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Determine the oxidation number for each atom in the following molecules 1.H 2 S 2.P 2 O 5 3.S 8 4.SCl 2 5.Na 2 SO 3 6. SO 4 -2 7. NaH 8.Cr 2 O 7 -2 9.SnBr.

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Presentation on theme: "Determine the oxidation number for each atom in the following molecules 1.H 2 S 2.P 2 O 5 3.S 8 4.SCl 2 5.Na 2 SO 3 6. SO 4 -2 7. NaH 8.Cr 2 O 7 -2 9.SnBr."— Presentation transcript:

1 Determine the oxidation number for each atom in the following molecules 1.H 2 S 2.P 2 O 5 3.S 8 4.SCl 2 5.Na 2 SO 3 6. SO NaH 8.Cr 2 O SnBr Ba(OH) 2 A

2 Intersection 14 12/05/06 Electrochemistry p A

3 December in Studio SMTuWThFS 12/5 Exam 3 12/6 Studio 12/8 Polymers; check out 12/11 Poster session, paper due 12/12 final IS 12/13 In- class assignment 12/17 Review session 7- 9 pm 12/19 Final exam 8-10 am A

4 Watershed Poster Session Monday, December 11 in USB 2165 Board (4 ft x 4ft), easel, pins Set up by 1:10 and 3:10 One person stationed at poster; others evaluate Rubric available Paper due same time A

5 Last In-Class Assignment Wednesday, December 13 th in studio Available on-line Read papers before coming to class; bring them with you. May make any notes you like on the papers Goal: to evaluate scientific method and data A

6 Outline Ed’s demos Balancing Redox Reactions Electrochemistry –Electrochemical cells and Standard Hydrogen Electrodes –Nernst –Quantifying current A

7 Ed’s Demos A

8 Oxidation States of Vanadium: Reduction of V 5+ to V +2 Reaction 1 –Zn (s) + 2 VO 3 - (aq) + 8 H 3 O + (aq) ↔ 2 VO 2 + (aq) + Zn +2 (aq) + 12 H 2 O (l) Reaction 2 –Zn (s) + 2 VO 2 + (aq) + 8 H 3 O + (aq) ↔ 2 V 3+ (aq) + Zn +2 (aq) + 6 H 2 O (l) Reaction 3 –Zn (s) + 2 V 3+ (aq) ↔ 2 V 2+ + Zn +2 (aq) V +5 (aq) → V +4 (aq)yellow to green V +4 (aq) → V +3 (aq)green to blue V +3 (aq) → V +2 (aq)blue to violet A

9 Oxidation States of Manganese: Mn +7, Mn +6, Mn +4, and Mn (purple) to +2 (colorless) –2 MnO 4 - (aq) + H + (aq) + 5 HSO 3 - (aq) ↔ 2 Mn +2 (aq) + 5 SO 4 -2 (aq) + 3 H 2 O(l) + 7 (purple) to +4 (brown) –OH MnO 4 - (aq) + 3 HSO 3 - (aq) ↔ 2 MnO 2 (s) + 3 SO 4 -2 (aq) + 2 H 2 O(l) + 7 (purple) to + 6 (green) –2 MnO 4 - (aq) + 3 OH - + HSO 3 - (aq) ↔ 2 MnO 4 -2 (aq) + SO 4 -2 (aq) + 2 H 2 O(l) A

10 Thinking back…. What happened when Na (s) was added to water? Na (s) + H 2 O (l)  Na + (aq) + H 2(g) + OH - (aq)  Determine the oxidation state of each reactant and product What was oxidized? What was reduced? A

11 Balancing Redox Reactions When KMnO 4 (potassium permanganate) is mixed with Na 2 C 2 O 4 (sodium oxalate) under acidic conditions, Mn +2 (aq) ions and CO 2 (g) form. The unbalanced chemical equation is: KMnO 4(aq) + Na 2 C 2 O 4(aq)  Mn +2 (aq) + CO 2(g) + K + (aq) + Na + (aq) K + and Na + are spectator ions, so we can ignore them at this point. MnO 4 - (aq) + C 2 O 4 -2 (aq)  Mn +2 (aq) + CO 2(g) M

12 Half-Reactions Reduction reaction Oxidation reaction MnO 4 - (aq) + C 2 O 4 -2 (aq)  Mn +2 (aq) + CO 2(g) M

13 Reduction reaction Step 1: Balance all elements other than oxygen and hydrogen. Step 2: Balance the oxygens by adding water. Step 3: Balance the hydrogens using H + Step 4: Balance the electrons Mn +7 on reactants side Mn +2 on products side Step 5: Check charge balance and elemental balance MnO 4 -  Mn +2 M

14 Oxidation reaction C 2 O 4 -2  CO 2 M

15 Combine Half Reactions 5 e - + 8H + + MnO 4 -  Mn H 2 O C 2 O 4 -2  2 CO 2 + 2e - We are assuming the reaction takes place under acidic conditions! M

16 Balancing in Base 5 e - + 8H + + MnO 4 -  Mn H 2 O C 2 O 4 -2  2 CO 2 + 2e - Change H + to water by adding OH - to each side M

17 M NO 2 - (aq) + Cr 2 O 7 -2 → Cr +3 (aq) + NO 3 - (aq) acidic soln

18 Electrochemical Cells A

19 Definitions Electrochemical cell: A combination of anode, cathode, and other materials arranged so that a product-favored redox reaction can cause a current to flow or an electric current can cause a reactant- favored redox reaction to occur Voltaic cell (battery): An electrochemical cell or group of cells in which a product-favored redox reaction is used to produce an electric current. Galvanic cell: A cell in which an irreversible chemical reaction produces electrical current Electrolytic cell: electrochemical reactions are produced by applying electrical energy A

20 A Copper-Zinc battery – What Matters? Consider reduction potentials: Cu e - → Cu(s) V Zn e - → Zn(s) V Place Zn electrode in copper sulfate solution – What happens? Cu e- → Cu(s) V Zn(s) → Zn e V Cu +2 + Zn(s) → Zn +2 + Cu(s)1.1 V E > 0, spontaneous Note, no need for electron to flow external to cell for reaction to occur!! Copper is plated on Zn electrode A

21 A Copper-Zinc battery – What Matters? Consider reduction potentials: Cu e- → Cu(s) V Zn e- → Zn(s) V Place Cu electrode in zinc sulfate solution – What happens? Cu(s) → Cu e V Zn e- → Zn(s) V Zn +2 + Cu(s) → Cu +2 + Zn(s)-1.1 V E < 0, not spontaneous No reaction occurs !! Zn doesn’t plate on copper electrode?! A

22 Fig. 19-3, p.918 A

23 What are the ½ reactions? What is the overall reaction? Identify the oxidation, reduction, anode, and cathode A

24 Fig. 19-7, p.922 SHE: Standard Hydrogen Electrode 2 H 3 O + (aq, 1.00 M) + 2e - H 2(g, 1 atm) + 2H 2 O (l) E o = 0V Standard conditions: 1M, 1atm, 25 o C A

25 Measuring Relative Potentials Table of Standard Reduction Potentials A

26 Standard Reduction Potentials What is the standard potential of a Au +3 /Au/Mg +2 /Mg cell? A

27 The half-reaction with the more positive standard reduction potential occurs at the cathode as reduction. The half-reaction with the more negative standard reduction potential occurs at the anode as oxidation. A


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