Collision frequency factor

The analysis of the curvature of the experimental parabola led to very reasonable determinations of the intrinsic barrier. The measured values are relatively large, ca. 10-13 kcal moP, i.e. larger than usually found in stepwise dissociative processes but still not as large as found with other dissociative-type acceptors, such as halides. On the other hand, if the intrinsic barriers are calculated by the Eyring equation (equation 4) the values are larger by a few kcal mol (using the collision frequency factor Z). This is because the heterogeneous ET is actually non-adiabatic (which means that the actual pre-exponential factor is smaller). This is a very important aspect, which will be covered below. 

Rate constant, rate law, concentration profile, experimental measurement, integrated rate laws, linear plots, half-lifes Theory of the rate constant (activation energy, orientation factor, collision frequency factor, Transition State Theory)... 

To evaluate the activation energy, E, and the collision frequency factor, A, recourse may be made to Semenov s equation (18). To accommodate noninteger orders of... 

This expression relates the second-order rate constant, k, for an outer-sphere electron transfer reaction to the free energy of reaction, AG°, with one adjustable parameter, X, known as the reorganization energy. Wis the electrostatic work term for the coulombic interaction of the two reactants, which can be calculated from the collision distance, the dielectric constant, and a factor describing the influence of ionic strength. If one of the reactants is uncharged, Wis zero. In exact calculations, AG should be corrected for electrostatic work. The other terms in equation 46 can be treated as constants (Eberson, 1987) the diffusion-limited reaction rate constant, k, can be taken to be 10 M" is the equilibrium constant for precursor complex formation and Z is the universal collision frequency factor (see Eberson, 1987). 

The value of the collision frequency factor, A, is very nearly constant over moderate temperature changes. Thus, In A can be interpreted as the constant term in the equation (the intercept). The slope of the straight line obtained by plotting In k versus 1/T equals —EJR. This allows us to determine the value of the activation energy from the slope (Figure 16-14). Exercises 57 and 58 use this method. 

As indicated, the terms multiplying the exponential are customarily denoted by A, the collision frequency factor, or simply the preexponential factor. Thus, the reaction rate coefficient consists of two components, the frequency with which the reactants collide and the fraction of collisions that have enough energy to overcome the barrier to reaction. 

Effective Activation Energy and Collision (Frequency) Factor... 

The isotopic substitution affects, in general, not only the rate constant, but also the effective activation energy and the effective collision (frequency) factor. 

Here /c, = A, = FiZ, and P is the steric factor and Zj the collision frequency factor [350]. We will consider only situations where the steric factor ensures that the kinetics are reaction-limited. We assume that the pool species A is in equilibrium, i.e., /0+a = /0, a = In dimensionless variables we obtain the following... 

Two theories are generally used to describe the temperature dependence of the rate constants of elementary reactions. According to the collision theory, the rate constant ki depends on the collision frequency factor/ , the steric factor Z, and the Boltzmann factor exp (—E IRT) ... 

The first of these was simple collision theory, in which reacting molecules were treated as if they were hard spheres, and the frequencies of their collisions were calculated on the basis of kinetic theory. This treatment leads to a collision frequency factor z, and the rate constant is then obtained by multiplying z by, which is... 

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