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Chapter 21

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  1. Chapter 21 Electrochemistry AP Chemistry Wolpa CDO

  2. Oxidation-Reduction Reactions • oxidation: loss of electrons LEO • reduction: gain of electrons GER • Can be determined from change in oxidation numbers. AP Chemistry Wolpa CDO

  3. Oxidation Number Charge an atom has or would have if all the bonding electrons were assigned to the most electronegative element. AP Chemistry Wolpa CDO

  4. Rules for assigning oxidation numbers 1). Oxidation number pure neutral element = 0 (e.g. Li, H2, C etc) 2). Oxidation number monatomic ion = ionic charge (Na+ = +1, Mg2+ = +2, etc) 3). Oxidation number of O = -2 in all its compounds except: those with O-O bond (peroxides e.g. Na-O-O-Na then ox. number = -1) superoxides, (e.g. KO2 then ox. number = -1/2) AP Chemistry Wolpa CDO

  5. Rules cont. 4). H is +1 in all its compounds (except metal hydrides then = -1) 5). F is - 1 in all its compounds (except, of course, F2) 6). Alkali metals are always +1; alkaline earths are always +2. 7). Sum of oxidation number is zero for neutral compounds, and equal to the overall charge for polyatomic ionic species. AP Chemistry Wolpa CDO

  6. Oxidation and Reduction must occur simultaneously • Zn(s) + Cu2+(aq) -------> Zn2+(aq) + Cu(s) AP Chemistry Wolpa CDO

  7. Electrochemical Cells • System consisting of electrodes that dip into an electrolyte and in which an oxidation-reduction reaction either uses or generates an electric current. AP Chemistry Wolpa CDO

  8. Types of Electrochemical Cell Voltaic or Galvanic • A spontaneous reaction generates an electric current Electrolytic • An electric current is used to drive a nonspontaneous reaction AP Chemistry Wolpa CDO

  9. AP Chemistry Wolpa CDO

  10. Electrodes • Cathode: electrode where reduction reaction occurs • Anode: electrode where oxidation reaction occurs. AP Chemistry Wolpa CDO

  11. Balancing Redox Reactions Neutral Solution 1. Split into two half-reactions 2. Balance each half-reaction a. Balance the elements b. Balance the charge 3. Combine half-reactions so that electrons cancel. AP Chemistry Wolpa CDO

  12. Cl2(g) + Br-(aq) --> Br2(l) + Cl-(aq) AP Chemistry Wolpa CDO

  13. Balancing Redox Reactions Acidic Solution 1. Split into two half-reactions 2. Balance each half-reaction a. Balance the elements b. Balance oxygens by adding water c. Balance hydrogens by adding hydrogen ions d. Balance the charge 3. Combine half-reactions so that electrons cancel. AP Chemistry Wolpa CDO

  14. Zn(s) + VO2+(aq) --> V2+(aq) + Zn2+(aq) AP Chemistry Wolpa CDO

  15. Balancing Redox Reactions Basic Solution 1. Split into two half-reactions 2. Balance each half-reaction a. Balance the elements b. Balance oxygens by adding water c. Balance hydrogens by adding hydrogen ions d. Neutralize the H+ ions by adding OH- ions e. Balance the charge 3. Combine half-reactions so that electrons cancel. AP Chemistry Wolpa CDO

  16. Zn(s) + Cl-(aq) --> Zn(OH)2(s) + Cl- (aq) AP Chemistry Wolpa CDO

  17. Voltaic Cell Description • anode: Zn ----------> Zn2+ + 2e- • cathode: Cu2+ + 2e- ----------> Cu • Consists of two half-cells that are electrically connected. The salt bridge consists of a tube of an electrolyte that is connected to two half-cells of the voltaic cell. Allows the flow of charge ions but prevents diffusional mixing of the different solutions. AP Chemistry Wolpa CDO

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  19. AP Chemistry Wolpa CDO

  20. Volatic Cell Key Points 1. There are always two separate half-cells connected by a wire and a salt bridge 2. One of the compartments is the anode and the other is the cathode 3. If a metal participates in a cell reaction it is ordinarily chosen as the electrode. If no metal is involved in the half-reaction an electrically conduction solid like platinum or graphite is used 4. Electrons flow from the anode to the cathode. 5. Cations move to the cation and anions to the anode via the salt bridge. AP Chemistry Wolpa CDO

  21. Cell Notation Zn/Zn2+ ll Cu2+/Cu anode reaction is shown at left salt bridge is indicated by ll cathode reaction is shown at right AP Chemistry Wolpa CDO

  22. Draw cell diagram and write the cell notation Fe3+(aq) + H2(g)  Fe2+(aq) + 2H+(aq) AP Chemistry Wolpa CDO

  23. Electrochemical Cells and Potentials Emf • Electrons generated at the site of oxidation of a cell are thought to be "driven" or "pushed" toward the cathode by an electromotive force, or emf. This force is due to the difference in electric potential of an electron at the two electrodes. AP Chemistry Wolpa CDO

  24. Standard Potential • Measure of the driving force of the cell reaction when all ions and molecules in the solution are at a concentration of 1 M and all gases are at a pressure of one atm. AP Chemistry Wolpa CDO

  25. Calculating the Potential Eo of an Electrochemical Cell • Eotot = Eoox + Eored • Eoox: standard voltage for the oxidation half-reaction • Eored: standard voltage for the reduction half-reaction AP Chemistry Wolpa CDO

  26. Standard Hydrogen Half-Cell • Cannot determine an individual half-reaction voltage therefore we arbitrarily take the standard voltage for the reduction of H+ ions to H2(g) to be zero (standard hydrogen half-cell): • 2H+(aq,1M) + 2e- ----------> H2(g, 1atm) Eored = 0 • With this assigned voltage others can be determined from measurement by hook-up with knowns. AP Chemistry Wolpa CDO

  27. Standard Hydrogen Half-Cells AP Chemistry Wolpa CDO

  28. Standard Potentials AP Chemistry Wolpa CDO

  29. Using Standard Potentials 1. The Eo values are for reactions written in the form "oxidized form + electrons --------> reduced form." The species on the left side of the reaction is an oxidizing agent, and the species on the right is a reducing agent. All potentials are therefore for reduction reactions. 2. When writing the reaction "reduced form --------> oxidized form + electrons," the sign of Eo is reversed, but the value of Eo is unaffected. 3. All the half-reactions are reversible. 4. The more positive the value of Eo for the reactions the better the oxidizing ability of the ion or compound. AP Chemistry Wolpa CDO

  30. 5. The more negative the value of the reduction potential Eo the less likely the reaction occurs as a reduction, and the more likely the reverse reaction occurs. 6. The reaction between any substance on the left in this table with any substance lower than it on the right is product-favored under standard conditions. 7. The algebraic sign of the half-reaction potential is the sign of the electron when it is attached to H2/H3O+ standard cell. 8. Electrochemical potential depends on the nature of the reactants and products and their concentrations, not on the quantities of material used. AP Chemistry Wolpa CDO

  31. Largenegative value meansoxidation strongly favored; strong reducing agent. • Largepositive value meansreduction strongly favored; strong oxidizing agent. • Relative values in table give an indication that one half-reaction favored over other. Summing half-cell reactions allow determination of standard cell potential. • Half-cell potential intensive property  independent of amount of material  we don’t use stoichiometric coefficients for determining standard cell potentials. AP Chemistry Wolpa CDO

  32. Determine the cell potential of Br2(l) + 2I(aq)  I2(l) + 2Br(aq) AP Chemistry Wolpa CDO

  33. Determine the cell potential of 2Ag+(aq) + Cu(s)  2Ag(s) + Cu2+(aq) AP Chemistry Wolpa CDO

  34. Determine cell potential: MnO4-(aq) + Fe(s)  Fe2+(aq) + Mn2+(aq) (balanced?) when it is operated galvanically. Which is the oxidizing agent? reducing agent? AP Chemistry Wolpa CDO

  35. Determine if the reaction below is spontaneous in the direction written. • Fe3+(aq) + Ag(s)  ? AP Chemistry Wolpa CDO

  36. Eotot and DGo 1. DG = free energy change = maximum amount of useful work = -wmax 2. Electric work = charge x potential energy difference = coulomb x volt AP Chemistry Wolpa CDO

  37. Eotot and DGo • wmax = nFE = coulombs x volts = electrical energy in joules where F is the Faraday constant, 9.65 x 104 J/V mol. DGo = -nFEo tot = -96.5 nEo tot (in KJ) • Because spontaneous reactions have a negative free energy change, DG, the negative sign in the equation above confirms that all product-favored electron transfer reactions have a positive Eo. AP Chemistry Wolpa CDO

  38. Electrochemical Cells at Nonstandard Conditions • Voltage is a measure of reaction spontaneity. • Hence: • voltage is increased by increasing concentrations of reactants or decreasing concentrations of products. • voltage is decreased by decreasing concentrations of reactants or increasing concentrations of products. AP Chemistry Wolpa CDO

  39. Quantitative: Nernst Equation aA + bB ----------> cC + dD Etot = Eotot -RT/nF ln [products]/[reactants] raised to the power of their coefficients where R = 8.314510 J/K mol, F = 9.645309 x 104 J/v mol and number of electrons transferred at 25 C AP Chemistry Wolpa CDO

  40. Nernst Equation Etot = Eotot - 0.0257/n ln[products]/[reactants] raised to the power of their coefficients n = number of electrons transferred [ ] use molarities for aqueous solutions, atm for gases AP Chemistry Wolpa CDO

  41. K: Equilibrium Constant Etot = Eotot - - 0.0257/n ln [products]/[reactants] raised to the power of their coefficients at equilibrium Etot = zero and K = [products]/[reactants] raised to the power of their coefficients therefore 0 = Eotot - 0.0257/n ln K AP Chemistry Wolpa CDO

  42. Eotot and K • ln K = nEotot/0.0257 AP Chemistry Wolpa CDO

  43. Determine free energy and equilibrium constant for reaction below (unbalanced). MnO4-(aq) + Fe(s)  Fe2+(aq) + Mn2+(aq) AP Chemistry Wolpa CDO

  44. Determine cell potential and equilibrium constant of Cl2/Br2 cell. AP Chemistry Wolpa CDO

  45. The following cell has a potential of 0.578 V at 25°C; determine Ksp. Ag(s)|AgCl(s)|Cl(1.0 M)||Ag+(1.0 M)|Ag(s). AP Chemistry Wolpa CDO

  46. Batteries • A battery is a package of one or more galvanic cells used for the production and storage of electric energy by chemical means. A galvanic cell consists of at least two half cells, a reduction cell and an oxidation cell. Chemical reactions in the two half cells provide the energy for the galvanic cell operations. • Each half cell consists of an electrode and an electrolyte solution. Usually the solution contains ions derived from the electrode by oxidation or reduction reaction. AP Chemistry Wolpa CDO

  47. Types of Batteries • Batteries can be divided into two types: primary or disposable batteries and secondary or rechargeable batteries. AP Chemistry Wolpa CDO

  48. Fuel cells are different from batteries in that they consume reactant, which must be replenished, whereas batteries store electrical energy chemically in a closed system. AP Chemistry Wolpa CDO

  49. Corrosion of Metals/Cathodic Protection Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. AP Chemistry Wolpa CDO

  50. AP Chemistry Wolpa CDO