Chapter 19: Oxidation and Reduction Oxidation originally was thought to be the gain of oxygen. Reduction originally was thought to be the loss of oxygen. Oxidation is really the loss of electrons. LEO Lose Electrons Oxidized Reduction is really the gain of electrons. GER Gain Electrons Reduced 2Mg + O2  2MgO Do the dot diagrams for Mg and O and show how they gain/lose electrons.

Redox Half-Reactions 2Na + Cl2  2NaCl The oxidation half reaction is the half of the reaction that involves loss of electrons. 2Na 2 Na+ + 2e- The reduction half-reaction is the half of the reaction that involves gain of electrons. Cl2 + 2 e-  2Cl- The net ionic equation would be the 2 added together: 2Na +Cl2 + 2e-  2 Na+ + 2e- + 2Cl-

Oxidation Numbers e- Oxidation numbers are numbers assigned to keep track of electron gain and loss. e- See page 686 in your MH text for rules guiding assignment of oxidation numbers. The oxidation number of an uncombined atom is 0. The oxidation number of a monatomic ion= its charge. The oxidation number of oxygen is -2 except -1 in peroxides. The oxidation number of + ions in IA,IIA, and IIIA are the + of the GP #. The oxidation number of hydrogen is +1 except where #4/#6 cause -1. The sum of oxidation numbers in a neutral compound=0. The sum of oxidation numbers in a polyatomic ion = its charge.

Practice Assigning Oxidation #s Assign oxidation numbers to each element in the following compounds or polyatomic ions. To really keep on your toes, see if you can name them as well. There should be a number/sign above each capital letter. MgBr2 Fe2O3 AlN SO3 PO43- Cr2O72- HClO2 CuSO4 Cl2 Na Ca2+ H2O2

Check Your Answers Assign oxidation numbers to each element in the following compounds or polyatomic ions. To really keep on your toes, see if you can name them as well. +2 -1 +3 -2 +3 -3 +6 -2 MgBr2 Fe2O3 AlN SO3 +5 -2 +6 -2 +1 +3 -2 +2 +6 -2 PO43- Cr2O72- HClO2 CuSO4 0 0 +2 -1 +1 Cl2 Na Ca2+ H2O2

Oxidation # Change Method for Determining Substance Oxidized/Reduced 1. Assign oxidation numbers. 2. Connect the substance with numbers that increase. 3. Connect the substances with numbers that decrease. 4. The substance that decreases is reduced…the one that increases is oxidized.

Oxidation # Change Method for Determining Substance Oxidized/Reduced

Oxidation Number Change Method Use the oxidation number change method to determine what is oxidized and reduced: 2Na + Br2  2NaBr