Presentation on theme: "A Determine the oxidation number for each atom in the following molecules H2S P2O5 S8 SCl2 Na2SO3 6. SO4-2 7. NaH Cr2O7-2 SnBr4 10. Ba(OH)2 For practice."— Presentation transcript:
1 ADetermine the oxidation number for each atom in the following moleculesH2SP2O5S8SCl2Na2SO36. SO4-27. NaHCr2O7-2SnBr410. Ba(OH)2For practice before class
2 12/05/06 Electrochemistry 19.9-19.13 p 941-955 AIntersection 1412/05/06Electrochemistryp
3 December in Studio S M Tu W Th F A 12/5 Exam 3 12/6 Studio 12/8 Polymers; check out12/11 Poster session, paper due12/12 final IS12/13 In-class assignment12/17 Review session 7-9 pm12/19 Final exam 8-10 am
4 Watershed Poster Session Monday, December 11 in USB 2165Board (4 ft x 4ft), easel, pinsSet up by 1:10 and 3:10One person stationed at poster; others evaluateRubric availablePaper due same time
5 Last In-Class Assignment Wednesday, December 13th in studioAvailable on-lineRead papers before coming to class; bring them with you.May make any notes you like on the papersGoal: to evaluate scientific method and data
6 Outline Ed’s demos Balancing Redox Reactions Electrochemistry Electrochemical cells and Standard Hydrogen ElectrodesNernstQuantifying current
10 Thinking back…. What happened when Na(s) was added to water? Na(s) + H2O(l) Na+ (aq) H2(g) + OH-(aq)Determine the oxidation state of each reactant and productWhat was oxidized?What was reduced?
11 Balancing Redox Reactions MBalancing Redox ReactionsWhen KMnO4 (potassium permanganate) is mixed with Na2C2O4 (sodium oxalate) under acidic conditions, Mn+2(aq) ions and CO2(g) form.The unbalanced chemical equation is:KMnO4(aq) + Na2C2O4(aq) Mn+2(aq) + CO2(g) + K+(aq) + Na+(aq)K+ and Na+ are spectator ions, so we can ignore them at this point.MnO4- (aq) + C2O4-2(aq) Mn+2(aq) + CO2(g)
12 Half-Reactions Reduction reaction Oxidation reaction M MnO4- (aq) + C2O4-2(aq) Mn+2(aq) + CO2(g)Reduction reactionOxidation reactionReduction rxn MnOxidation C
13 Reduction reaction M MnO4- Mn+2 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 electronsMn+7 on reactants sideMn+2 on products sideStep 5: Check charge balance and elemental balance
19 ADefinitionsElectrochemical 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 occurVoltaic 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 currentElectrolytic cell: electrochemical reactions are produced by applying electrical energy
20 A Copper-Zinc battery – What Matters? Consider reduction potentials:Cu+2 + 2e- → Cu(s) VZn+2 + 2e- → Zn(s) VPlace Zn electrode in copper sulfate solution – What happens?Copper is plated on Zn electrodeCu+2 + 2e- → Cu(s) VZn(s) → Zn+2 + 2e VE > 0, spontaneousCu+2 + Zn(s) → Zn+2 + Cu(s) 1.1 VNote, no need for electron to flow external to cell for reaction to occur!!
21 A Copper-Zinc battery – What Matters? Consider reduction potentials:Cu+2 + 2e- → Cu(s) VZn+2 + 2e- → Zn(s) VPlace Cu electrode in zinc sulfate solution – What happens?Zn doesn’t plate on copper electrode?!Cu(s) → Cu+2 + 2e VZn+2 + 2e- → Zn(s) VE < 0, not spontaneousZn+2 + Cu(s) → Cu+2 + Zn(s) -1.1 VNo reaction occurs !!
23 AWhat are the ½ reactions?What is the overall reaction?Identify the oxidation, reduction, anode, and cathode
24 SHE: Standard Hydrogen Electrode 2 H3O+(aq, 1.00 M) + 2e- <->H2(g, 1 atm) + 2H2O(l)Eo = 0VStandard conditions:1M, 1atm, 25oCRedox Reactions & Galvanic CellsIn order to compare the potential difference when different metals are used for the anode and the cathode, each half reaction is compared to a standard half reaction. This process is like choosing par in golf. The par for a hole in golf is set at the standard. Golfers scores are then measured relative to par.The freedom to set the cell potential difference equal to zero for this reaction is the same freedom we have to set any height equal zero wherever is convenient when we work with gravitational potential energy. We have this freedom because we are not interested in the absolute potential of a reaction, rather we are only interested in the magnitude of the change in potential.Fig. 19-7, p.922
25 Measuring Relative Potentials Measuring Relative PotentialsTable of StandardReduction Potentials
26 Standard Reduction Potentials What is the standard potential of a Au+3/Au/Mg+2/Mg cell?
27 AThe 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.