Química Ambiental e Processos

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18 Chemistry of the Environment Solutions to Exercises The hydroxide precipitate is composed of very small, evenly dispersed particles that do not settle quickly. This is required to remove very small bacteria and viruses from all parts of the liquid, not just the sites of solid formation. 18.55 (a) Trihalomethanes are a class of molecules with one central carbon atom bound to one hydrogen and three halogen atoms. They are produced by the reaction of dissolved chlorine with organic matter naturally present in water, and are byproducts of water disinfection via chlorination. (b) H-C-Br Cl Cl 18.56 If THMs are easily removed from water by aeration, this implies that, at a given temperature, their vapor pressure is greater than that of water. Air bubbles provide a vehicle to carry THM vapor away from the liquid water. Green Chemistry (section 18.5) 18.57 The fewer steps in a process, the less waste (solvents as well as unusable by-products) is generated. It is probably true that a process with fewer steps requires less energy at the site of the process, and it is certainly true that the less waste the process generates, the less energy is required to clean or dispose of the waste. 18.58 Catalysts increase the rate of a reaction by lowering activation energy, Ea. For an uncatalyzed reaction that requires extreme temperatures and pressures to generate product at a viable rate, finding a suitable catalyst reduces the required temperature and/or pressure, which reduces the amount of energy used to run the process. A catalyst can also increase rate of production, which would reduce the net time and thus energy required to generate a certain amount of product. 18.59 (a) C CH₂ + H2C + H2 C CH₂ (b) Prevention (1). The alternative process eliminates production of 3- chlorobenzoic acid by-product, chlorine-containing waste that must be treated. Atom Economy (2). Most of the starting atoms are in the final product. Less hazardous chemical synthesis (3) and Inherently safer for accident prevention (12). The starting material of the alternative process is not shock-sensitive, and the by-product is nontoxic water. The low molar mass of water means that a small amount of "waste" is generated. 562