxygen Fe3+ + 3e-  Fe -Gain of Electrons - Reduction

Oxidation and Reduction must occur simultaneously: If one element is oxidized then another must be reduced! Example: 2AgNO3(aq) + Cu(s)  2Ag(s) + Cu(NO3)2(aq) Net Ionic: 2Ag+ + Cu  2Ag + Cu2+ Nitrates ions are spectator ions! Half Reactions: 2Ag+ + 2e-  2Ag Reduction Cu  Cu2+ + 2e- Oxidation These reactions occur simultaneously and the electrons lost must equal the electrons gained. Remember that silver and copper II nitrate are soluble and hence are dissociated ions AgNO3(aq) is Ag+(aq) and NO3-(aq)

When silver tarnishes it forms a black silver sulfide compound according to the following reaction: 2Ag(s) + S(s)  Ag 2 S(s) Observe the following video and write the oxidation and reduction half reactions: http://www.youtube.com/watch?v=a6RR4kPsnlE&feature=related Ag  Ag+ + 1e- (oxidation) S + 2e-  S2- (reduction)

Chapter 20.1- Read pages 631  638 Complete question #1, #2 page 634 Complete Worksheet – Redox - 1