The rates of many atmospheric reactions are accelerated by

Chapter , Problem 14.122

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The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride: Reaction 1: CH41g2 + Cl21g2 CH3Cl1g2 + HCl1g2 .This reaction is very slow in the absence of light. However, Cl21g2 can absorb light to form Cl atoms: Reaction 2: Cl21g2 + hv 2 Cl1g2 Once the Cl atoms are generated, they can catalyze the reaction of CH4 and Cl2, according to the following proposed mechanism: Reaction 3: CH41g2 + Cl1g2 CH31g2 + HCl1g2 Reaction 4: CH31g2 + Cl21g2 CH3Cl1g2 + Cl1g2 The enthalpy changes and activation energies for these two reactions are tabulated as follows: Reaction h 1kJ,mol2 Ea 1kJ,mol2 3 +4 17 4 -109 4 (a) By using the bond enthalpy for Cl2 (Table 8.4), determine the longest wavelength of light that is energetic enough to cause reaction 2 to occur. In which portion of the electromagnetic spectrum is this light found? (b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4. (c) By using bond enthalpies, estimate where the reactants, CH41g2 + Cl21g2, should be placed on your diagram in part (b). Use this result to estimate the value of Ea for the reaction CH41g2 + Cl21g2 CH31g2 + HCl1g2 + Cl1g2. (d) The species Cl(g) and CH31g2 in reactions 3 and 4 are radicals, that is, atoms or molecules with unpaired electrons. Draw a Lewis structure of CH3, and verify that it is a radical. (e) The sequence of reactions 3 and 4 comprises a radical chain mechanism. Why do you think this is called a chain reaction? Propose a reaction that will terminate the chain reaction.