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  3. About a century ago, the Swedish physical scientist Arrhenius proposed a low

About a century ago, the Swedish physical scientist Arrhenius proposed a low

游客2025-01-03  29
问题    About a century ago, the Swedish physical scientist Arrhenius proposed a low of classical chemistry that relates chemical reaction rate to temperature. According to his equation, chemical reactions are increasingly unlikely to occur as temperature approaches absolute zero, and at absolute zero, reactions stop. However, recent experiment evidence reveals that although the Arrhenius equation is generally accurate in describing the kind of chemical reaction that occurs at relatively high temperature, at temperatures closer to zero a quantum-mechanical effect known as tunneling comes into play; this effect accounts for chemical reactions that are forbidden by the principles of classical chemistry. Specifically, entire molecules can tunnel through the barriers of repulsive forces from other molecules and chemically react even though these molecules do not have sufficient energy, according to classical chemistry, to overcome the repulsive barrier.
   The rate of any chemical reaction, regardless of the temperature at which it takes place, usually depends on a very important characteristic known as its activation energy. Any molecule can be imagined to reside at the bottom of a so-called potential well of energy. Some chemical reaction corresponds to the transition of a molecule from the bottom of one potential well to the bottom of another. In classical chemistry, such a transition can be accomplished only by going over the potential barrier between the well, the height of which remain constant and is called the activation energy of the reaction. In tunneling, the reacting molecules tunnel from the bottom of one to the bottom of another well without having to rise over the harrier between the two wells. Recently researchers have developed the concept of tunneling temperature: the temperature below which tunneling transitions greatly outnumber Arrhenius transitions, and classical mechanics gives way to its quantum counterpart.
   This tunneling phenomenon at very low temperatures suggested my hypothesis about a cold prehistory of life: formation of rather complex organic molecules in the deep cold of outer space, where temperatures usually reach only a few degrees Kelvin. Cosmic rays might trigger the synthesis of simple molecules, such as interstellar formaldehyde, in dark clouds of interstellar dust. Afterward complex organic molecules would be formed, slowly hut surely, by means of tunneling. After I offered my hypothesis, Hoyle and Wickramashinghe argued that molecules of interstellar formaldehyde have indeed evolved into stable polysaccharides such as cellulose and starch. Their conclusions, although strongly disputed, have generated excitement among investigators such as myself who are proposing that the galactic clouds are the places where the prebiological evolution of compounds necessary to life occurred. [br] In which of the following ways are the mentioned chemical reactions and tunneling reactions alike?
选项 A、In both, reacting molecules have to rise over the barrier between the two wells.
B、In both types of reactions, a transition is made from the bottom of one potential well to the bottom of another.
C、In both types of reactions, reacting molecules are able to go through the barrier between the two wells.
D、In neither type of reaction does the rate of a chemical reaction depend on its activation energy.
答案 B
解析 两类反应中，都有一个从一个势阱底部到另一个势阱底部的跃迁。见第二段第三句起“化学反应跟分子从一个势阱的底部到另一个势阱的底部的跃迁相类似。在经典化学中，这种跃迁只有跨过两阱之间的势垒才能完成。位垒之高度为常数(固定不变)。这种跃迁叫做能量活化．在贯穿势垒效应中作反应的分子从一个势阱的底部通到另一个势阱底部不需要上升跨越两阱之间的位垒。”
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