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LECTURE 6

LECTURE 6. AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON. THERMODYNAMICS (continued). Wark and Warner Chapter 7,8 Seinfeld & Pandis, Chapter 3 From the last lecture, you will remember the concepts of enthalpy (heat), entropy (disorder) and Gibbs Free Energy (the criterion feasibility).

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LECTURE 6

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  1. LECTURE 6 AOSC 434 AIR POLLUTION RUSSELL R. DICKERSON

  2. THERMODYNAMICS (continued) Wark and Warner Chapter 7,8 Seinfeld & Pandis, Chapter 3 From the last lecture, you will remember the concepts of enthalpy (heat), entropy (disorder) and Gibbs Free Energy (the criterion feasibility). How much CO is in equilibrium with CO₂ in auto exhaust? Again, consider the combustion of isooctane (C₈H₁₈)

  3. The dissociation of CO₂ produces CO: What is the concentration of CO₂ if there is complete combustion? [CO₂] = 8 / (8 + 9 + 47) = 0.125 If P = 1.00 atm, then P(CO₂) = 0.125 atm. What is the equilibrium constant? Equilibrium lies far to the left!

  4. If we assume that all the CO and O₂ come from the dissociation of CO₂ then: [CO₂] >> [CO] = 2[O₂] If total pressure is 1.00 atm then = [CO₂]. This is not much to worry about, but combustion does not occur at room temperature, it occurs at about 2000 K. What are ΔG and at 2000 K? Assume that ΔH and ΔS are independent of temperature. We want

  5. A positive ΔS for the reaction means that the products are favored by the entropy term in the Gibbs Free Energy, and the equilibrium will be pushed to the right as temperature increases. At P = 1.00 atm [CO] = 0.31% = 3100 ppm It takes only 600 ppm of CO to cause death in humans; see Seinfeld, p.55. The CO is produced quickly and does not reform CO₂ fast enough to be destroyed as the exhaust gases cool. But a car’s engine does not run at 1.00 atm pressure, the typical compression ratio is 8:1 (120 psi). If P = 8 atm and we maintain a stoichiometric air/fuel mixture,

  6. This may look like more than we had at 1.0 atm, but at 8.0 atm total pressure: [CO] ≈ 1.6 x 10⁻³ = 1600 ppm Only half as much! Higher compression reduces the amount of CO produced. What happens if we adjust the carburetor to produce a lean burn? Let there be 10% excess oxygen, and leave the total pressure at 8.0 atm. We will assume all the O₂ comes from the excess oxygen.

  7. Rearranging: [CO] = 330 ppm This is much less than the 3100 ppm we calculated for a stoichiometric mixture!

  8. CONCLUSIONS For minimal CO production: • Burn as cool as possible. • Burn as lean as possible. • Burn as high compression as possible. EXTRA EXAMPLE How much molecular hydrogen will be formed by the decomposition of water in the exhaust gas? This example uses water recombination instead of the water dissociation reaction, whole number coefficients instead of ½, and units of kcal mole⁻¹ instead of kJ mole⁻¹.

  9. Assume that the reaction takes place at one atmosphere pressure, 2000 K and slightly lean, 1% excess oxygen. (The concentration of H₂ would be somewhat more for a stoichiometric mixture.) Let P(O₂) ~ 1%, P(H₂O) ~ 10% and T = 298 K, Equilibrium lies far to the right and H₂O is strongly favored over H₂. But at a combustion temperature of 2000 K what happens to ΔG?

  10. This difference of 36 kcal (about 30%) makes a large difference in the calculated equilibrium concentration of H₂. In general, for the Gibbs free energy at some temperature other than 298 K.

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