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Equilibrium Constants

Equilibrium Constants Chlorofluorocarbons, haloalkanes, or Freons TM , a Dupont trademark for which they were a $5 billon dollar business, are alkanes in which all the hydrogens have been replaced by fluorine and chlorine atoms .

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Equilibrium Constants

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  1. Equilibrium Constants Chlorofluorocarbons, haloalkanes, or FreonsTM, a Dupont trademark for which they were a $5 billon dollar business, are alkanes in which all the hydrogens have been replaced by fluorine and chlorine atoms. FreonTM 12, CF2Cl2, is an example of a halomethane. How many different halomethanes are there? They are unreactive gases under ordinary conditions and are ideal for use as propellants in pressurized products such as hair sprays, shaving creams, etc. They are also non-inflammable compounds with high heat capacities and have found wide use in halon fire extinguishers and as coolants in refrigerators and air conditioning systems. Because they are unreactive these gases eventually mix into the stratosphere where they are subject to photodecomposition by ultraviolet light: CF2Cl2 (g) + hn (l < 220 nm) ------> CF2Cl (g) + Cl (g) The neutral radical chlorine atoms formed in this decomposition are very reactive (why?) and react readily with ozone in the two step mechanism: Cl (g) + O3 (g) ------> ClO (g) + O2 (g) Kp1 = 1.1 x 10+23 ClO (g) + O (g) ------> O2 (g) + Cl (g) Kp2 = 7.7 x 10+45 to give a net reaction involving oxygen atoms in which a molecule of ozone is converted to 2 molecules of oxygen: O3 (g) + O (g) ------> 2 O2 (g) Kp, net = ? What role do the Cl (g) atoms play in this mechansim? What role do the ClO (g) molecules play? 35.1

  2. We can begin the calculation of Kp, net by writing out in detail the equilibrium constant expression for the net reaction: Kp, net = (PO2)2 / ( PO3 PO ) = [PClO PO2 / (PCl PO3)] [PCl PO2 / (PClO PO)] How did we accomplish this last step? = Kp1 Kp2 = (1.1 x 10+23) (7.7 x 10+45) = 8.5 x 10+68 In general, when reactions are added to give some net reaction, how are the equilibrium constants of the individual reactions related to the equilibrium constant of the net reaction? At equilibrium is oxygen or ozone the favored molecular species in the reaction O3 (g) + O (g) ------> 2 O2 (g) ? The chemistry that we have just described is cause for some concern, since the stratospheric ozone that is removed as a result of these reactions absorbs much of the short wavelength solar ultraviolet light and protects us from this energetic radiation. The WEB site http://www.epa.gov/Ozone/science/hole/holehome.htmlprovides a good discussion of the causes and dynamics of ozone depletion in the stratosphere. Ozone levels are often measured in Dobson units.One Dobson unit, DU, corresponds to the ozone that would be present in a column of the atmosphere if it were compressed to a height of 0.0100 mm at STP. A “hole” in the ozone is said to occur when the ozone has fallen below 220 DU. How many ozone molecules are present in a column of the atmosphere 1.00 mm2 in area that has an ozone level of 220 DU? 35.2

  3. What is Kp for the reaction: 1/2 O3 (g) + 1/2 O (g) ------> O2 (g) Kp = ? in which the net ozone depletion reaction that we just discussed is scaled by a factor of 1/2? In general, when a reaction is scaled by some common factor, how is the equilibrium constant of the scaled reaction related to the equilibrium constant of the original reaction? What is Kp for the reaction: 2 O2 (g) ------> O3 (g) + O (g) Kp = ? in which net ozone depletion reaction that we previously discussed is reversed? In general, when a reaction is reversed, how is the equilibrium constant of the reversed reaction related to the equilibrium constant of the original reaction? 35.3

  4. Sulfur dioxide, SO2, formed during the combustion of sulfur containing fossil fuels, reacts with atmospheric oxygen and water droplets to form an areosol of sulfuric acid, i.e., acid rain: SO2 (g) + 1/2 O2 (g) + H2O (l) ------> H2SO4 (aq) K = ? Calculate the equilibrium constant for this reaction at 25.0 oC, using the known equilibrium constants for the reactions: 2 SO2 (g) + O2 (g) ------> 2 SO3 (g) K1 = 6.8 x 10+24 H2SO4 (aq) ------> SO3 (g) + H2O (l) K2 = 2.1 x 10-53 35.4

  5. During World War I Fritz Haber, a German scientist, developed a method of synthezing ammonia from atmospheric nitrogen and hydrogen known as the Haber synthesis of ammonia: N2 (g) + 3 H2 (g) ------> 2 NH3 (g) This reaction is an example of the chemical fixation of nitrogenin which normally unreactive molecular nitrogen is incorporated into a chemical compound. The ammonia was used produce ammonium nitrate for use in munitions. In the long run this chemistry has had a much greater impact in the peacetime production of fertilizer and has contributed greatly to feeding the world’s population. While the equilibrium constant for this reaction, Kp = 821 atm-2, is favorable for the production of ammonia at 25.0 oC, unfavorable kinetics requires that the reaction be run at substantially higher and more costly temperatures. Mimicking the cheap room temperature biological fixation of nitrogen by plant enzymes is still an active and unrealized goal of biochemists. The equilibrium constant based on concentration is defined for this reaction as: Kc = [NH3]e2 / ( [N2]e [H2]e3 ) Assuming that the gases can be treated as ideal, we can express the gas concentrations in terms of their partial pressures, e.g., for ammonia: [NH3] = PNH3 / R T Could you write similar relations for [N2] and [H2]? 35.5

  6. These relations allows us to relate Kc to Kp: Kc = [NH3]e2 / ( [N2]e [H2]e3 ) = (PNH3, e / R T)2 / [(PN2, e / R T) (PH2, e / R T)3 )] = Kp (R T)+2 = (821 atm-2) [(0.008205 L atm / mole K) (298.2 K)]+2 = 4.91 x 10+3 L2 / mole2 = 4.91 x 10+3 M-2 Could you write a general equation describing the relation between Kc and Kp for any reaction involving ideal gases? An equilibrium constant based on mole fractions, Kx, can also be defined using the relation between mole fractions and partial pressures that exists for ideal gases: Xi = Pi / Ptotal What is the value of Kx for the Haber synthesis of ammonia? Do any of the equilibrium constants K, Kc, Kp, or Kx depend onpressure? Two likely structures for fulminic acid, HCNO, are: C = N - O - H and H - C  N = O Use thermodynamics to argue which one of these structures is expected to be the most stable at 25 oC. You will have to look up data to answer this question. 35.6

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