TEXT E 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. In which of the following ways are the mentioned chemical reactions and tunneling reactions alike
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.