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A little more Thermodynamics, Redox, Electrochemistry, and Radioactivity

A little more Thermodynamics, Redox, Electrochemistry, and Radioactivity. --Whew. Entropy -a measure of disorder in a system. In general, in terms of entropy: (s)<(l)<(aq)<<(g) Entropy (S) measured in ( J/mol k) Def: entropy of a pure substance, perfect crystal, at absolute 0 = 0 J/ mol k.

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A little more Thermodynamics, Redox, Electrochemistry, and Radioactivity

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  1. A little more Thermodynamics, Redox, Electrochemistry, and Radioactivity --Whew.

  2. Entropy -a measure of disorder in a system • In general, in terms of entropy: (s)<(l)<(aq)<<(g) • Entropy (S) measured in (J/mol k) • Def: entropy of a pure substance, perfect crystal, at absolute 0 = 0 J/ mol k

  3. Is DS (+) or (-) ? • 2NaHCO3 (s) Na2CO3 (s) + H2O(g) • CaCO3 (s) CaO (s) +CO2 (g) • H2 (g) +Cl2 (g) 2HCl (g) • N2(g) + 3H2 (g) 2NH3 (g) Just check the states

  4. Changes in entropy A: A system can lose entropy, becoming more ordered. B: As it does, the rest of the Universe becomes less ordered • B is always greater than A Entropy always increases

  5. Gibbs Free Energy • For any reaction: DG=DH-TDS or DGo=DHo-TDSo (“o”=standard conditions) and DG for a reaction = -DG for the reverse reaction

  6. Standard conditions (thermo) 25oC (298k) Solutes at 1.0M Gasses at 1.0 atm.

  7. DGo and Spontaneity

  8. Spontaneity • DGo<0: the system will proceed forwardfrom standard conditions. This is called spontaneous. • DG=0. concentrations are stable. A system in equilibrium has no gain or loss of enthalpy or entropy . • DGo>0: Rxn is not spontaneous, the reverse rxn is.

  9. What is DSo ,DHo ,andDGo? • 2NaHCO3 (s) Na2CO3 (s) + H2O(g) • CaCO3 (s) CaO (s) +CO2 (g) • H2 (g) + Cl2 (g) 2HCl (g) • N2(g) + 3H2 (g) 2NH3 (g)

  10. At what T does the spontaneity change? • 2NaHCO3 (s) Na2CO3 (s) + H2O(g) • CaCO3 (s) CaO (s) +CO2 (g) • H2 (g) + Cl2 (g) 2HCl (g) • N2(g) + 3H2 (g) 2NH3 (g)

  11. If DHo is & DSo is thenDGo… - + -spontaneous at any T + - +Not spontaneous at any T + + - spontaneous at high T (or) +Not spontaneous at low T - - +Not spontaneous at high T (or) - spontaneous at low T

  12. In (other) words… Situation 1 • If an exothermic reaction leads to an increase in entropy, then free energy is released, and the reaction is spontaneous at any temperature

  13. In (other) words… Situation 1 • If an exothermic reaction leads to an increase in entropy, then free energy is released, and the reaction is spontaneous at any temperature • (Can you state the other three situations?)

  14. In (other) words… Situation 2 • If an endothermic reaction leads to an decrease in entropy, then free energy is absorbed, and the reaction is nonspontaneous at any temperature

  15. In (other) words… Situation 3 • If an endothermic reaction leads to an increase in entropy, then free energy is released at high temperatures, and the reaction is spontaneousonly when it is hot enough

  16. In (other) words… Situation 4 • If an exothermic reaction leads to an decrease in entropy, then free energy is released only at low temperatures, and the reaction is spontaneousonly when it is cool enough (It may be very slow at that temperature)

  17. So, how do you make products? N2(g) + 3H2 (g) 2NH3 (g)

  18. So, how do you make products? • Non-spontaneous does not mean that reactants won’t make products. (It just won’t make a whole lot before they start decomposing just as fast.) • Use Le Chatelier’s principle. Remove the products (including heat), and the system will keep making more.

  19. Why must a non-spontaneous reaction make some product?

  20. Why must a non-spontaneous reaction make some product? • Entropy.

  21. Why must a non-spontaneous reaction make some product? • Entropy. • Pure reactants have less entropy than a mixture of reactants and products

  22. Come, let us reason together… • Spontaneous=proceeding forward (more products) from standard conditions • Standard conditions=conc. of 1 M (or 1 atm.) Q (std) =[products] / [reactants] = (1)x/(1)y=1 • and K=Q at eq. therefore…

  23. …if a reaction is spontaneous… …K>1

  24. DG<0 and K>1 for spontaneous reactions DG>0 and K<1 for non-spontaneous reactions

  25. Gibbs free energy and equilibrium • If a system is spontaneous: • It loses free energy if it proceeds forward from standard conditions—so it will • and K>1 • In any case: DGo = -RT ln K

  26. What is K (at 298k)? • 2 NaHCO3 (s) Na2CO3 (s) + H2O(g) • CaCO3 (s) CaO (s) +CO2 (g) • H2 (g) +Cl2 (g) 2 HCl (g) • N2(g) + 3H2 (g) 2 NH3 (g)

  27. Does it go forward from where it is? If Q<K, yes! If you know DGo, use the relationship: DG = DGo + RT ln Q If DG<0, it proceeds forwards If DG=0, it is in equilibrium (Q=K) If DG>0, it proceeds in reverse

  28. Three Laws of Thermodynamics 1st Law: Energy is neither created nor destroyed 2nd Law: Entropy (system and surroundings) always increases 3rd Law: The entropy of a pure substance, perfect crystal, at absolute 0 = 0 kJ/ mol k

  29. Redox—Review and Ch. 19 • A reduction is a gain of electrons, an oxidation is a loss of electrons • A reduction is always conjoined with an oxidation (e- ’s are conserved, charges mustbalance) • Remember: “OILRIG” or “LEO says GER”

  30. OILRIG Oxidation is loss of electrons Reduction is gain of electrons

  31. LEO says GER Loss of electrons is oxidation Gain of electrons is reduction

  32. Does a redox reaction occur? • Look for an oxidizing agent and a reducing agent. • If there is one of each, then ask, “Can this oxidizing agent oxidize this reducing agent” • Answer by comparing reduction potentials. Don’t memorize a rule. Compare the values to a reaction you know will occur

  33. Does a redox reaction occur? If you combine… • Na+ and Fe+3? • Cl- and Ag? • Cu and K+ ? • Pb+2 and I- ? • Fe+2 and Mg?

  34. Does a redox reaction occur? If you combine… • Na+ and Fe+3?—No. There is no reducer. • Cl- and Ag?—No. There is no oxidizer. • Cu and K+ ?—No. This oxidizer can’t do it • Pb+2 and I- ?—No, but it will precipitate. • Fe+2 and Mg?—Yes. Fe+2 + Mg  Fe and Mg+2

  35. Redox—half reactions • Balance the atoms • Rectify the electrons • Add H2O and H+ to balance oxygen and hydrogen • Check that charges are balanced • (Add OH- if the reaction is specified as in a basic solution)

  36. Try it. Sodium thiosulfate and nitric acid yield… Hydrogen peroxide and iron (II) sulfate Potassium dichromate and potassium iodide Potassium permanganate and ethanol 

  37. Try it. • S2O3-2 + NO3-  • H2O2 + Fe+2  • Cr2O7-2 + I-  • MnO4- + C2H5OH 

  38. Try it. • S2O3-2 + NO3-  SO4-2 + NO • H2O2 + Fe+2  H2O + Fe+3 • Cr2O7-2 + I- Cr+3 + I2 • MnO4- + C2H5OH  Mn+2 + CO2 + H2O

  39. Try it. 3x(S2O3-2 + 5H2O 2 SO4-2 +10H + + 8 e-) 8x(NO3- +4H + + 3 e- NO + 2 H2O) 3S2O3-2 + 15H2O 6 SO4-2 +30H + + 24 e-) 8NO3- +32H + + 24 e- 8NO + 16H2O) 3S2O3-2+8NO3-+2H+6SO4-2 +8NO+H2O

  40. Reduction potentials • Standard reduction potentials are measured as compared to: 2H++2e-H2(0.00V – by definition) • Half reactions that accomplish this have (-) reduction potentials (Eo<0) • Half reactions that force the reverse have (+) reduction potentials (Eo>0)

  41. Reduction potentials • Specifically: Magnesium reduces H+ • While bromine oxidizes hydrogen gas Write each reaction. What is the sign on Eo?

  42. Reduction potentials • Specifically: Magnesium reduces H+ • While bromine oxidizes hydrogen gas 2H+ + Mg  Mg+2 + H2 which implies that Mg+2+ 2 e-  Mg has Eo<0 H2+ Br2 2H+ +2Br -which implies that; Br2 + 2 e-  2Br – has Eo>0

  43. Electrochemical Cells • Half reactions are separated, and electrons are connected in a circuit. • A salt bridge is needed to allow charges to migrate to offset the motion of electrons • An electrode (anode or cathode) carries electrons to or from a half reaction Cathode means reduction

  44. Electrochemical Cells What is happening? Pb Cd Pb+2 Cd+2 The lead / cadmium battery

  45. Electrochemical Cells What is happening? Pb Cd Pb+2 Cd+2 Pb+2 + Cd  Cd+2 +Pb

  46. electrons Salt bridge Cathode gains mass Cathode = reduction Anode = oxidation Pb Cd Pb+2 Cd+2 Anode loses mass Pb+2Pb Cd  Cd+2

  47. Electrochemical Cells What is happening? Cu Zn Cu+2 Zn+2 The copper/zinc battery

  48. Shorthand notation • The Danielle Cell, using copper and zinc, Zn|Zn+2||Cu+2|Cu …makes 1.1 V

  49. Zn|Zn+2||Cu+2|Cu (or, in general) product reactant Anode of of cathode oxidation reduction

  50. If non-metals are used… Pt|H2|H2O||O2|H2O|Pt • The (non-reactive) metal electrode is noted outside the bars

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