A closer look at a selected reaction: Mg + CuSO4 MgSO4 + Cu Charges of Metal Reactants Charges of Metal Products # of Electrons Lost/Gained Mg: 0

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1 AP Chemistry Ms. Ye Name Date Block A closer look at a selected reaction: Mg + CuSO4 MgSO4 + Cu Charges of Metal Reactants Charges of Metal Products # of Electrons Lost/Gained Mg: 0 Mg: +2 Mg: Cu: +2 Cu: 0 Oxidation: the by an atom or ion during a chemical reaction Reduction: the by an atom or ion during a chemical reaction Oxidation and Reduction occur together in a reaction. Oxidation-reduction reactions are known as Redox reactions. OXIDATION NUMBERS (state) +, -, or neutral (0) numbers that can be assigned to atoms Used to identify how many are being or by an atom/ion when they Assigning Oxidation Numbers: Rules 1. Free Elements (elements not bonded to a different element) ALWAYS have an oxidation number of 2. Ions have the oxidation number as the charge on the ion 3. When combined with another element (within a compound): a. Group 1 metal ions ALWAYS have an oxidation number of b. Group 2 metal ions ALWAYS have an oxidation number of c. Fluorine ion ALWAYS has an oxidation number of 4. Oxygen usually has an oxidation number of a. Exceptions: OF2, H2O2 5. Hydrogen s oxidation number is different depending on whether it s in an ionic or covalent compound a. H bonded with a nonmetal (covalent): the oxidation number will be b. H bonded with a metal (ionic): the oxidation number will be 6. The oxidation numbers must add up to the charge of the compound (0) or polyatomic ion Ex: 2 MgF2 SO Total: -2-1 Total: Note: When assigning oxidation numbers, start with your ALWAYS rules first Cu: Element Oxidation Number Mg 0 H2 0 Ion Mg F 1-1 Oxidation Number

2 Assigning Oxidation Numbers: Practice Determine the oxidation states for each atom present in the given formulas 1. Cl2 7. H2SO4 Cl= H= 2. Cl 1 Cl= S= 3. Na Na = 8. NO3 1 N= 4. Na KCl 6. CaO Na= K= Cl= Ca= 9. Cr2O H2S 11. NO2 Cr= H= S= N= More Oxidation Numbers Practice Determine the oxidation numbers for each element in the following molecules: a. Na2O d. Cl2 b. H2O e. NO2 _ c. SO4 2- f. CH4

3 In a redox reaction, one element electrons while another element electrons. OIL RIG O I L R I G Writing Half Reactions Ex 1: LEO the lion goes GER L E O G E R Ca + CrF2 CaF2 + Cr Step 1. Assign oxidation numbers Step 2. Determine how many electrons are gained/lost by each species Ca gains/loses electrons. Ca is oxidized/reduced. Cr 2+ gains/loses electrons. Cr 2+ is oxidized/reduced. Step 3. Write two separate reactions for the oxidation and the reduction. CHARGES MUST BE BALANCED! Ex 2: Mg + ZnCl2 MgCl2 + Zn Step 1. Assign oxidation numbers Step 2. Determine how many electrons are gained/lost by each species Mg gains/loses electrons. Mg is oxidized/reduced. Zn 2+ gains/loses electrons. Zn 2+ is oxidized/reduced. Step 3. Write two separate reactions for the oxidation and the reduction. CHARGES MUST BE BALANCED!

4 Ex 3: Al + Cu 2+ Al 3+ + Cu Step 1. Assign oxidation numbers Step 2. Determine how many electrons are gained/lost by each species Al gains/loses electrons. Al is oxidized/reduced. Cu 2+ gains/loses electrons. Cu 2+ is oxidized/reduced. Step 3. Write two separate reactions for the oxidation and the reduction. CHARGES MUST BE BALANCED! 4. Write out the oxidation and reduction half reactions for: 2Fe 3+ + Sn 2+ 2Fe 2+ + Sn 4+

5 Redox Problem Set 1. Determine the oxidation number of each element in the following ions or compounds: a) BrO3 - d) CaH2 b) C2O4 2- e) H4SiO4 c) F2 f) SO Determine the oxidation number of each element in the following ions or compounds: a) SF6 d) N2O4 b) H2AsO4 - e) PCl4 + c) UO2 + f) XeO Which of the following reactions is (are) oxidation-reduction reactions? Explain your answer briefly. classify the remaining reactions. a) Zn(s) + 2 NO3 - (aq) + 4 H + (aq) Zn 2+ (aq) + 2 NO2(g) + 2 H2O(l) b) Zn(OH)2(s) + H2SO4(aq) ZnSO4(aq) + 2 H2O(l) c) Ca(s) + 2 H2O(l) Ca(OH)2(s) + H2(g)

6 4. Which of the following reactions is (are) oxidation-reduction reactions? Explain your answer briefly. classify the remaining reactions. a) CdCl2(aq) + Na2S(aq) CdS(s) + 2 NaCl(aq) b) 2 Ca(s) + O2(g) 2 CaO(s) c) Ca(OH)2(s) + 2 HCl(aq) CaCl2(aq) + 2 H2O(l) 5. In each of the following reactions, decide which reactant is oxidized and which is reduced. Designate the oxidizing agent and reducing agent. a) 2 Mg(s) + O2(g) 2 MgO(s) b) C2H4(g) + 3 O2(g) 2 CO2(g) + 2 H2O(g) c) Si(s) + 2 Cl2(g) SiCl4(l) 6. In each of the following reactions, decide which reactant is oxidized and which is reduced. Designate the oxidizing agent and reducing agent. a) Ca(s) + 2 HCl(aq) CaCl2(aq) + H2(g) b) Cr2O7 2- (aq) + 3 Sn 2+ (aq) + 14 H + (aq) 2 Cr 3+ (aq) + 3 Sn 4+ (aq) + 7 H2O(l) c) FeS(s) + 3 NO3 (aq) + 4 H + (aq) 3 NO(g) + SO4 2- (aq) + Fe 3+ (aq) + 2 H2O(l)