Orientation probability factor

Reaction rates are characterized by the number of successful collisions between reactants, which is represented by the product of the total number of collisions of the reactants, the number of collisions that have sufficient energy to cause a reaction event (energy factor), and the fraction of collisions that have the proper orientation (probability factor). The theoretical reaction rate (k) is given by... 

This term is called the frequency factor, the product of the collision frequency Z and an orientation probability factor, p, which is specific for each reaction A = pZ. The factor p is related to the structural complexity of the colliding... 

Of the five collisions shown, only one has an orientation in which the N of NO makes contact with an O of NO3. Actually, the orientation probability factor p value) for this reaction is 0.006 only 6 collisions in every 1000 (1 in 167) have an orientation that can lead to reaction. 

Entropy of activation (AA ), also called the probability factor. Not all collisions between molecules possessing the requisite AW result in reaction. Often collisions between molecules must also occur in a certain orientation, reflected by the value of AA. The more organized or less random the required orientation of the colliding molecules, the lower the entropy of activation and the slower the reaction. 

Let us now deduce the factors that control the rate of conversion of B and C to D and E by imagining the transformation process is portrayed well by what is known as a collision rate model. (Strictly speaking, the collision rate model applies to gas phase reactions here we use it to describe interactions in solution where we are not specifying the roles played by the solvent molecules.) First, in order to be able to react, the molecules B and C have to encounter each other and collide. Hence, the rate of reaction depends on the frequency of encounters of B and C, which is proportional to the product of their concentrations. The rate is also related to how fast B and C move in the aqueous solution. Next, the rate is proportional to the probability that B and C meet with the right orientation to be able to react, which we may refer to as the orientation probability . Third, only a fraction of collisions have a sufficient amount of energy (greater then or equal to Ea) to break the relevant bonds in B and C... 

The nature of the probability factor is very poorly understood. Since our. two reactions are quite similar, however, we might expect them to have similar probability factors. Experiment has shown this to be true whether chlorine or bromine atoms are involved, about one in every eight collisions with methane has the proper orientation for reaction. In general, where closely related reactions are concerned, we may assume that a difference in probability factor is not likely to be the cause of a large difference in reactivity. 

Entropy corresponds, roughly, to the randomness of a system equilibrium tends to favor the side in which fewer restrictions are placed on the atoms and molecules. Entropy of activation, then, is a measure of the relative randomness of reactants and transition state the fewer the restrictions that are placed on the arrangement of atoms in the transition state—relative to the reactants—the faster the reaction will go. We can see, in a general way, how probability factor and entropy of activation measure much the same thing. A low probability factor means that a rather special orientation of atoms is required on collision. In the other language, an unfavorable (low) entropy of activation means that rather severe restrictions arc placed on the positions of atoms in the transition state. 

The idea behind this is that even those collisions having the requisite energy may not produce reaction originally it was supposed that the molecules had to collide in a particular configuration, hence the name steric factor. This idea has some validity, especially since the low rates of reaction are usually observed with c,omplex molecules. Presumably two complex molecules will have less chance of colliding in the correct orientation for reaction than will two simple molecules. We will see shortly that the probability factor receives a more acceptable interpretation in terms of the entropy of activation of a reaction. In particular, the collision theory offers no explanation for abnormally fast reactions in which P would have to be greater than unity. 

Probability factor. If a primary hydrogen is to be abstracted, the alkane must be so oriented at the time of collision that the halogen atom strikes a primary hydrogen. Likewise, if a secondary hydrogen is to be abstracted, the alkane must be so oriented that the halogen atom strikes... 

Results obtained using the order parameter determined by SALS, as the value of the orientation efficiency factor, are always closer to the lower bound values of the Young s modulus of the composites. The order parameter is determined as a function of the angle of the fibers to the shear direction, meaning that its value will be more accurate for an in-plane distribution of the fibers. This would probably be the case if the fibers had higher aspect ratio. Nevertheless, SALS seems to be a promising... 

Thus, die main reaction takes place on the esteratic site of the oizyme but its anionic site facilitates the orientation of the ACh molecule, providing a high-probability factor that the reaction will occur. ... 

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