Unimolecular elementary reaction

The equation for the decay of a nucleus (parent nucleus - daughter nucleus + radiation) has exactly the same form as a unimolecular elementary reaction (Section 13.7), with an unstable nucleus taking the place of a reactant molecule. This type of decay is expected for a process that does not depend on any external factors but only on the instability of the nucleus. The rate of nuclear decay depends only on the identity of the isotope, not on its chemical form or temperature. 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

The fraction of the ds-2-butene molecules present that react during each one-minute interval is the same. That is, the rate of reaction is constant on a per molecule basis. As the reaction proceeds, however, fewer ds-2-butene molecules remain, causing the overall rate of reaction to decrease. This is characteristic of unimolecular elementary reactions. The rate per molecule is constant, but if the number of reactant molecules is cut in half, the rate of reaction is cut in half, as well. 

A unimolecular elementary reaction is a fragmentation or rearrangement of one chemical species, so its rate law contains the concentration of only that species ... 

A unimolecular elementary reaction occurs when one molecule or ion reacts. For example, when one molecule of chlorine absorbs ultraviolet light, the Cl—Cl bond breaks. The product is two chlorine atoms. 

Irreversible First-Order Reaction If a reactant A is converted to a product P by an irreversible unimolecular elementary reaction... 

Figure 2.12 illustrates schematically the essential features of the thermodynamic formulation of ACT. If it were possible to evaluate A5 ° and A// ° from a knowledge of the properties of aqueous and surface species, the elementary bimolecular rate constant could be calculated. At present, this possibility has been realized for only a limited group of reactions, for example, certain (outer-sphere) electron transfers between ions in solution. The ACT framework finds wide use in interpreting experimental bimolecular rate constants for elementary solution reactions and for correlating, and sometimes interpolating, rate constants within families of related reactions. It is noted that a parallel development for unimolecular elementary reactions yields an expression for k analogous to equation 128, with appropriate AS °. 

A unimolecular elementary reaction involves only a single reactant molecule. An example is the dissociation of energized N2O5 molecules in the gas phase ... 

This form is the same as a unimolecular elementary reaction, with an unstable nucleus taking the place of an excited molecule. 

Consider two substances, A and B, that are linked by simple unimolecular elementary reactions. ... 

The rate laws for elementary reactions can be written immediately. Under any prescribed set of conditions, the probability of a molecule A falling into fragments in unit time is a constant. So for the unimolecular elementary reaction... 

Because there is only one molecule present, this is a unimolecular elementary reaction. It follows that the larger the number of A molecules present, the faster the rate of product formation. Thus, the rate of a unimolecular elementary reaction is directly proportional to the concentration i, ... 

Fig. 7.4 In a unimolecular elementary reaction, an energetically excited species decomposes into products or undergoes a conformational change. Shown is an example of the latter process the isomerization of energetically excited retinal (denoted with an asterisk). In the protein rhodopsin, bound retinal undergoes a similar isomerization when excited by light, initiating the cascade involved in vision.

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