![]() Chlorine atoms are generated when chlorocarbons or chlorofluorocarbons-once widely used as refrigerants and propellants-are photochemically decomposed by ultraviolet light or react with hydroxyl radicals. Molina and Rowland demonstrated that chlorine atoms from human-made chemicals can catalyze ozone destruction in a process similar to that by which NO accelerates the depletion of ozone. The work of Molina and Rowland was instrumental in the adoption of the Montreal Protocol, an international treaty signed in 1987 that successfully began phasing out production of chemicals linked to ozone destruction. As chemical reactions deplete the amount of ozone in the upper atmosphere, a measurable “hole” forms above Antarctica, and an increase in the amount of solar ultraviolet radiation- strongly linked to the prevalence of skin cancers-reaches earth’s surface. The ozone layer protects earth from solar radiation by absorbing ultraviolet light. In 1974, Molina and Rowland published a paper in the journal Nature detailing the threat of chlorofluorocarbon gases to the stability of the ozone layer in earth’s upper atmosphere. (credit a: courtesy of Mario Molina credit b: modification of work by NASA) Although the catalyzed reaction mechanism for a reaction needn’t necessarily involve a different number of steps than the uncatalyzed mechanism, it must provide a reaction path whose rate determining step is faster (lower E a).įigure 12.20 (a) Mexican chemist Mario Molina (1943 –) shared the Nobel Prize in Chemistry in 1995 for his research on (b) the Antarctic ozone hole. This difference illustrates the means by which a catalyst functions to accelerate reactions, namely, by providing an alternative reaction mechanism with a lower activation energy. The uncatalyzed reaction proceeds via a one-step mechanism (one transition state observed), whereas the catalyzed reaction follows a two-step mechanism (two transition states observed) with a notably lesser activation energy. ![]() The reaction mechanisms, however, are clearly different. Consistent with the fact that the two diagrams represent the same overall reaction, both curves begin and end at the same energies (in this case, because products are more energetic than reactants, the reaction is endothermic). Inspection of the diagrams reveals several traits of these reactions. The concepts introduced in the previous section on reaction mechanisms provide the basis for understanding how catalysts are able to accomplish this very important function.įigure 12.19 shows reaction diagrams for a chemical process in the absence and presence of a catalyst. List examples of catalysis in natural and industrial processesĪmong the factors affecting chemical reaction rates discussed earlier in this chapter was the presence of a catalyst, a substance that can increase the reaction rate without being consumed in the reaction.Explain the function of a catalyst in terms of reaction mechanisms and potential energy diagrams. ![]() By the end of this section, you will be able to:
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