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Chemistry 12 Electrochemistry I Name: Date: Block: 1. Oxidation Numbers 2. Electron gain and loss 3. Agents Electrochemistry is the study of the interchange of chemical and electrical energy. Reactions with electron transfers are commonly called oxidation-reduction reactions (redox reactions) Not all reactions involve an electron transfer – these reactions are referred to as metathesis reactions Oxidation Numbers Oxidation number is the real or apparent charge of an atom or ion when all bonds in the species containing the atom are or ion are considered to be ionic. Also called “combining capacity”. Rules for Assigning Oxidation Numbers (simplified): 1. 2. 3. 4. 5. 6. 7. Hint!! It might be helpful to break up a compound in a dissociation equation first! Ex: KMnO4 Ex: Na2Mo2O5 Example: Assign the oxidation number of each atom in the following species a) WBr4 b) AsO4 3- c) Pb(NO3)2 d) C4H10 e) Al2(SO4)3 f) CO3 2- g) NH4+ h) Na2Cr2O7 MORE PRACTICE YAY! (Answers found under “Worksheet 1” Determine the oxidation number for each of the following elements in each compound: F2 Fe2O3 S2Cl2 BrO2- CaH2 H2 S2O32- CN- ZnO NH2OH P4 Cs2O2 S2O8 2- N2H4 H2AsO3 - PO3 3- N2O5 WBr4 K2S SeO3 2- SF6 NO2 MnO2- Na2Mo2O5 K4P2O7 H2CO OH- Cl2O O3 Ba2XeO4 PCl3 P2O5 U3O8 Pb NaAuCl4 S2O3 2- (NH4)2SeO4 CH3OH LiAlH4 CH3COO- FeCl3 (NH4)2C2O4 BF3 SiO4 4- Hebden Workbook Pg. 194 #3-5 Electron gain and loss Loss Oxidation number increases Called “oxidation” Gain Oxidation number decreases Called “reduction” L G E E O R O R I I L G Fe (s) + Cu 2+ (aq) Fe 2+ (aq) + Cu(s) Practice: Consider the following reaction: 2 Al + Fe2O3 Al2O3 + 2 Fe Determine the oxidation numbers for each atom and write the value on top of the element in the reaction. 1. Are electrons gained or lost by each iron (III) ion? ________________________ a. How many? ______ 2. Are electrons gained or lost by each Al atom? ________________________ a. How many? ______ 3. How many electrons were transferred in total during the reaction? ______ 4. What happened to the oxide ion, O2- during the reaction? The number of electrons ________________ by the species being oxidized must always equal the number of electrons ________________ by the species being reduced. Agents Another way of looking at it is that one species causes the electron loss or gain. A species that is being oxidized causes the other species to gain electrons and be reduced. A substance that is reduced acts as an _________________________ agent, while a substance that is oxidized acts as a _________________________ agent. Practice: Indicate the oxidizing and reducing agents in each of the following reactions. Assign oxidation numbers to all atoms in the equation. For each reaction write the oxidation and reduction half reaction: 1. Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) Oxidizing agent: Reducing Agent: 2. 4 NH3 (g) + 7 O2 (g) 4 NO2 (g) + 6 H2O (l) Oxidizing agent: Reducing Agent: 3. C(s) + 2 H2 (g) CH4 Oxidizing agent: Reducing Agent: 4. 3 Sr (s) + 2 FeBr3 (aq) 2 Fe (s) + 3 SrBr2 (aq) Oxidizing agent: Reducing Agent: 5. 5 CO (g) + Cl2O5 (s) 5 CO2 (g) + Cl2 (s) Oxidizing agent: Reducing Agent: 6. 4 PH3 (g) P4 (g) + 6 H2 (g) Oxidizing agent: Worksheet Reducing Agent: