Reaction rates orientation factor

In this expression, A is a constant having the same units as the rate constant. It is equal to the fraction of collisions with the proper orientations when all reactant concentrations are one molar. R is the universal gas constant, expressed with the same energy units in its numerator as are used for For instance, when E is known in J/mol, the value R = 8.314 J/mol-K is appropriate. Here the unit mol is interpreted as mole of reaction, as described in Chapter 15. One important point is the following The greater the value of E, the smaller the value of k and the slower the reaction rate (other factors being equal). 

The reaction rate will be influenced not only by the surface concentration of reacting species bnt also by other factors, which may include tlie orientation of these species on the snrface and their bonding to neighboring species. This implies that an analysis of electrocatalytic phenomena must include a full consideration of all fea-tnres of adsorption. 

It is very difficult in view of the vast amount of experimental data to draw general conclusions that would hold for different, let alone all electrocatalytic systems. The crystallographic orientation of the surface undoubtedly has some specific influence on adsorption processes and on the electrochemical reaction rates, but this influence is rather small. It can merely be asserted that the presence of a particular surface orientation is not the decisive factor for high catalytic activity of a given electrode surface. 

The first possibility is an increase in the pre-exponential factor, A, which represents the probability of molecular impacts. The collision efficiency can be effectively influenced by mutual orientation of polar molecules involved in the reaction. Because this factor depends on the frequency of vibration of the atoms at the reaction interface, it could be postulated that the microwave field might affect this. Binner et al. [21] explained the increased reaction rates observed during the microwave synthesis of titanium carbide in this way ... 

Both parts (a) and (b) of Example 6-1 illustrate that rates of molecular collisions are extremely large. If collision were the only factor involved in chemical reaction, the rates of all reactions would be virtually instantaneous (the rate of N2-02 collisions in air calculated in Example 6-l(a) corresponds to 4.5 X107 mol L-1 s-1 ). Evidently, the energy and orientation factors indicated in equation 6.4-2 are important, and we now turn attention to them. 

In semiclassical ET theory, three parameters govern the reaction rates the electronic couphng between the donor and acceptor (%) the free-energy change for the reaction (AG°) and a parameter (X.) related to the extent of inner-shell and solvent nuclear reorganization accompanying the ET reaction [29]. Additionally, when intrinsic ET barriers are small, the dynamics of nuclear motion can limit ET rates through the frequency factor v. These parameters describe the rate of electron transfer between a donor and acceptor held at a fixed distance and orientation (Eq. 1),... 

In the intramolecular reactions studied by Bruice and Koshland and their co-workers, proximity effects (reduction in kinetic order and elimination of unfavourable ground state conformations) and orientation effects might give rate accelerations of 10 -10 . Hence, these effects can by themselves account for the enhancements seen in most intramolecular reactions. However, a factor of 10 -10 is less than the rate acceleration calculated for many enzyme reactions and certain intramolecular reactions, for example, hydrolysis of benzalde-hyde disalicyl acetal (3 X 10 ) (Anderson and Fife, 1973) and the lactonization reaction of[l] (10 ) where a trimethyl lock has been built into the system. If hydrolysis of tetramethylsuccinanilic acid (Higuchi et al., 1966) represents a steric compression effect (10 rate acceleration), then proximity, orientation, and steric compression... 

Here the pre-exponential factor At is the product of a temperature-dependent constant (ksT/h) = 2 X 10 °Ts where ke and h are the Boltzmann and Planck constants, and a solvent-specific coefficient that relates to both the solvent viscosity and to its orientational relaxation rate. This coefficient may be near unity for very mobile solvent molecules but may be considerably less than unity for viscous or orientationally hindered highly stractured solvents. The exponential factor involves the activation Gibbs energy that describes the height of the barrier to the formation of the activated complex from the reactants. It also describes temperature and pressure dependencies of the reaction rate. It is assumed that the activated complex is in equilibrium with the reactants, but that its change to form the products is rapid and independent of its environment in the solution (de Sainte Claire et al., 1997). 

When the symmetry factor was introduced by Volmer and Erdey-Gruz in 1930, it was thought to be a simple matter of the fraction of the potential that helps or hinders the transfer of an ion to or from the electrode (Section 7.2). A more molecularly oriented version of the effect of P upon reaction rate was introduced by Butler, who was the first to apply Morse-curve-type thinking to the dependence of theenergy-dis -tance relation in respect to nonsolvent and metal—hydrogen bonds. 

It is well known that the surface orientation of crystals and imperfections in the surface, like grain boundaries or dislocations, affect largely the reaction rates at electrodes made of metals or semiconductors. Such effects are most pronounced in those reactions where atoms leave their position in a crystal lattice or have to be incorporated into such one. These processes are connected with activation barriers which are particularly high for semiconductors where the chemical bonds between the components of the crystal lattice are highly directed and localized. If we consider photoelectrochemical reactions at semiconductors we have additionally to discuss the influence of these factors on light absorption and its consequences. 

In another example of SPE, a single crystal Si substrate with a 1000X thick Pd layer covered with lym of amorphous Si was heated to produce Pd9Si (13). As the Pd2Si reaction proceeds the thickness of the amorphous layer decreases and epitaxial Si is grown under the silicide layer. RBS, SIMS and AES were used to study the kinetics of the reaction. Substrate orientation and the concentration of carbon in the amorphous layer were found to be important factors in the rate of SPE growth. 

As you should recall from general chemistry, a favorable equilibrium constant is not sufficient to ensure that a reaction will occur. In addition, the rate of the reaction must be fast enough that the reaction occurs in a reasonable period of time. The reaction rate depends on a number of factors. First, the reactants, in this case the acid and the base, must collide. In this collision the molecules must be oriented properly so that the orbitals that will form the new bond can begin to overlap. The orientation required for the orbitals of the reactants is called the stereoelectronic requirement of the reaction. (,Stereo means dealing with the three dimensions of space.) In the acid-base reaction, the collision must occur so that the atomic orbital of the base that is occupied by the unshared pair of electrons can begin to overlap with the is orbital of the acidic hydrogen. 

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... 

Observed rates can be factors of 10 smaller than those predicted from simple reaction dynamics suggesting that a steric factor, which accounts for the relative orientations of the reactants, is extremely important in determining reaction rates. 

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